Read-only memory (ROM, ROM), also called firmware, is an integrated circuit that is programmed with specific data during manufacture. ROMs are used not only in computers, but also in most other electronic devices.

Before talking about specific types of modern memory chips, we need to recall a little the past and understand the basic principles of electronic memory and its addressing features.

Computers, unlike people who use the decimal number system, use binary arithmetic, that is, any bit of a machine number can contain either "0" - no, or "1" - yes. Accordingly, each cell of the computer's electronic memory must remember one of two values ​​- 0 or 1. The simplest memory device is a set of toggle switches or relays that close or open an electrical circuit. If you recall, then the old computers were just used for random access memory relays, and ordinary toggle switches were used as ROM (and this is not surprising, since even minicomputers of the 80s of the last century had a panel with a set of toggle switches for entering commands).

The development of semiconductor technologies has led to the fact that in most cases silicon integrated circuits are used for the electronic memory of a personal computer. And the minimum memory cell in a microcircuit is a flip-flop, which in the simplest case is assembled on two transistors. But since trigger control requires control circuits, the elementary storage cell of modern static memory, which is used, in particular, for cache memory, sometimes contains up to a dozen transistors. For an example in fig. 12 shows a diagram of a memory cell of a CMOS chip. In it, of the six CMOS transistors, only transistors V3 and V5 are responsible for storing information, and the rest are used for other purposes.

Since in modern computer microcircuits containing hundreds of thousands of cells are used, then to simplify control, the storage cells are grouped into square matrices. To access a specific memory cell, an address is used that is formed from the row and column number (Fig. 13). As soon as the correct address of the desired cell is set on the column and row buses, the voltage corresponding to the information recorded in the memory cell will appear at the output of the matrix. Note that this addressing principle is also used to read or write a byte in RAM, but at the same time, each bit of a byte or word is responsible for its own storage matrix, which, most often, is located in a separate microcircuit.

To write information to a specific cell of the microcircuit, only one output is intended. When the correct address of the memory cell is set on the address bus, although the write signal will be applied to all cells, only the cell that is currently selected (addressed) will be written.

Figure 12. Diagram of a CMOS Memory Cell

The principle of writing and reading memory cells in a memory matrix is ​​well illustrated by the example of a ferrite memory (Fig. 14). At the dawn of the computer era, it was a small ferrite rings, located in knots wire mesh. To form a read and write signal, a separate wire was threaded through all the rings. Note that to write "1" and "0" we used the property of ferromagnets to remagnetize under the action of electric current. The smallest ferrite rings were only about 1 mm in diameter. With the advent of semiconductor memory chips, ferrite memory was forgotten for a long time, but FeRAM chips have recently appeared, which combine silicon chip production technology and the property of ferromagnetic materials to change their resistance depending on the applied magnetic field.

Processors have a data bus that is a multiple of 8 bits, for example, 8, 16, 32, or 64. In old personal computers, electronic memory was assembled from microcircuits having, for example, 64, 128, 256, etc. cells. On motherboards personal computers The IBM PC could see rows of memory chips taking up too much space there. In order to reduce the number of microcircuits and simplify their electrical connections with each other, several separate arrays of memory cells began to be created on a single silicon chip. The most popular options were when the memory chip has a bit depth equal to 4 and 8, which made it possible to reduce the number of cases on the board.

Figure 13

The documentation and price lists for memory chips always indicate not only its total volume, but also how the memory cells are organized. For example, below are the lines from the price list for microcircuits of dynamic DDR memory and SDRAM:

DDR 256Mb, 32Mx8, 266MHz;

DDR 128Mb, 1bMx8, 266MHz;

· SDRAM 256Mb, 32Mx8, 133MHz;

· SDRAM 128Mb, 16Mx8, 133MHz.

Figure 14. The principle of writing and reading memory cells in a storage matrix

Note that at the beginning there is a symbol for the type of microcircuit, and at the end it indicates the maximum clock frequency of the bus on which they can operate. The amount of memory in the chip is indicated in two versions: 256Mb -- the total number of memory cells in the chip; 32Mx8 - this designation shows that each bit has 32 MB (the term "address space depth" is also used, from English, address depth). If you multiply 32 MB by 8, you get 256 MB

Memory is always a very complex structure that includes many elements. True, the internal structure of the memory is regular, most of the elements are the same, the connections between the elements are relatively simple, so the functions performed by the memory chips are not too complicated.

Memory, as its name implies, is designed to remember, store some arrays of information, in other words, sets, tables, groups of digital codes. Each code is stored in a separate element of memory called a memory cell. The main function of any memory is precisely to issue these codes to the outputs of the microcircuit upon an external request. And the main memory parameter is its volume, that is, the number of codes that can be stored in it, and the capacity of these codes.

The following special units are used to indicate the number of memory cells:

• 1K is 1024, that is, 2 10 (read "kilo-" or "ka-"), approximately equal to one thousand;
• 1M is 1048576, that is, 2 20 (read "mega-"), approximately equal to one million;
• 1G is 1073741824, that is, 2 30 (pronounced "giga-"), approximately equal to one billion.

Principle memory organization is written as follows: first, the number of cells is written, and then through the multiplication sign (oblique cross) - the capacity of the code stored in one cell. For example, memory organization 64Kx8 means that the memory has 64K (that is, 65536) cells and each cell is eight-bit. AND memory organization 4M x 1 means that the memory has 4M (that is, 4194304) cells, with each cell having only one bit. The total amount of memory is measured in bytes (kilobytes - KB, megabytes - MB, gigabytes - GB) or in bits (kilobits - Kbps, megabits - Mbps, gigabits - Gbps).

Depending on the method of entering (recording) information and on the method of its storage, memory chips are divided into the following main types:

• Persistent memory(ROM - persistent storage device, ROM - Read Only Memory - read-only memory), in which information is entered once at the stage of manufacturing the microcircuit. Such memory is also called mask ROM. The information in the memory does not disappear when the power is turned off, which is why it is also called non-volatile memory.
• Programmable Read Only Memory(PROM - programmable ROM, PROM - Programmable ROM), into which information can be entered by the user using special methods (a limited number of times). The information in the PROM also does not disappear when its power is turned off, that is, it is also non-volatile.
• RAM(RAM - random access memory, RAM - Random Access Memory - memory with random access), the recording of information into which is the simplest and can be done by the user as many times as desired throughout the life of the chip. Information in the memory is lost when the power is turned off.

There are many intermediate types of memory, as well as many subtypes, but these are the most important, fundamentally different from each other. Although, the difference between ROM and PROM from the point of view of the developer of digital devices, as a rule, is not so great. Only in some cases, for example, when using the so-called flash memory (flash-memory), which is a PROM with repeated electrical erasure and rewriting of information, is this difference really extremely important. It can be considered that flash memory occupies an intermediate position between RAM and ROM.

In general, any memory chip has the following information outputs (Fig. 11.1):

Rice. 11.1.

• Address pins (input) that form the memory address bus. The code on the address lines is the binary number of the memory cell that is currently being accessed. The number of address bits determines the number of memory cells: with the number of address bits n, the number of memory cells is 2 n .
• Data pins (output) that form the memory data bus. The code on the data lines represents the contents of the memory cell that is being accessed at the moment. The number of data bits determines the number of bits of all memory cells (usually it is equal to 1, 4, 8, 16). As a rule, the data outputs are of the output stage type OK or 3C.
• In the case of random access memory, in addition to the output data bus, there may also be a separate input data bus, to which the code is fed, which is written to the selected memory cell. Another possible option is to combine the input and output data buses, that is, a bidirectional bus, the direction of information transfer over which is determined by control signals. A bidirectional bus is usually used when the number of data bus bits is 4 or more.
• Control pins (input), which determine the mode of operation of the microcircuit. In most cases, the memory has a CS chip selection input (there may be several of them, combined by the AND function). RAM also necessarily has a WR write input, active signal level which puts the chip in write mode.

We will not, of course, study all possible types of memory chips in this lecture; an entire book is not enough for this. In addition, this information is contained in numerous reference books. Memory chips are produced by dozens of companies all over the world, so even listing all of them is not too easy, not to mention considering their features and parameters in detail. We will just consider various schemes for switching on typical memory chips to solve the most common tasks, as well as design methods for some nodes and devices based on memory chips. This is what has a direct bearing on digital circuitry. And it is precisely the methods of switching on microcircuits that little depend on the characteristic features of a particular microcircuit of a particular company.

In this section, we will not talk about flash memory, as this is a separate big topic. We will limit ourselves to the simplest ROM and PROM chips, information in which is entered once and for all (at the manufacturing stage or by the user himself). We will also not consider here the features of equipment for programming EPROMs (the so-called programmers), the principles of their construction and use - this is a separate big topic. We will assume that the information we need can be recorded in ROM or PROM, and when, how, in what way it will be recorded is not very important to us. All these assumptions will allow us to focus specifically on the circuitry of nodes and devices based on ROM and PROM (for simplicity, we will call them simply ROM in the future).

We will only mention here that PROMs are divided into reprogrammable or reprogrammable

Very often, various applications require the storage of information that does not change during the operation of the device. This is information such as programs in microcontrollers, bootloaders (BIOS) in computers, tables of digital filter coefficients in signal processors, and tables of sines and cosines in NCO and DDS. Almost always, this information is not required at the same time, so the simplest devices for storing permanent information (ROM) can be built on multiplexers. Sometimes read-only memory devices are referred to in translation literature as ROM (read only memory). A diagram of such read-only memory (ROM) is shown in Figure 1.

Figure 1. Schematic read-only memory (ROM) built on a multiplexer.

In this scheme, a permanent storage device is built for eight single-bit cells. Storing a specific bit in a single-bit cell is done by soldering the wire to the power source (writing one) or soldering the wire to the body (writing zero). On circuit diagrams such a device is designated as shown in Figure 2.

Figure 2. Designation of a read-only memory device on circuit diagrams.

In order to increase the capacity of a ROM memory cell, these microcircuits can be connected in parallel (the outputs and the recorded information naturally remain independent). The scheme of parallel connection of single-bit ROMs is shown in Figure 3.

Figure 3. Scheme of a multi-bit ROM (ROM).

In real ROMs, information is recorded using the last operation of chip production - metallization. Metallization is performed using a mask, so such ROMs are called mask ROMs. Another difference between real microcircuits and the simplified model above is the use of, in addition to the multiplexer, also. This solution makes it possible to convert a one-dimensional memory structure into a two-dimensional one and, thereby, significantly reduce the volume of the decoder circuit required for the operation of the ROM circuit. This situation is illustrated by the following figure:

Figure 4. Schematic of the mask read-only memory (ROM).

Masked ROMs are depicted on circuit diagrams as shown in Figure 5. The addresses of the memory cells in this microcircuit are fed to pins A0 ... A9. The chip is selected by the CS signal. Using this signal, you can increase the amount of ROM (an example of using the CS signal is given in the discussion). The chip is read by the RD signal.

Figure 5. Mask ROM (ROM) in schematic diagrams.

The mask ROM is programmed at the factory, which is very inconvenient for small and medium production runs, not to mention the device development stage. Naturally, for large-scale production, mask ROMs are the cheapest type of ROM, and therefore are widely used at present. For small and medium production series of radio equipment, microcircuits have been developed that can be programmed in special devices - programmers. In these ROMs, the permanent connection of conductors in the memory matrix is ​​replaced by fusible links made of polycrystalline silicon. During the production of the ROM, all jumpers are made, which is equivalent to writing logical units to all ROM memory cells. In the process of programming the ROM, increased power is supplied to the power leads and outputs of the microcircuit. In this case, if the supply voltage (logical unit) is applied to the output of the ROM, then no current will flow through the jumper and the jumper will remain intact. If, however, a low voltage level is applied to the ROM output (connected to the case), then a current will flow through the memory matrix jumper, which will evaporate it, and when information is subsequently read from this ROM cell, a logical zero will be read.

Such chips are called programmable ROM (PROM) or PROM and are depicted on circuit diagrams as shown in Figure 6. As an example of PROM, microcircuits 155PE3, 556RT4, 556RT8 and others can be called.

Figure 6. Programmable read-only memory (PROM) symbol on circuit diagrams.

Programmable ROMs have proved to be very convenient for small- and medium-scale production. However, when developing electronic devices, it is often necessary to change the program written to ROM. In this case, the ROM cannot be reused, therefore, once written ROM, in case of an erroneous or intermediate program, one has to throw it away, which naturally increases the cost of developing equipment. To eliminate this shortcoming, another type of ROM was developed that could be erased and reprogrammed.

ROM with UV erasure is built on the basis of a memory matrix built on memory cells, the internal structure of which is shown in the following figure:

Figure 7. ROM memory cell with ultraviolet and electrical erasure.

The cell is a MOS transistor with a polycrystalline silicon gate. Then, during the manufacturing process of the microcircuit, this gate is oxidized and as a result it will be surrounded by silicon oxide - a dielectric with excellent insulating properties. In the described cell, with the ROM completely erased, there is no charge in the floating gate, and therefore the transistor does not conduct current. When programming the ROM, a high voltage is applied to the second gate, located above the floating gate, and charges are induced in the floating gate due to the tunnel effect. After the programming voltage is removed, the induced charge remains on the floating gate, and hence the transistor remains in the conductive state. The charge on the floating gate of such a cell can be stored for decades.

Structural scheme described read-only memory does not differ from the previously described mask ROM. The only difference is that the cell described above is used instead of a fusible link. This type of ROM is called reprogrammable read-only memory (EPROM) or EPROM. In the EPROM, the erasure of previously recorded information is carried out by ultraviolet radiation. In order for this light to pass unhindered to the semiconductor crystal, a quartz glass window is built into the housing of the ROM chip.

When the EPROM chip is irradiated, the insulating properties of silicon oxide are lost, the accumulated charge from the floating gate flows into the semiconductor volume, and the storage cell transistor goes into the closed state. The erasing time of the RPZU chip ranges from 10 to 30 minutes.

The number of write-erase cycles of EPROM chips is in the range from 10 to 100 times, after which the RPZU chip fails. This is due to the destructive effect of ultraviolet radiation on silicon oxide. As an example of EPROM microcircuits, we can name microcircuits of the 573 series of Russian production, microcircuits of the 27sXXX series of foreign production. The EPROM most often stores the BIOS programs of mainframe computers. RPZU are depicted on circuit diagrams as shown in Figure 8.

Figure 8. Conventional graphic designation of RPZU (EPROM) on the circuit diagrams.

So so, cases with a quartz window are very expensive, as well as a small number of write-erase cycles, led to the search for ways to erase information from the EPROM electrically. Many difficulties were encountered along the way, which have now been practically resolved. Now, microcircuits with electrical erasure of information are quite widespread. As a memory cell, they use the same cells as in the EPROM, but they are erased by electric potential, so the number of write-erase cycles for these microcircuits reaches 1,000,000 times. The time for erasing a memory cell in such ROMs is reduced to 10 ms. The control circuit for electrically erasable programmable ROM turned out to be complex, so two directions for the development of these microcircuits have been outlined:

1. EEPROM (EEPROM) - electrically erasable programmable read-only memory
2. FLASH ROM

Electrically erasable EPROMs (EEPROMs) are more expensive and smaller in size, but they allow you to overwrite each memory cell separately. As a result, these microcircuits have the maximum number of write-erase cycles. The scope of electrically erasable ROM is the storage of data that should not be erased when the power is turned off. These microcircuits include domestic microcircuits 573РР3, 558РР3 and foreign EEPROM microcircuits of the 28cXX series. EEPROMs are designated on circuit diagrams as shown in Figure 9.

Figure 9. Electrically erasable read-only memory (EEPROM) symbol on circuit diagrams.

Recently, there has been a tendency to reduce the size of the EEPROM by reducing the number of external microcircuit pins. To do this, the address and data are transferred to and from the chip via a serial port. In this case, two types of serial ports are used - SPI port and I2C port (microcircuits 93cXX and 24cXX series, respectively). The foreign series 24cXX corresponds to the domestic series of microcircuits 558РРX.

FLASH-ROMs differ from EEPROMs in that the erasure is not performed on each cell separately, but on the entire microcircuit as a whole or on the memory matrix block of this microcircuit, as was done in the EPROM.

Figure 10. Conventional graphic designation of FLASH memory on circuit diagrams.

When accessing a permanent storage device, you must first set the address of the memory cell on the address bus, and then perform a read operation from the microcircuit. This timing diagram is shown in Figure 11.

Figure 11. Timing diagrams of signals for reading information from ROM.

In Figure 11, the arrows show the sequence in which the control signals should be generated. In this figure, RD is the read signal, A is the cell address select signals (because individual bits in the address bus can receive different meanings, then the transition paths are shown both to the single and to the zero state), D is the output information read from the selected ROM cell.

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If there are RISC processors, then they are close to executing one instruction in each clock cycle.

Also, with the simplification of the CPU, the number of transistors required for its implementation decreases, therefore, the crystal area decreases. This results in a reduction in cost and power consumption.

It should also be borne in mind that, due to their simplicity, RISC processors are not patentable. This also contributes to their rapid development and wide production. Meanwhile, the reduced set of RISC included only the most commonly used commands. A number of rare CISC processor instructions are executed by RISC processor instruction sequences.

Later, the concept of MISC processors appeared, using a minimal set of long instructions. Following them, VLIW processors appeared, working with extra-long instructions. The speed of processors is defined in millions of operations per second MIPS.

Memory in microprocessor devices

AT In microprocessor devices, memory is used to store the initial data of information processing programs of intermediate and final calculation results.

There are two main types of memory:

∙RAM is random access memory used to store data, so this memory is also called data memory. The number of read and write cycles in the RAM is not limited, but when the supply voltage is turned off, all information is lost;

AT In modern microprocessors, RAM memory is a multi-level system in which there are levels of super-rapid memory (SRAM), RAM, buffer memory (BRAM) and external memory(VZU).

Each subsequent level differs from the previous one in capacity and speed.

The capacity is called maximum amount information that can be stored in memory.

Performance is characterized by the duration of read and write operations, the two main operations performed by memory.

For the specified memory levels, the capacity grows in the direction from SRAM to OVC, and the performance in the opposite direction.

∙ROM is a read-only memory device designed to store programs, so this memory is often called code or program memory. ROM chips are capable of retaining information when the power is turned off, but can only be programmed once or a very limited number of times.

Key Features of Semiconductor Memory

The main characteristics of memory to consider when designing systems are:

∙Memory capacity is determined by the number of bits of stored information. The chip capacity is usually also expressed in bits. An important characteristic crystal is the information organization of the memory crystal MxN, where M is the number of words, N is the word length. For the same access time, a memory with a larger sample width has a greater information capacity.

∙Time characteristics of memory.

1.1 Access time - the time interval determined from the moment when the central processor put the address of the required memory cell on the address bus and sent an order to read or write data via the control bus, until the addressed cell is connected to the data bus.

o The recovery time is the time required to restore the memory to its original state after the CPU has removed the address from the SHA, the “read” or “write” signal from the SHA, and the data from the SDS.

∙The unit cost of a storage device is determined by the ratio of its cost to information capacity, i.e. is determined by the cost of a bit of stored information.

∙Energy consumption (or power dissipation) is given for two modes of operation of the crystal: passive storage mode and active mode, when write and read operations are performed at nominal speed.

∙Packing density is determined by the area of ​​the memory element and depends on the number of transistors in the element circuit and the technology used. The highest packing density has been achieved in dynamic memory crystals.

∙The allowable ambient temperature is usually specified separately for active operation, for passive storage, and for non-operating with power off. The type of housing is indicated if it is standard, or a drawing of the housing indicating all dimensions, marking and numbering of contacts if the housing is new. Operating conditions are also given: working position, mechanical influences, permissible humidity and others.

Types of read-only memory (ROM) chips

There are the following main types of ROM:

∙mask ROMs - they are programmed during their manufacture by applying a mask of closed (high level) and open jumpers (low level), this type of ROM is the cheapest, but when manufactured in large quantities;

∙ ROM with fusible jumpers or electrically programmable (EPROM) - these microcircuits are programmed by the consumer by passing current pulses until the jumpers corresponding to the bits are destroyed, which should become zero;

∙ reprogrammable ROM with electrical recording of information and erasing by ultraviolet radiation (UFPROM) - the basis of the microcircuit memory cell of this type- MOSFET with a fully insulated "floating" gate, during programming, the oxide breaks through and a charge accumulates on the gate, which remains there until the microcircuit is subjected to UV irradiation, under its action the oxide becomes conductive; the resistance of the transistor channel depends on the charge on the gate and will determine the bit written to the cell;

∙ electrically erasable ROMs (EEPROM) are designed similarly to UFPROM, but erasing occurs, like recording, when voltage pulses are applied; this is the most expensive, but also the most convenient type of ROM.

∙FLASH memory is the most popular at present. Its main advantage is that it is built on the principle of electrical reprogrammability, that is, it allows for multiple erasing and recording of information using programmers. The minimum guaranteed number of write/erase cycles usually exceeds several thousand. This significantly increases the life cycle and increases the flexibility of microprocessor systems, as it allows you to make changes to the microprocessor program, both at the system development stage and during its operation in a real device.

Types of RAM chips

There are two types of RAM chips:

∙static RAM, in which the trigger serves as the basis of the storage cell;

RAM (English RAM) and ROM (English ROM) are digital storage media. They are used if the internal resources of the MK are insufficient for one reason or another. For comparison, the MK data memory size is 0.5…8 KBytes, the program memory size is 2…256 KBytes. You can also connect to the MK one or more external RAM chips with a capacity of 32 ... 512 KB or a flash ROM with a capacity of 0.5 ... 128 MB. The increase in computing resources is evident.

The generalized block diagrams of RAM and ROM are largely the same (Fig. 3.8). The base is a rectangular matrix of memory cells, which is accessed through the address lines AO…An, and read/write through the bidirectional data bus I/OO…I/Ok. Numerous varieties of RAM and ROM differ from each other in the logic of generating control signals CS, WR, RD, as well as the presence or absence of address line multiplexing.

Rice. 3.8. Structural diagram of RAM (ROM).

External ROMs are better to use "low voltage" electrically writable ( keyword Flash). Their programming voltage is 5 V, unlike 12 ... 27 V in the old "high-voltage" ROMs 27C256, KR573RF6A, which now do not make sense to use together with MK.

Typical storage time in flash ROM reaches 10...40 years with 0.1...1 million rewriting cycles. Distinguish between serial and parallel flash ROMs. The first of them are small-sized, low-output, but they have a low access speed and low capacity. Example - series 24Cxxx, 93Cxx. To connect such ROMs to MK, two or three-wire PC, SPI interfaces are used. In contrast, parallel flash ROMs have a large amount of memory, good performance, but require a lot of pins (two or three free 8-bit ports) to interface with MK. Example - series 28Fxxx, 29Cxxx.

External RAMs have high write and read speeds, but the information in them is lost when the power is turned off. Both lines of ports are used to interface the RAM with the MK. Sometimes it is more profitable to transfer them to the special “External RAM” mode, in which the external RAM area is included in a common memory card. Whether a particular MK supports this mode can be determined by the specific port line names in its part number. For example, in Fig. 3.9 is "AP0" ... "AP7" (data / address bus), "A8" ... "A15" (high bits of the address bus), "ALE", "WR", "RD" (control signals).

On Fig. 3.10, a ... and diagrams for connecting external memory to MK are given.

a) DS1 chip (Samsung) is an "intelligent" flash ROM with its own command system. It is used, in particular, in USB drives;

Rice. 3.9. Pin layout and signal names in MK Atmel ATmega8515.

b) 16-bit information in dynamic RAM DS1 (OKI) is transmitted/received through pins "1/01" ... "1/04" sequentially in time by four blocks;

Rice. EVIL. Diagrams for connecting external memory to MK (continued):

c) the address bus "A0" ... "A18" and the data bus "U0" ... "U7" of the static RAM DS1 (Samsung) are multiplexed by registers DD1, DD2. On the edges of the signals F1, F2 in the registers, the full address of the cell is latched twice. Missing addresses are formed directly from MK ("R0" ... "R2"). When reading / writing RAM ("*RD" / "*WR"), 8 top MK lines work;

d) DS1 is a FRAM (Ramtron) ferroelectric serial RAM/ROM connected to the MK via the PC bus. When powered on, FRAM is equivalent to RAM, and when powered off, it is equivalent to ROM. The number of overwrites is not limited (!), the storage time of information is 45 years;

e) connection of serial RAM DS1 (64Kx8) to MK via a three-wire interface and anti-ringing resistors R2…R4;

Rice. 3.10. Diagrams for connecting external memory to MK (continued):

f) the lower 8 bits of the address bus are stored in the DD1 register. The upper 7 bits are fed from the MK directly to the Hitachi DS1 RAM. MK works in "External RAM" mode. Instead of a common wire, you can apply an enable signal from the free output MK to the “CE” input of the DS1 RAM. This reduces the overall power consumption from the power supply, because when the CE input is HIGH, the DS1 chip enters an economical data storage mode;

g) connection to the MK serial flash RAM DS1 from Atmel. If the switch S1 is closed, then no data can be written to the RAM, this is a protection mode. Resistors R3, R4 are missing in some circuits. DS1 replacement - any higher/lower capacity RAM from Atmel's DataFlash AT45DB family, including legacy AT45DB081B-CNU models;

Rice. 3.10. Diagrams for connecting external memory to MK (end):

h) Direct connection of DS1 Flash ROM (AMD) to MK requires a large number of free lines of ports. Some MK output lines, for example, "A7", can be simultaneously used to control other nodes, however, this is allowed only when there is no access to the ROM, i.e. at a HIGH level of the “OE” signal;

i) increasing the capacity of RAM due to the parallel connection of DSl ... DSn microcircuits. Each of the connected RAM has its own network program address, which is determined by different logic levels at the inputs "AO", "A1", "A2".