WriteROM accomplishes the impossible, allowing one to write a ROM in place while installed in a circuit without a write line. No flying leads or pigtails are required. WriteROM allows writing to the FLASH using only read accesses.
In normal operation, WriteROM works just like a normal 28 pin JEDEC ROM. However, once a "knock" sequence of reads are performed on the device, the device enters into a programming mode, where address bits 7-0 are interpreted as the data, and bits 11-8 are considered the "operation". The following operations are supported:
| 10:8 | Description |
|---|---|
| 0 | set low byte of address |
| 1 | set middle byte of address |
| 2 | set high byte of address |
| 6 | read from stored address |
| 7 | write to address |
As expected, programming times will be increased due to the additional level of abstraction, but such a device can be cleanly inserted into any memory map, even those not allowing writes to a specific range of memory.
Currently, the design includes a 4Mbit (512kB) FLASH ROM, though the design can be extended to larger sizes if desired.
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