Step 5: Attempt to connect CPU + Flash RAM + ACIA + Simple address decoder and experience voltage sag

Goal: Get to the point where my desktop computer will talk to the 6502

In the days of yore, you’d wire up an RS232 port to your computer. This meant that you needed an extra chip to do the voltage conversion, generally a MAX232. These days, pretty much everyone standardized on the FTDI connector, which is 2 signal lines, the RX and TX lines, and power and ground, in a 6 pin header that all of the popular USB serial port breakout boards connect to.

So I get my USB serial breakout adaptor connected, the Flash RAM and the ACIA.

The memory map here is pretty much:

  • 0x0000-0x7FFF - The ACIA, repeated every 4 bytes
  • 0x8000-0xFFFF - The first 32k of the flash ROM

It didn’t work.

I had a few wires not correctly inserted into the right places, so I fixed that.

But the clock wasn’t getting output from the Phi-1 and Phi-2 pins, and everything was stopped.

It took me a while. I kept suspecting that the RESET line was the wrong one because it didn’t seem to be triggering. I figured out after a while that the power LED I’d helpfully added on the board felt dim, so I pulled the voltmeter out and realized that the voltage was around 3v, and the MAX707 triggers at 4.65V.