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FIELD UPDATES FOR FLASH BASED ST7 APPLICATIONS USING A PC COMM PORT

1 FLASH MEMORY SECTORS

We will divide flash memory within the ST7 into two sectors by selection within the option byte

when the chip is programmed using a standard programmer. These sectors are referred to as

sector0 and sector1. The selection at program time is to decide what will be the size of

sector0, with the balance of the available memory going to sector1. The boot loader program

is less than 500 bytes in size so when programming the part select the smallest size for

sector0, which is 0.5K or 512 bytes. The only real distinction between sector0 and sector1 is

that sector0 is always write protected as it pertains to in application programming (under direct

ST7 program control). Sector0 can always be programmed by a dedicated programmer but it

cannot be written to by the MCU itself. Sector1 can be programmed either by a dedicated pro-

grammer or by software executing on the MCU itself. Sector0 is always the uppermost part of

memory so that it includes the reset and interrupt vectors (data that one always wants to be

write protected in order to make a self updateable system “bulletproof” in the face of catastro-

phes such as power failures in the middle of an update). Since the entire boot loader code and

the vectors reside in the write protected sector0 it is always possible to recover from a problem

during an update transfer by simply resetting the ST7, which will get us back to the boot

loader. The example program was written for the ST72264 which has 8K of flash so sector1

occupies from $E000 to $FDFF and sector0 from $FE00 to $FFFF.

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