Small update

A project log for FPGA Serial Terminal

Instant-on VT100-style serial terminal implemented in minimal FPGA board, with VGA display and PS/2 keyboard.

Howard JonesHoward Jones 11/03/2015 at 14:300 Comments

I've got some standalone serial and VGA breakout boards now, so I decided to try moving over to the smaller EP2C5 board that was the intended target. This has a 2-bit resistor DAC per RGB channel for the VGA, and a MAX232 chip for the serial, so it can talk to the rest of the world. Except that it doesn't, even to my buspirate. So I (literally) dusted off my 1980s oscilloscope to see if I was even generating the right kind of thing...

Where it turned out I was out by a factor of 10 in my clock counting process, so running at more like 960 baud instead of 9600. With that fixed, I can see bits on the scope, but obviously not the right ones as the PC and Bus Pirate both still show nothing. The top trace is the 9600 baud clock, and the bottom one is the transmit line. I've just got a 10 bit buffer being rotated to send a start, ASCII 'A' and stop bit, currently. It might be time to get a modern DSO (and win back some bench space), because I can't really get this to trigger nicely to show a full word.

While I was fooling around with the scope, I also noticed that the resistors on my VGA DAC aren't very well chosen. The scope can't show an actual signal (it maxes out at 10MHz, I think, and the VGA dot clock is 50MHz), but I'm pretty sure these 4 lines should be evenly spaced: