Friday, 3 July 2009

First Run...

Yesterday I finished wiring the processor board and I wired up a new board with a 24 LEDs and buiffers to help diagnose any problems (they cover the control and IO[0..7] bus signals) I'm going to also wire up 6 x 7 segment displays to display the current A[0..15] and D[0..7] lines as hex (well as the odd hexadecimal that the 74LS47 makes)

However I couldn't resist (it was taking so long to wire up the seven segment displays) so had to plug in and give the thing a test.

First there was a problem with the thing just hanging which I managed to trace back to the WAIT signal always being asserted, this due to me not wiring the pull up resistors to Vcc but instead to GND. Wiring up all those LEDs has already proved its worth! I'd never have spotted this so quickly without them!

Next I tried running at full speed, the ROM is blown with a program that should reset the memory controller then access each of the IO1..7 lines (not 0) over and over. When I ran it these all lit up dimly! Success...maybe

Next I tried single stepping, this was less successful firstly it gets stuck after a reset.....then I found that reset only works by holding down reset and doing a number of clock cycles. Then it kind of seems to work but is not running the program I intended there's a few M1 cycles showing up but sometimes it will do a lot of cycles without and M1. I'm not sure if this is due to the single step switch bouncing - I'll try a bigger debounce cap or if there's something bad happening. Also the IO1..7 lights don' light up as expected so I reckon something odd is going on!

Next I tried wiring up the most significant digit of the address lines to it's 7 segment display this should be showing E all the time but actually jumps about all over the place which suggests to me that there's a problem....anyway I'll try and connect up the other digits and see if I can work out what's going on.

Back later with some pictures...

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