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158 lines
8.1 KiB
Markdown
158 lines
8.1 KiB
Markdown
---
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title: PROM Types
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description: "PROM types supported by TommyPROM - EPROM, EEPROM, Flash"
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has_children: false
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nav_order: 40
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---
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TommyPROM can be used to read and write several different types of PROM chips. Due to
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differences in the technologies of these chips, some features of TommyPROM may work
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differently or may not be applicable at all. The standard code can read most types of
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PROM, even if there is no specific software support for them.
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# PROM Types
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## EPROM - Erasable Programmable Read-only Memory
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EPROMs can be written once, but must then be erased by exposing them to UV light for
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several minutes. They are usually easily read by TommyPROM, but there is limited
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support for programming.
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## EEPROM - Electrically Erasable Programmable Read-only Memory
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EEPROMs are the easiest PROMs to use. Modern EEPROMs usually can be erased and reprogrammed
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electrically at the individual byte level. This makes them appear similar to a slower
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static RAM. All of the interactive features of TommyPROM work well with EEPROMs. Due
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to their complexity, EEPROMs typically come in smaller sizes than other technologies.
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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
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operation. Programming and erasing for these chips usually requires voltages higher than 5V.
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## Flash ROM
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Flash is similar to EEPROM, in that it can be electrically erased and reprogrammed. Unlike
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EEPROMs, Flash cannot erase single bytes individually. Instead, the chip is arranged into
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memory blocks called sectors. The programmer can erase individual sectors or, sometimes,
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the entire chip. Some chip families require an explicit erase command and others initiate
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an erase at the start of a programming operation.
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Most of the features of TommyPROM are supported for Flash, but some operations that would
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do multiple writes to the same sector work differently.
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## OTP ROM - One-time Programmable ROM
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One-time Programmable (OTP) ROM is written once, either at the factory or in the field.
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These chips cannot be erased and re-written, but they can be read by TommyPROM. Interesting
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trivia - some of the field-programmable OTPs are just EPROMs packaged without the erase
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window.
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# Supported Chip Families
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## 28C EEPROM
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The 28C series EEPROMs, specifically the 28C256, are the original chips that TommyPROM
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supported. As a result, these have the best support, with all of the interactive features
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of the command line. In addition the basic read and write operations, there are also
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commands to enable and disable the [Software Data protection (SDP)](../28C256-notes)
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features of 28C chips. There is also support for the fast block write mode of these
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chips, allowing a 32KB chip to be programmed in just a few seconds.
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## 27C EPROM
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The 27C EPROMs use a variety of programming algorithms. Code exists for some of these,
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but very few of the chips have been tested.
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Most, if not all, 27C series chips are not 5V only and require a higher voltage to program
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them. Some need the voltage constantly applied while programming and others use high
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voltage pulses for each byte. For constant voltage chips, it is probably easiest to just
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add an external power supply and manually assert the voltage before starting a write. For
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those with switched voltages, some elements of the 8755 hardware may be leveraged to build
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a version of the programer that supports these chips.
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## Atmel SST39FS Flash
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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.
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The SST39FS chips use fixed 2KB sectors that must be manually erased before a new program
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operation, but the code manages this transparently. Whenever a write is started to a new
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segment, the driver first initiates an erase of that sector. A second write to the same
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sector will not cause an erase, so is is possible to write to a segment multiple times
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with no additional steps as long as the writes are to different parts of the sector. For
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example, 256 bytes could be written to the start of a sector from one file and then 512
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bytes could be written to the end of the sector from another file.
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Writing data that spans multiple sectors also works with no additional steps. When
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a sector boundary is crossed, the new sector is erased and set as the current sector.
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The SST39FS driver supports a manual erase from the command line using the E command.
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This is only needed if data will be rewritten to the same location after a previous write
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to that sector.
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## Misc Flash
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**29C Series**
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The 29C series flash chips are similar to the 28C EEPROMs, except they must erase an
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entire sector before programming new data. The start of a block write operation causes
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the target sector to be erased before the block is written. If the entire sector is not
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written in one operation, there is no way to write additional data to the sector because
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a new block write will erase the sector again.
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The write algorithm for the 29C chips is similar to 28C, so it is possible to use that
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code to write these chips as well with some restrictions. All 29C010 and 29C020 chips
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that have been encountered use a sector size of either 64 bytes or 128 bytes. A single
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XModem packet holds 128 bytes of data, so these chips will program without issue.
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The 28C040 chips from some manufacturers use a 256 byte sector. This will not currently
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work with TommyPROM, because each of the 128 byte XModem packets received will do its own
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block write. The first write will erase 256 bytes and then write data into the first half
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of the sector. The second write will then erase the data just written and then write 128
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bytes into the second half of the sector. To fix this, the code would need to be modified
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to keep 256 bytes of data from two packets before initiating a block write.
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**28F Series**
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The 28F series flash do not have sectors. The entire chip must be erased before writing
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new data. Data is written a byte at a time using a command register to control the
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programming. **These chips require a 12V programming voltage that must be present during
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the programming or erase operations**. TommyPROM does not currently support 28F chips,
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but it would not be difficult to write a driver. The default 28C driver will read 28F
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chips.
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**29F Series**
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The 29F series flash have sectors that must be explicitly erased before writing new
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data. Data is written a byte at a time using a command register to control the
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programming. TommyPROM does not currently support 29F chips, but it would not be
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difficult to write a driver. The default 28C driver will read 29F chips.
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## Intel 8755/8355 EPROM
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TommyPROM has a driver for Intel 8755 EPROMs. This driver replaces the 28C driver at
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compile time. See configure.h to enable a different driver. This driver will also read
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Intel 8355 OTP EPROMs. Also note that the TommyPROM hardware for the 8755 is drastically
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different from the other versions.
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The Intel 8755 is a peripheral chip designed for 8085 systems. It is a 40 pin device that
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contains a 2KB EPROM plus two general-purpose I/O ports. The 8355 is a one-time
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programmable version of the 8755. The 8755 requires a 25V programming pulse for each byte
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to be written.
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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.
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The chip only needs 8 connects that are shared for address and data, plus three dedicated
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address lines. The Arduino has enough pins to drive all of these directly, without the
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need for shift registers to create address lines.
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The 8755 build of TommyPROM also has a circuit to control the 25V programming pulses.
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# Verified Chips
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|Model |Manufacturer |Type |Module |Notes|
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|:--- |:--- |:--- |:--- |:--- |
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|28C256 |Atmel, others|EEPROM |28C |Fully supported|
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|SST39SF040|Microchip |Flash |SST39SF|All SST39SF0x0 supported|
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|27E257 | |EEPROM |27 |Continual 12V or 14V for program/erase|
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|29C010 | |Flash |28C |Only with 128 byte or less sector size|
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|8755A |Intel |EPROM |8755A |Requires 25V pulses to program|
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