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

6 APPENDEX2: BLOW BY BLOW SOFTWARE DESCRIPTION

6.1 POWER-UP INITIALIZATION

At power up reset the vector at $FFFE is fetched, which branches execution to the code label

BANK0. At BANK0 interrupts are disabled, the UART baud rate is set to 9600 and the UART

transmitter and receiver are enabled. Next the 40 byte routine at label FLASHPROGRAMMER

is copied from its location in flash to a block in RAM. This is the code which actually writes to

the flash and it is necessary that this code be executed out of RAM since, once the write se-

quence to flash is started, data cannot be read out of flash until the write is completed.

6.2 TO UPDATE OR NOT TO UPDATE

At label BOOTUPWAIT a 24 bit counter made up of concatenated registers x:y:a is load with

$550000 and then the polling loop starting at label CHARCHECK is cycled through a total of

5,570,560 times, each time checking to see if a UART character has been received. With an

8 MHZ effective clock, this polling will take about five seconds. If no character is detected then

the “jp $FDFC” instruction branches execution to the normal sector1 program via the skip-

jump table. At label CHARCHECK, if a character is detected then execution branches to label

DOUPDATE.

6.3 UPDATE

To begin the flash update process a sequence of two unique “keys” are written to control reg-

ister FCSR. This exact data in this exact sequence must be written to this register in order to

remove write protection from the flash. If any other data is written, the protection will “lock up”

until the next reset of the MCU. Next, whatever character had been received in order to acti-

vate the update sequence is echoed back to the operator terminal as a confirmation.

6.4 INTERPRETING THE S-RECORDS FILE

At label B001 the subroutine WAITFORCHAR is called. This routine will poll indefinitely

waiting for a character to be received by the UART and then echo back the character to the

terminal and return it in the a register. Since an S-Records line must start with an upper case

S, the program will loop back through label B001 until an S is seen. Next the program looks for

either a 1 or a 9 to indicate an S1 or S9 record. Anything else is discarded as an error and ex-

ecution goes back to label B001 to look for an S again. At label GOTS1 subroutine GETBI-

NARY is called. This routine will read two characters from the character stream, convert them

from hex/ascii to binary format, and return the number in register a. This number is the nn

record (see appendix 3) which is the total number of bytes in the record. To get the number of

actual data bytes in the record, 3 is subtracted from the value to account for the two address

and one checksum byte that are included in the total count. This is saved in the variable