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42 lines
1.9 KiB
Markdown
42 lines
1.9 KiB
Markdown
---
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title: PROM Types
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permalink: /docs/prom-families
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exerpt: "PROM types supported by TommyPROM - EPROM, EEPROM, Flash"
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---
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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
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reprogrammed electrically at the individual byte level. This makes them appear similar to
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a slower static RAM. All of the interactive features of TommyPROM work well with EEPROMs.
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Due 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.
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Some older EEPROMs cannot be reprogrammed at the individual byte level and are instead
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bulk erased before a new write operation. Programming and erasing for these chips usually
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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.
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Interesting trivia - some of the field-programmable OTPs are just EPROMs packaged without
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the erase window.
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