Sunday, September 19, 2010

68HC11 Bootstrap mode

In 1999, I received a Super T-Comp; a 68HC11 based microcontroller board that is similar to the Handyboard.
While working with it I released the sacred smoke. I blew out 3 chips.
I repaired the damage, but I couldn't get it to work right again. So it sat in my toybox waiting for me to try again.

In 2010, I am taking a class in embedded systems; so it is time to try again.

Inspecting the board again, I found a bad chip that I missed; a NAND gate driving the external RAM. I will fix that this week.

Regardless I should be able to talk to T-Comp in Bootstrap mode. PCBug11 is the original tool for talking to 68HC11. It doesn't run on modern (fast) computers.

So I tried TeraTerm terminal software. Finally did established communication this morning.

68HC11 has 4 modes:
Single Chip: no external RAM
Expanded: external RAM
Bootstrap: single chip, using bootstrap loader in ROM
Test: Uh? don't know

Bootstrap Mode
Hold ModA and ModB low
Toggle Reset
Release ModA and ModB
within a second
send a byte over SCI (serial port) at 1200 Baud, No parity, 8 bits, 1 stop bit.

It is now ready to accept a 256 byte program. 68HC11 will echo bytes. Once it recieves 256 bytes, the program counter resets to $0000 and starts to run the loaded program. In theory.

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