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

4 DECISION POINT

At the label “bootupwait” the program enters a five second loop where it continually polls to

see if a UART character has been received. If a character is detected then execution jumps to

the label “doupdate” where the flash memory is unlocked and the program waits patiently to

receive the S-Records update file. If the five second period expires without detection of a

UART character then execution escapes to the entry point for sector1 code (the “jp $FDFC” in-

struction). While this is a good method to use for an example program because it will run prop-

erly “out of the box” on any ST7 based system that has a RS232 port it may not be the best

choice in all situations. It may be undesirable, for example, that the system will always delay

for five seconds after power up before starting to execute the normal program or it may be dif-

ficult to arrange for the triggering character to be send to the board within five seconds after

power up reset.

One alternative method would be to dedicate a general purpose port pin to control the func-

tion. In the hardware, we could include a pull-up resistor to Vcc at the pin and provide a jumper

that would ground the pin if installed. The jumper would normally be installed and we would

only remove it if an update was desired. The code at “bootupwait” can be changed so that the

port pin is examined. If the input is logic low, execute the jump to $FDFC without delay to start

normal operation. If the pin is high then the program can wait until the jumper is re-installed

and then jump to “doupdate” to receive a new program. Normally, the jumper would always be

installed. To do an update we would remove the jumper, power up (reset) the ST7, replace the

jumper and then transmit the file. After the file has been received a cold reset will follow and,

because the jumper is installed, normal execution will commence.

If an application is tight on I/O and cannot afford to dedicate a pin to this function another ap-

proach is to insert a jumper between the output of the RS232 line receiver and the Rx input of

the ST7 and then provide a 10k pull down resistor to ground at the pin of the ST7. Normally,

the jumper would be installed. Since the normal marking state of the Rx signal is logic high, the

code at the decision point can examine the pin (as just a general I/O pin) and, if it is high, exit

normally to sector1. If the pin is low (jumper has been removed) then the program would wait

until the pin goes high, then jump to doupdate and wait to receive a new file. To perform an up-

date one would then remove the jumper and power up (reset) the ST7, install the jumper, and

then transmit the file. After the file has been received a cold reset will follow and, because the

jumper is installed, normal execution will commence.