EEPROM Programming

Writing to Flash EEPROMs is much slower than writing to RAM, and requires writing control codes to the address space of the chip to perform these operations. Apparently the Flash EEPROM used is the "SHARP LH28F160S3-L", which has many features (some may be disabled) including writing (multiple) words/bytes, erasing 64kb blocks, and locking of blocks.

For more information on this chip see Johan Borg's file at http://d188.ryd.student.liu.se/ftp/calculator/ti89/tech/flashrom.txt, he also has put up the datasheet at ftp://d188.ryd.student.liu.se/calculator/ti89/tech/f160s3.pdf which is worth a look.

Fortunately only a subset of this chips features are used; writing words and erasing blocks. Write operations take 12.95 micro-seconds, read operations 100ns, so you can see why a special mode of operation is required.

To program the EEPROM we need to enter a special mode by writing certain control codes to the address range. Two operations can be performed, erase (64kb) sector, and write a word. In this write mode reading from the address range gives the status register of the EEPROM, generally the only value that is used is that bit 7 of the read word will be set when complete. After the operations are complete Read mode is then set by writing another control code.

The drawback of this method is that the ROM cannot be read whilst write operations are being performed. Therefore the writing code must execute from the RAM. Trap 11 handles all of this on the calculator, copying the relevant portions to a buffer then executing them.

The following subset of operations is used:

Code	Function
0x5050	Clear Status Register
0x1010	Write Setup (next word will be written)
0x2020	Erase Setup
0xD0D0	Erase Conform (address)
0xFFFF	Read Memory

Erasing and writing require to writes, e.g. 0x2020, 0xD0D0 to perform erase to prevent accidental operations.

Note

The flash memory is, by default, write protected by the system. And can only be written to under special circumstances. See Intelligent Memory.

Examples

;erase 64kb block in which ERASE_ADDR resides

lea (ERASE_ADDR),a2

move.w #0x5050,(a2) ;Clear Status Register

move.w #0x2020,(a2) ;Erase Setup

move.w #0xD0D0,(a2) ;Erase Confirm

write_state_busy:

move.w (a2),d0 ;Read Status Register

btst #7,d0 ;1=Ready

beq write_state_busy

move.w #0xFFFF,(a2) ;Read

;write VALUE to WRITE_ADDR

lea (WRITE_ADDR),a2