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};

    POKE(0xC000,n);

    puts ("Hello! Testing port I/O #0 and latch 74LS374.\n\r");

    puts ("Press ENTER and observe the blinking lights...\n\r");

    gets(buf1);

    for (n=1, i=1; i<256; n++,i++) {

       buf1[0] = buf2[0] = 0;

       strcpy(buf1, itoa (i, buf2, 10));

       puts(buf1); puts("\r");

       POKE(0xC000,n);

       my_pause(2000);

    }

    for (i=0; i<10; i++) {

       POKE(0xC000,0);

       my_pause(2000);

       POKE(0xC000,0xFF);

       my_pause(2000);

    }

    POKE(0xC000,0);

    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.

12/9/2015

MK