Wednesday, August 15, 2012

Parallax Propeller + COSMAC 1802: the Saga Continues...

Monday morning I posted about hooking up a Parallax Propeller QuickStart Board to an 1802 Microprocessor. My hopes are to make the Propeller behave as a RAM for the 1802, primarily to be able to display a bitmap of that segment of memory as video. The idea is to make it compatible software-wise with the 1861 Pixie chip (the 1802's native video adapter), which is presently unavailable.

Since then I've spent more time on this project. A lot of that time got tied up in side issues related to tying a Propeller to an 1802, as well as some time spent dealing with the test equipment I want to use to keep an eye on the circuit and see what it's up to.

First, here's how it looks tonight:

Propeller QuickStart hooked up to an old CDP1802 microprocessor.
The second stage of the Propeller/1802 mashup circuit. Click for full size.
I've got a closer relationship between the two processors now. The Propeller is actually doing something. Before, the 1802 was getting power and a fixed clock signal off the QuickStart board, but the Propeller was not itself involved.

Clocking the 1802
One thing I put a fair bit of time into (too much, that is) is using the Propeller to control the clock of the 1802. There are a few reasons I think this would be nice:

The clock can be varied in software depending on the user's desire and the individual 1802's capabilities.
At some later point, the Prop can have a greater degree of control over the 1802's operation in general. With the ability to stop it, start it, and even single-step it.

On my first stab at this a couple of days ago, I just threw a repeat loop into the Prop's code to pulse an output line. I didn't have very good control of the rate, I didn't have the constants set right, and it didn't work right off the bat so I decided to drop it and just use another clock.

Let's Try a Timer, Because...That's What They're For
This time I took time to read up on using the counter/timers on the Prop. Each cog has one, and I played around with it for a while before feeding its output to the 1802's clock input. After a while I was able to get the results I wanted, and was able to fine-tune that bit of the code.

Then I hooked it up to the 1802 with a frequency of 1.25MHz...well, I should say I hooked it up to a 4049, passed it through a pair of inverters to square it up a bit and buffer it. Then I passed the signal to the 1802. Which ran just fine.

At this point I still had the data bus of the 1802 hard wired to a $C4, which is a NOP instruction for the 1802. That way I can watch LEDs on the address bus and see that the chip is running through its address space. The pulse rate of the high order LED gives me an immediate idea of how fast the 1802 is getting clocked, too.

Then I started playing with the constant for the counter's frequency to see what speeds this 1802 chip would be good at with a 5V Vdd and 3.3V Vcc (the 1802 can use split supply voltages for its core and I/O. Not bad for 1976, eh?)

Overclocking! 4.8MHz Baby! Yeah, uh, Megahertz.
This one ran up to 3.6MHz without a problem. I didn't run it up until it stopped, the other day I think I had it running at about 4MHz when I was playing with the repeat loop. Above 3.6MHz it seemed to start running hot, so I stepped it back down again.

Above 3.6MHz it was noticeably warm after a while, and seemed to be getting warmer. Between 3.2 and 3.6MHz it was warm, but maintaining its temperature just fine. At 3.2MHz it was solid as a rock and the heat was barely detectable to a calibrated fingertip. If I can get it to run at this speed reliably with the Propeller providing video then popular Elf software like Tiny Basic, Forth, CHIP-8, and the programs written in them are going to rock on this system.

Edit: I've run it up to 4.807MHz now, and it didn't seem all that hot this time. It really shouldn't have any problem with heat at these voltages, it normally runs at 5MHz at higher voltages. And now it doesn't seem to have a problem. When I tried to take it over 4.8MHz it ran sometimes but not others, or hung occasionally. So this is the limit for stable operation at this voltage for this chip (an RCA CDP1802CE.)

Getting Data to the 1802

OK, so back to the problem of making the Propeller look like a RAM. Frankly, when I did my first look at the timing involved, I figured this would be completely trivial to implement. Push in data and address bus wires, connect up /MRD (memory read signal) from the 1802 to the Prop, crank a little Spin code, and I'd have the Prop acting like a ROM.

Put in /MWR and a little more code, BAM, the Prop is a RAM.

Hook up TPA to catch the high order address byte, tweak the code, and the Prop is ready to map more than 256 bytes. All I'd have to do is add the video code then figure out how much Prop RAM was available then expand to fit.

Well, it wasn't that easy. If it had been, I'd be playing with the video now.

I tried to leap forward at first. I plugged in the address and data bus wires, plugged in /MRD, and added some code to my clock driver program. It had a 256 byte array that it set up and initialized to $C4 in every position (NOP again, but this time it would be a soft NOP rather than a hard-wired one.) Then I wrote the program to wait for the Propeller I/O line I'd selected for /MRD to go low. Once that happened, it would get the address off the address bus, look up the appropriate element in the RAM array in memory, and put it out on the data bus until /MRD came back up again.

Easy-peasy, right?

Well, it didn't work.

Oscilloscope signal trace of one of the data lines and /MRD on the Propeller/1802 mashup.
Top: Data from Propellor pretending to be a memory. Bottom: Memory Read Request. Result: Memory Too Slow! (Click for full size.)

When I was doing my timing calcs and looking at the 1802's leisurely timing--it doesn't mind waiting just under a millisecond for its memory to come back when it's running at its normal speed (about 1.78MHz.) Hey, an 80MHz processor could be running a version of interpreted BASIC written in LISP and keep up with that, right?

I guess not. At least not the way I did it.

I clocked the 1802 at 1.25MHz for the test. This is an easy, slow speed that the Propeller can generate easily. I figured it'd work, I'd push up the frequency to 2.5MHz, check timings on the scope, and discover that I couldn't make the 1802 go too fast for the Prop.

Unfortunately the trace above shows different.

I Can Do Lots of Things Wrong, All at Once
I'm sure this is the result of something I'm doing improperly. I can think of several possibilities already:

I'm using WAITPNE and WAITPEQ to respond to the pin change. Perhaps, because they do some power-saving activity on the cog on which they're invoked, they just aren't meant to respond this quickly. Maybe if I just go to an active polling loop I'll be OK?

I'm using SPIN, an interpreted language. Perhaps I need to just insert a bit of Propeller assembly to tighten up the timing?

Perhaps there's some option or configuration thing I've not done?

Maybe this cog has to do something active with the timer/counter (I'm using the same cog as the one that's running the 1802's clock.) Perhaps if I move the RAM functions to another cog I'll be fine?

Those are just what comes off the top of my head. I'm far from being an expert on the Propeller, and I haven't picked up the books on it yet, just the free manual and datasheet downloads. But things like this create an opportunity to learn.

I also had a few other things I needed to figure out on the way, minor little things like the order you mention the output pins in when writing your code statements (I was getting $23 out instead of $C4 initially. Yeah, oops.) That and the whole thing is a rather fragile lash-up right now. I'm going to go to solder as soon as I get the RAM moving data to and from the 1802. But not before, because I hate rework, and if I don't test it on a breadboard first, there'll be rework. And this lets me do a few things like change which lines I'm using for address and data easily. I moved the data lines to P16..23 so that I can watch the data on the QuickStart board's LEDs, like a little front panel. Before, I had the address lines here, but I've already got LEDs on the breadboard showing me the address line states. So a quick change of a few constants in software, and I get a data display with no re-soldering.

Slightly Off-Task Tasks
I also took some time out to search out a line cord for the new frequency counter I got at the Ham Radio Club night before last that didn't have one (found one, after searching high and low. It's an oddball, not an HP cord.) And I checked out the two portable NonLinear Systems oscilloscopes I bought, too (one works, one doesn't, as expected.) The frequency counter was very nice to have while I was figuring out how to use the Prop's counter/timer as a numerically controlled oscillator, so the time was well spent even if it did eat into my Propeller-as-RAM time.

Looking Forward to Round 3
I'll be back at this before the end of the week (I'd like to tomorrow, but it'll be a busy day so I may not be able to.) If you have any suggestions or salient experience, your comments or emails would be much appreciated!


I've had a look at the Propeller docs. It looks like the timing of hub instructions is my problem. I may just insert WAIT states for the 1802 and see what that does.


  1. Hi there! I just got here because of a retweet by @Parallaxinc of one of your tweets about this. Looks like you and I are working on very similar projects, except totally different because I'm using a 6502 instead of an 1802 :-)

    Spin is not going to help you much with high speed hardware control like in your project (and mine): you're going to have to throw in some Assembler.

    The main problem I have with Assembler and Spin is that you can't mix it on the same cog. That's why I'm in the process of switching to C as the main language (and SimpleIDE as the development tool) instead of Spin: that way, mundane tasks can still be written in a higher language but I can insert the Assembly loop in there as inline assembler.

    Anyway, if you need any help with the Assembly code, feel free to send me a DM on twitter or GooglePlus or something. Also feel free to use any of the code that I already have; it's on Github and licensed under MIT.

    My project is at

  2. Mark, I have managed this in assembly. Cogs for clock and managing a cycle in assembly, and the main one in spin to manage it all. The prop emulates ram, rom, ports, generates the clock, and provides i/o via the serial port. If you want the source code/general structure drop me a line:
    Good luck,
    Roger Blott

  3. I have been thinking along the same lines of doing a Propeller with a 1802. Why not use one of the 16 PLL counters of the Propeller for the 1802 clock. Can you post your schematic?

  4. Hello:

    With a lot of microprocessers in our 'junk' boxes these days I am trying the same stuff with a Z80 that I have laying around. The propeller is great for this and I wonder if thier is other microprocessers out there that could benefit from such tinkerings.

  5. Did you give up on this project. I was looking forward to more, and completion.

    For people who want to mess with a COSMAC Elf 1802-based system easily, they can play with my online simulator: