To PLD or not to PLD

When I start something interesting, I tend to overdo it. I originally contemplated using standard gates for my glue logic. But then chose to go the EEPLD (Electrically Erasable Programmable Logic Device) route. Wow, was I ambitious? Not only I was to build the 6502 single-board computer from the ground up, but I had to also get to know PLDs up close and personal at the same time. Well, it turns out it's not all that hard, if you find the right tutorials on the web, and if you have a precise memory map defined.

I had this really particular memory map planned. But for my second iteration of the SBC, I opted to go simply and work my way up. 

The map, for now, is 32K ROM, 16K IO space, and 16K RAM. This will change in the future. And this is the beauty of PLDs, there is no need to rewire anything. It's just the software that needs changing.

I chose the ATF22V10C, a G22V10 equivalent, because of it's many input and output options: 12 dedicated inputs, and 10 configurable input/output.

ATF22C10C EEPLD


The documentation is non-existant, and not many decent tutorials exist out there. Download the software from Microchip's website.  The programming is bound to change as I get new I/O components on the breadboard, but for now, this is how it's going to be for the time being.

Here's what my WinCupl file looks like:

Name       Mainboard Logic Decode ;
PartNo     ATF22V10C ;
Date       8/13/2020 ;
Revision   01 ;
Designer   The Micro Hobbyist ;
Company    Self ;
Assembly   6502 mainboard logic ;
Location   U105 ;
Device     G22V10;

/* *************** INPUT PINS *********************/

PIN 1   =  CLK;     /* Clock */
PIN 2   =  A15;
PIN 3   =  A14;
PIN 4   =  A13;
PIN 5   =  A12;
PIN 6   =  A11;
PIN 9   =  IRQ0;    /* VIA IRQ */
PIN 10  =  IRQ1;    /* ACIA IRQ */
PIN 11  =  IRQ2;    /* Future expansion */
PIN 13  =  RW;     /* Read Write */

/* *************** OUTPUT PINS *********************/

PIN 23  =  !ROM;    /* ROM select */
PIN 22  =  !RAM;    /* RAM select */
PIN 21  =  !RTC;    /* Realtime Clock select */
PIN 20  =  !VIA;    /* Parallel select */
PIN 19  =  !ACIA;   /* Serial select */
PIN 14  =  !OE;     /* Output Enable, or !Read */
PIN 15  =  IRQ;     /* IRQ merge */

/* *************** EQUATIONS *********************/

ROM   =  A15;
RAM   =  CLK & !A15 & !A14;
RTC   =  CLK & !A15 & A14 & !A13 & !A12 & !A11;
VIA   =  !A15 & A14 & A13 & !A12 & !A11;
ACIA  =  !A15 & A14 & !A13 & !A12 & A11;
OE    =  RW;
IRQ   =  IRQ0 & IRQ1 & IRQ2;

Comments

Post a Comment

Popular posts from this blog

TMS9918A and PS/2 Keyboard

Image
  I've been away from my project, but this week I've made a few additions to the SBC. Instead of using an old TRS-80 keyboard, I decided a while back to use a PS/2 keyboard (interfaced through the VIA). Things worked relatively well, except that the SBC froze if no keyboard was present. Last week, I changed from the fully software-driven PS/2 interface, to an MCU-based interface. This offloads a lot of the work, and only sends characters (instead of scan codes) via an interrupt to a buffer.  And today, I decided to add a video component to the SBC. A Texas Instruments TMS9918A VDP (Video Display Processor). It outputs a composite signal to a standard-definition television. Although hard to see in this picture, you can see a bit of text with lots of color fringing. I will need a good monitor to see if it's a problem with my cheapy LCD monitor, or the video signal itself. I did notice some jitter in the oscilloscope, so fixing this might not be so easy.
Read more

The dawn of a new adventure

Image
In comes the insanity!  Well, that's it! I'm not one of "those" people that reminisce and rides down memory lane in the nostalgia bandwagon. Yes, my mid-life crisis has gone and past. But I find myself wanting and yearning to have a good start in a new hobby, micro-electronics. And what better way than to start building an 8-bit microcomputer. And yes, I do dabble with microcontrollers also, so I have a good start. When I was in school in the late '80s I learned to program on the  6809  in assembly language on a single board computer (SBC). My initial thought was to recreate the aforementioned SBC, but the availability of parts is diminishing. In comes  Western Design Center , makers of the new  WD65C02  chips, still in production today.  Off the shelf components is important to my endeavor, in case I want to replicate my design. Trying to scrounge vintage and obsolete parts is not part of the plan. So, welcome to my insanity project, keep in touch, and come back
Read more