TommyPROM/docs/prom-families.md

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PROM Types	PROM types supported by TommyPROM - EPROM, EEPROM, Flash	false	40

TommyPROM can be used to read and write several different types of PROM chips. Due to differences in the technologies of these chips, some features of TommyPROM may work differently or may not be applicable at all. The standard code can read most types of PROM, even if there is no specific software support for them.

PROM Types

EPROM - Erasable Programmable Read-only Memory

EPROMs can be written once, but must then be erased by exposing them to UV light for several minutes. They are usually easily read by TommyPROM, but there is limited support for programming.

EEPROM - Electrically Erasable Programmable Read-only Memory

EEPROMs are the easiest PROMs to use. Modern EEPROMs usually can be erased and reprogrammed electrically at the individual byte level. This makes them appear similar to a slower static RAM. All of the interactive features of TommyPROM work well with EEPROMs. Due to their complexity, EEPROMs typically come in smaller sizes than other technologies. The largest EEPROM in the 28C family is 32K bytes. Some older EEPROMs cannot be reprogrammed at the individual byte level and are instead bulk erased before a new write operation. Programming and erasing for these chips usually requires voltages higher than 5V.

Flash ROM

Flash is similar to EEPROM, in that it can be electrically erased and reprogrammed. Unlike EEPROMs, Flash cannot erase single bytes individually. Instead, the chip is arranged into memory blocks called sectors. The programmer can erase individual sectors or, sometimes, the entire chip. Some chip families require an explicit erase command and others initiate an erase at the start of a programming operation.

Most of the features of TommyPROM are supported for Flash, but some operations that would do multiple writes to the same sector work differently.

OTP ROM - One-time Programmable ROM

One-time Programmable (OTP) ROM is written once, either at the factory or in the field. These chips cannot be erased and re-written, but they can be read by TommyPROM. Interesting trivia - some of the field-programmable OTPs are just EPROMs packaged without the erase window.

Supported Chip Families

28C EEPROM

The 28C series EEPROMs, specifically the 28C256, are the original chips that TommyPROM supported. As a result, these have the best support, with all of the interactive features of the command line. In addition the basic read and write operations, there are also commands to enable and disable the Software Data protection (SDP) features of 28C chips. There is also support for the fast block write mode of these chips, allowing a 32KB chip to be programmed in just a few seconds.

27C EPROM

The 27C EPROMs use a variety of programming algorithms. Code exists for some of these, but very few of the chips have been tested.

Most, if not all, 27C series chips are not 5V only and require a higher voltage to program them. Some need the voltage constantly applied while programming and others use high voltage pulses for each byte. For constant voltage chips, it is probably easiest to just add an external power supply and manually assert the voltage before starting a write. For those with switched voltages, some elements of the 8755 hardware may be leveraged to build a version of the programer that supports these chips.

Atmel SST39FS Flash

TommyPROM has a driver for Atmel SST39FS flash chips. This driver replaces the 28C driver at compile time. See configure.h to enable a different driver.

The SST39FS chips use fixed 2KB sectors that must be manually erased before a new program operation, but the code manages this transparently. Whenever a write is started to a new segment, the driver first initiates an erase of that sector. A second write to the same sector will not cause an erase, so is is possible to write to a segment multiple times with no additional steps as long as the writes are to different parts of the sector. For example, 256 bytes could be written to the start of a sector from one file and then 512 bytes could be written to the end of the sector from another file.

Writing data that spans multiple sectors also works with no additional steps. When a sector boundary is crossed, the new sector is erased and set as the current sector.

The SST39FS driver supports a manual erase from the command line using the E command. This is only needed if data will be rewritten to the same location after a previous write to that sector.

Misc Flash

29C Series

The 29C series flash chips are similar to the 28C EEPROMs, except they must erase an entire sector before programming new data. The start of a block write operation causes the target sector to be erased before the block is written. If the entire sector is not written in one operation, there is no way to write additional data to the sector because a new block write will erase the sector again.

The write algorithm for the 29C chips is similar to 28C, so it is possible to use that code to write these chips as well with some restrictions. All 29C010 and 29C020 chips that have been encountered use a sector size of either 64 bytes or 128 bytes. A single XModem packet holds 128 bytes of data, so these chips will program without issue.

The 28C040 chips from some manufacturers use a 256 byte sector. This will not currently work with TommyPROM, because each of the 128 byte XModem packets received will do its own block write. The first write will erase 256 bytes and then write data into the first half of the sector. The second write will then erase the data just written and then write 128 bytes into the second half of the sector. To fix this, the code would need to be modified to keep 256 bytes of data from two packets before initiating a block write.

28F Series

The 28F series flash do not have sectors. The entire chip must be erased before writing new data. Data is written a byte at a time using a command register to control the programming. These chips require a 12V programming voltage that must be present during the programming or erase operations. TommyPROM does not currently support 28F chips, but it would not be difficult to write a driver. The default 28C driver will read 28F chips.

29F Series

The 29F series flash have sectors that must be explicitly erased before writing new data. Data is written a byte at a time using a command register to control the programming. TommyPROM does not currently support 29F chips, but it would not be difficult to write a driver. The default 28C driver will read 29F chips.

Intel 8755/8355 EPROM

TommyPROM has a driver for Intel 8755 EPROMs. This driver replaces the 28C driver at compile time. See configure.h to enable a different driver. This driver will also read Intel 8355 OTP EPROMs. Also note that the TommyPROM hardware for the 8755 is drastically different from the other versions.

The Intel 8755 is a peripheral chip designed for 8085 systems. It is a 40 pin device that contains a 2KB EPROM plus two general-purpose I/O ports. The 8355 is a one-time programmable version of the 8755. The 8755 requires a 25V programming pulse for each byte to be written.

A new hardware build was created to support the 8755 chips. Because the 8755 has a multiplexed data and address bus, the usual shift registers are not used for addressing. The chip only needs 8 connects that are shared for address and data, plus three dedicated address lines. The Arduino has enough pins to drive all of these directly, without the need for shift registers to create address lines.

The 8755 build of TommyPROM also has a circuit to control the 25V programming pulses.

Verified Chips

Model	Manufacturer	Type	Module	Notes
28C256	Atmel, others	EEPROM	28C	Fully supported
SST39SF040	Microchip	Flash	SST39SF	All SST39SF0x0 supported
27E257		EEPROM	27	Continual 12V or 14V for program/erase
29C010		Flash	28C	Only with 128 byte or less sector size
8755A	Intel	EPROM	8755A	Requires 25V pulses to program