Programming FLASH-memory chips. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Microcontrollers

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Reprogrammable permanent memory microcircuits with electrical data erasure, made using FLASH technology, have taken a strong position in electronic and computer technology, displacing other types of non-volatile storage devices. Their main advantage is the possibility of reprogramming "in the system" without unsoldering the chip from the printed circuit board or removing it from the panel. A large allowable number of reprogramming cycles makes it possible to build "FLASH-disks" with a volume of tens of megabytes on such microcircuits, which differ from conventional hard or floppy disk drives in the complete absence of moving parts. As a result, they are durable and able to operate in high vibration environments such as vehicles and other moving objects. The published article is devoted to the programming of FLASH-memory chips.

FLASH-memory chips differ from other types of ROMs by the presence of a built-in "programmer" directly on the chip - an erasing and writing machine (AC3). It eliminates the need in the programming process to apply increased voltage to the microcircuit outputs, to form certain sequences of pulses. AC3 does all this independently and imperceptibly for the user, who only needs to report the address of the cell and the code that should be written into it using the appropriate command, and wait for the operation to complete. In many cases, a lengthy operation (such as erasing a block of data) can be paused, read the desired information from another area of ​​memory, and then continue.

Today, many companies (the most famous are Intel, AMD, Atmel, Winbond) produce a wide range of FLASH memory chips up to 4 MB. Their external interface is either parallel or serial. Chips with a serial interface are mainly intended for storing small amounts of data in small-sized or specialized devices, for example, for storing fixed settings for a radio receiver or a program for operating a household appliance.

Next, we will talk about "parallel" FLASH microcircuits, which, in terms of the physical and logical device of the interface with the processor, do not differ in any way from conventional ROMs, except that they, like RAM, have a write enable input. It is in these microcircuits that the BIOS codes of modern computers are stored. Data organization is either eight or 16 bits. Often it can be chosen by connecting a specially provided output to a common wire or power source. In addition to the address and data buses, three control signals are supplied to the microcircuits: crystal select (CE), output enable (OE), and write enable (WE). The latter - only if the chip needs to be programmed. The minimum duration of the read cycle is 70... 150 ns.

In the first FLASH microcircuits, the array of memory cells was a single block, and data could only be erased entirely from the entire array. In many modern microcircuits, memory is divided into blocks, and erasing data in one of them does not affect the data stored in others. Block sizes are very different - from 128 bytes to 128 KB or more. However, when reading data, the entire memory of the microcircuit is considered as a single array, and that's it. that it is physically divided into blocks does not matter.

Typically, the blocks are the same and equal, but they can be different. For example, Intel's 28Fxxx series chips have a so-called boot block of 16 KB and two parameter blocks of 8 KB each. This is followed by a block of 96 KB, and the rest of the memory consists of blocks of 128 KB. The properties of these blocks are somewhat different. Boot has hardware write and erase protection. It is turned on by applying the appropriate logic level to a specially provided output of the microcircuit. Parameter blocks are designed to store frequently changed data and withstand a greater number of erase / write cycles than others.

Each of the microcircuits of the series under consideration is made in two versions, differing in the placement of blocks in the address space. In microcircuits with index B (bottom), they are located, starting from address zero, in the order indicated above. In products with the T index (top), the order is reversed (boot - in the area of ​​​​higher addresses).

The FLASH-memory microcircuits currently produced are designed for nominal supply voltages from 2.7 to 5 V. They do not require an increased voltage (12 V) at all or are necessary only in some special modes. In the passive ("unselected") state, such microcircuits consume a current of no more than 1 mA from the power source (in most cases, ten times less). Sometimes a special sleep mode is provided, in which the consumption is negligible. True, it is impossible to read data from a "sleeping" microcircuit, but in order to "wake" it up. sometimes it takes several tens of microseconds. The current consumed in active mode is tens of milliamps, and if you put a chip in a passive state, the AC3 of which performs a long operation (for example, erases data), the current will not decrease until it is completed.

Much attention is paid to protecting the data stored in the FLASH-memory from accidental change, especially under the influence of noise and transients when the power is turned on and off. In most cases, there are three types of hardware protection. The first one is. that the microcircuit does not respond to pulses in the WE circuit with a duration of less than 15 ... 20, the second is that with a low logic level at the OE input, no signal manipulations at other inputs can cause a recording, the third is that. that when the supply voltage drops below a certain level, AC3 turns off. For microcircuits of different types, the shutdown threshold is in the range of 1.5 ... 3.8V.

Sometimes it is possible to completely prohibit changing and erasing the entire data array or its parts. The imposition or removal of such a ban usually requires "extraordinary" measures (for example, briefly applying increased voltage to certain terminals).

Software protection is also provided. To change the contents of a cell, FLASH memory is not enough, as in conventional RAM. write one code to one address. A command is required, consisting of several codes written to specific addresses.

Any FLASH-chip is able to tell its type to the device in which it is installed, which allows you to automatically select the necessary algorithms for writing and erasing data. The appropriate commands are provided to programmatically enable and disable the ID reading mode. Having turned it on, the manufacturer's identifier is read at the address OH, and the devices are read at the address 1H (the identifiers of some microcircuits are given in the table). In the same mode, but at other addresses, in some cases you can get additional information, for example, about the state of hardware write protection.

It is possible to switch to the mode of reading identifiers without a command, by applying a voltage of +9 V to the address input A12. The permissible deviation of its value for microcircuits of different types is different. In some cases it is not more than ±5%. in others, it is enough that the voltage only exceeds a certain value, for example, 10 V. The identifiers are read at the above addresses, setting them without taking into account the A9 discharge. Usually this method is used in universal programmers.

AC3 of most flash memory chips accept commands given in accordance with the so-called JEDEC standard, although there are exceptions. Sometimes, when upgrading chips, their command system is supplemented with standard code combinations, while retaining, however, the old commands (this is necessary so that the upgraded chips can work in previously released devices). Intel uses its own command system.

Before considering the commands in detail, let's talk a little about connecting FLASH chips. Microcircuits of the same type, as a rule, are produced in several types of packages, differing in location, pitch and number of pins. Often, "mirror" options are provided, allowing you to install microcircuits on any side of the board without changing the topology of the printed conductors.

The pin numbers in the diagrams below are typical for 512K memory chips in the most common 32-pin PLCC and PDIP packages. The "pinout" of microcircuits of a smaller volume is similar, but the conclusions of the highest digits are not connected to them (for example, the 29th and 010st are free for Am30F1).

A scheme similar to that shown in Fig. 1 are used if it is necessary to erase and write data without removing the chip from the microprocessor system.

It is assumed that the system data bus is eight-bit, addresses are 16-bit. The ROM is allocated in the address space of 32 KB, the rest of it can be occupied by RAM Since the amount of memory of the Am29F040 is 512 KB, a FLASH memory page register is provided that controls the upper bits of the address. To read and write data, you can use the following simple procedures (written in Pascal):

If it is necessary to program the FLASH-chip outside the device in which it will work, it can be connected to a personal computer. The easiest way to do this is to install an optional parallel I/O card in your computer. Such boards such as PCL-731 from Advantech, DIO-48 from IOP DAS or PET-48DIO from ADLink are commercially available. As a rule, they have 48 inputs / outputs and they work similarly to two 8255 microcircuits (KP5806V55A) in O mode with the same information and control ports, even if in reality there are no such microcircuits in their composition. If necessary, the parallel input/output board can be made independently, using the article by N. Vasiliev "PC Interface Extender" ("Radio", 1994, No. 6, pp. 20, 21).

For reading or programming, the FLASH chip is connected to the ports of two 8255 chips according to the circuit shown in Fig. 2. The PA port of the first of them is used for data input/output, separate bits of its PC port are used for outputting control signals CE, OE and WE. Ports PA, PB, and PC second form the 24-bit address bus of the FLASH chip. If a smaller bit width of this bus is sufficient, the corresponding number of high-order bits of the PC port is not connected.

I/O board ports and auxiliary constants must be described in the program as follows:

And the procedures for accessing FLASH memory described above are replaced by the following:

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Now - actually about programming FLASH-microcircuits. According to the JEDEC standard, each command begins with the entry of the OAAN code at address 5555H. Next, code 55H is written at address 2AAAH and, finally, the code of the operation to be performed at address 5555H.

Speaking, for example, about the 40H command, we will mean just such a sequence with the number 40H as the operation code.

After turning on the power, any FLASH microcircuit automatically enters this mode and there is no need to set it with a special command. However, it is required, for example, to return from the read identifiers mode. It is sometimes referred to as a reset or initial setup command. To transfer some microcircuits to the array reading mode, one cycle of writing the 0F0H code to any address is enough.

The write cycle following the 0A0H command contains the address of the programmable cell and the code written to it. In most cases, writing to each cell requires a separate command. Keep in mind that, like conventional EEPROMs, in the bits of a programmable cell, you can only replace logical ones with zeros. To perform the reverse operation, it is usually necessary to first erase the contents of the entire memory block and repeat the programming of all its cells. Note that the AC3s of many FLASH chips do not recognize these errors and report success. To make sure that the programming is correct, a control reading of the recorded data is necessary.

In Winbond chips with 128-byte blocks, the programming of any cell is automatically preceded by the erasure of all data in the containing block. Therefore, you should always first copy the block to RAM, make the necessary changes to the copy and reprogram all 128 bytes. Having received the OOH command, the address and the first of the programmable bytes, AC3 enters it into the block's internal buffer and waits for 200 µs without starting programming. If during this time one more OOH command and the next byte are received, it will also go into the buffer, and AC3 will wait for the next 300 µs. This continues until then. until all 128 bytes of the block are received or the pause exceeds the allowable value (300 μs). The AC3 then erases the block and starts the actual programming. The sequence of writing to the buffer of data intended for different cells of the block does not matter, but those cells for which data has not been received will contain codes 0FFH after programming.

There are two ways to write programming data to such a chip. The first of them (usual for others) is called software-protected. Each byte to be written must be preceded by an OOH command. However, the protection can be disabled by issuing the 80H and 20H commands in sequence.

After that, the byte written to any address enters the internal buffer of the microcircuit, and this mode is maintained even after turning off and on the power. Get out of it at the command of OON.

There are two equivalent command options for writing data to an Intel FLASH chip. First of all, one of the codes 40H or 10H is written at any address. and then - the programmable code at the desired address.

"Erase all memory" command.

AC3 of the FLASH microcircuit starts this important operation by receiving a sequence of two commands - 80H and 10H.

Intel microcircuits are given a similar command by writing to arbitrary addresses of codes 20H and 0D0H

Erasing the entire contents of memory takes from tens of milliseconds to several seconds. Some microcircuits provide the ability to suspend this process by writing the OVON code to any address. After writing (also to any address) code 30H (for Intel chips - ODOH), erasing will continue.

Delete block command. To erase the contents of a memory block, two commands must be issued. The first of them is 80H, the second differs in that its operation code 90H must be written not at address 5555H, but at the address of any of the cells of the erased block.

"Read identifiers" command. To switch to this mode, the command 90H is used, but sometimes a sequence of two commands is required - 80H and 60H.

In Intel microcircuits, it is enough to write the code 90H at any address. This mode is exited by the command "Read data array" discussed above.

How to check the completion of the execution of "long" programming and erasing commands? The easiest way is to use the reference data of the microcircuit and provide for the software formation of the corresponding delays. But the actual execution time of certain operations often differs significantly from the reference values ​​even for different cells and blocks of the same microcircuit, increasing as the latter "ages".

Reading the status register AC3 allows you to accurately find out the moment the end of a particular operation. The FLASH chip outputs the contents of this register to the data bus as long as the AC3 is busy performing an erase or programming procedure. There are two signs that the process is not finished. The first is that the value of bit D7 of the status register is inverse with respect to the value written to the same bit of the memory cell (during erasing, it is equal to 0). At the end of the operation, it will match the recorded one. The second symptom is the "flickering" of bit D6 (its value changes with each read of the register until the operation is completed).

As a rule, both signs are observed, however, there are exceptions. For example, in Intel chips there is no "flickering" bit, and the D7 bit is 0 during programming, regardless of the code being written. The end of the operation in this case is evidenced by D7=1. In microcircuits with block recording (for example, from Winbond), the value of bit D7 is inversely analogous to the bit of the last code written to the block buffer.

Usually, upon completion of programming or erasing, the FLASH chip automatically returns to the data array read mode, but Intel chips require a corresponding command for this.

If the chip fails, the "long" operation may never complete, causing the programming computer to "hang". To avoid this, it is necessary to provide for checking the duration of the erasing and programming operations and, in case of exceeding a reasonable value, an "emergency" output with a fault message.

Sometimes, especially when working with microcircuits that have gone through a number of erasing / programming cycles close to the limit, it makes sense to repeat the unsuccessful operation several times. One of the attempts may be successful.

In conclusion, a few words about utilities that allow you to update the computer's BIOS stored in FLASH memory. They are developed for each type of system (motherboard) boards and take into account the peculiarities of connecting FLASH microcircuits to system buses. Therefore, attempts to use a utility designed for one type of board to update the BIOS of another often lead to a complete failure of the computer.

The utility is launched as a normal application program, specifying as a parameter the name of the file containing the codes of the new BIOS version. It reads this file, creating an array of data in RAM to be written to FLASH memory. Then it determines the type of microcircuit and selects the appropriate procedures for working with it. After that, the erasing of old and recording of new data begins, and at this time the program cannot use any BIOS functions, including displaying information on the screen or polling the keyboard. If it is still necessary to do this, the necessary subroutines are introduced into the utility itself. Once the programming is complete and verified to be correct, the computer is usually restarted and starts a "fresh life" with an updated BIOS.

Author: A. Dolgiy, Moscow

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