Error on the inverter for the clock module ?

[wawachief]

Hi,
first, I want to thank you for the great job you did on this project. I am a big fan of Ben Eater's videos on the 8 bit computer but not everyone is a breadboard master like he is ! Your project allows me to play with this computer... as soon as I get it toi work properly !

I have several issues I try to figure out. First one is with the inverter you use on the clock module : The 74ls06 needs puul-ups as its outputs are open collectors. Ben uses 74ls04. Is it a mistake in the silkscreen or do you really mean 74ls06 ? I had to add pullups on the pins 2 - 4 - 6 to get a clock pulse.

[dangrie158]

Ha! you are completely right. Problem was I only had an LS06 in my parts library and changed the value for all parts simply to 04 as the pinout and symbol is the same. Forgot this one, thanks for spotting!

I updated the silkscreen, schematic, BOM and gerbers

[jpbiley]

I had made a board using the Gubers downloaded on Nov 2019. Is this just a chip swap or a more significant retrofit of the PCB?

On my build the clock "appears" to be working ,I can add values to ram, read values in ram (using the dip switches), run the clock, single step and even get numeric output on the 7 segment displays. however every instruction seem to cause a halt condition in the third step in the microcode.

Any hints would be appreciated. Thank you for making this system available.

[softtribe]

@dangrie158
Hi Daniel,
Thank you for sharing your design and for the awesome documentation.
Like @jpbiley I have manufactured 2 computers using the gerbers (PCB layout) from August 2019.

• The clock is working fine using 74LS06 and 74LS04,
• The fetch cycle is executing correctly
• Afterwards the computer freezes - the HLT flag is enabled

It seems like @jpbiley I have the same issues with both boards : the halt flag appearing on the 3rd micro-step. I don't think the source cause is my EEPROMS because I have swapped them, verified their content, and recompiled the uCode using your Python library and tools.

Have you any suggestion for further debugging ?

To investigate the issue I have replaced the two uCode EEPROMS with a breadboard to manually drive the control word (CW) but I'm finding new problems (I'm not able to simulate correctly the fetch cycle: the sequence RO (RAM Out) II (Instruction Register In) don't drive correctly the data from memory via the W Bus into the Instruction Register, and CE doesn't increment the Program Counter)

Thank you!

[dangrie158]

Hi,
Did you have a look at #2 ?
Make sure that i1-i4 are not connected via the buffer to the database but instead directly from the Flipflops to the micro code decoders.

Maybe check ne new Gerbers or if you have eagle installed the schematic. Sounds like you have the old version of the board

[dangrie158]

Maybe check this picture : https://twitter.com/danielgriehaber/status/1160595950110461952?s=21

[softtribe]

@dangrie158 Thank you Daniel.
The issue was #2 (problem with the instruction register buffer connection to the bus). With the correction that you have already implemented the computer works fine now.
The only issue remaining : for programs containing the OUT instruction, the output segments are displaying 0224 when OI is enabled (on the micro-step T2) instead of keeping its original content before updating the new one. Is it normal ?
I'm so excited to play with the programming of the processor, thank you again !

[jpbiley]

@dangrie158
I added the suggested jumpers and reprogramed the LUT's with the Jan 14 code. Now everything works correctly, thank you for these updates!