Wednesday, December 9, 2015

Update on MKHBC-8-R 6502 computer (now on revision #2) - prototyping RTC and banked RAM.

I had very productive 2 weeks or so with this project. 

To make long story short:

  • I assembled CPU and UART cards.
  • I discovered and mostly corrected errors in my design and technical problems in manufactured boards (bus contention, wrong address decoding scheme, bad solder joints/electrical contact problem on CPU bus, connector B2).
  • I built and tested RTC circuit prototype (based on DS1685 and Chris Ward's design of connecting this multiplexed bus type chip to 6502 bus).
  • I built and tested prototype of RAM bank switching register (based on 74LS374 latch register).

More details in my YouTube video: MKHBC-8-R2 - part 4.

My address decoding circuits design incorporated Phi2 signal in the decoding scheme. I consulted literature and expertise of knowledgeable people on the 6502 subject and decided this is not the correct approach. I removed the Phi2 signal from address decoders input. The Phi2 signal is now only used to synchronize reading/writing access cycles.


I tested two variants of RTC circuit:



and decided to go with 2-nd one (seems to be more reliable and less susceptible to problems resulting from long propagation times).

My I/O bus (data lines) was not properly separated from CPU bus resulting in bus contention and data corruption. Here is the corrected circuit:

I also have a concept of banked RAM:

The code testing the 74LS374 latch register alone (I just had LED-s connected to the data outputs):

#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <peekpoke.h>
#include "mkhbcos_serialio.h"

char buf1[10], buf2[10];

void my_pause(uint16_t delay)
    int i = 0;

    for(i=0; i < delay; i++);

int main(void)
    int i = 0;
    unsigned char n = 0;
    char buf[5] = {0};

    puts ("Hello! Testing port I/O #0 and latch 74LS374.\n\r");
    puts ("Press ENTER and observe the blinking lights...\n\r");


    for (n=1, i=1; i<256; n++,i++) {
       buf1[0] = buf2[0] = 0;
       strcpy(buf1, itoa (i, buf2, 10));
       puts(buf1); puts("\r");

    for (i=0; i<10; i++) {

    puts ("Test finished.\n\r");

    return 0;

Pictures of my prototype for your enjoyment (electronics hardware porn :-)) and some screenshots of serial command interface:



That would be all for today.

Thanks for visiting my blog.