The Compact Flash interface was quite easy to check, however plugging a CF card into the socket before power-up prevented the whole computer from starting. Something as major as that should be easy to diagnose, and so it was. The CF Card is configured at reset by checking the status of the two select pins in the interface, and the first problem was that the reset wasn't connected to the interface.... fixing that didn't help much, however, as the two select pins were swapped. With that fixed the computer started and I could start to write a program to read and write to the memory card.

Assist09 provides several functions that are handy when writing a program, such as read a character from the console, print to the console and print memory locations. Software interrupts control this and I soon had the bare bones of a disk editor working. I will continue to improve this over the next week or two, but I could see enough to know the hardware was working correctly. With the Chinese New Year getting close I added all the changes to the PCB and added a few features, and sent the board off to be made.

The new PCB has a few new features an addition to bug fixes. I have opted for a better reset circuit as the simple resistor, capacitor and push-button switch isn't so good for any chips that don't have Schmitt-trigger inputs (and I can't be sure the signal met the CF Card specification). A small expansion connector will let me add features in the future without making a whole new card. I would like to add a video and keyboard interface in the future (and a second CF Card socket), and this expansion port will let me do this. I was also able to squeeze a real-time clock chip onto the board too, with the thought that, should I get Flex running, I can use this to set the date.

Quite a successful couple of weeks, and I hope I have time to complete the CF Card editor before the PCB's arrive, so that I can put it in the EEPROM along with the monitor. With the design becoming pretty stable now I have added a parts list for the new design, and added the schematic too.