New PCB, new bugs (but very minor)

A project log for 6809 SBC

A 6809-based SBC with DES encryption hardware.

philmacphailPhil.macphail 02/27/2021 at 18:450 Comments

The new PCB arrived before the Chinese New Year holiday, and I even found time to assemble one. All the modifications listed in the previous logs worked without problem, and I was able to get the monitor prompt straight after reset. The real-time clock circuit is new, and I very quickly noticed a missing connection - one of the CS pins is floating and should be connected to the reset line. With this fixed with a small wire the chip could be read and written to, but to use it properly needs a little more complex code, so that can wait. The code for the CF card isn’t finished yet but is moving forward. I can read the card details into a buffer and list them calling the memory dump routines in the monitor. I can also fill the buffer with a constant and read a block from the card. The reads aren’t consistent, however, so I want to fix this before writing to the card. I did notice that the configuration data seems to be a little scrambled, configured for Intel-style 16-bit reads. Reading 8 bits at a time, the LSB of the address needs to be inverted. Without this, the manufacturer for the card I have is reported as OTHSBI A, rather than TOSHIBA.

A kind reader correctly pointed-out that the TSC pin is floating in my design, and it should be grounded. Another wire again fixed this, but I had been lucky that the chips I used (all Thomson) seemed to ‘float-low’ on this pin and work. I had noticed that at power-on the CPU required a press of the reset button to work, and assumed this was due to how the PSU ramped. That is one of the reasons I added the reset chip to the new design. 
My priority now is to finish the CF Card code, then write the routines for I/O for Flex. Actually getting Flex onto the CF cards is something I haven’t yet resolved, but that isn’t a concern right now.