Building TK-Pie by Victor Trucco

GozdniJezek
Aug 25, 2019 · 8 min read

My friend and I built TK-Pie for Spectrum ZX 48k, a home computer from 1983, which is an edge connection add-on that enables you to connect your Spectrum to a modern HDMI TV/monitor. This is a quick instruction that will hopefully help you build your own.

A picture of the finished product (version 2, which takes power from the 9V line).

How it works?

The TK-Pie watches the video memory addresses in Spectrum RAM (memory address &4000 onward) and uses raspberry pi and some clever software to generate an image that is than displayed through raspberry pi HDMI output. More on Spectrum video memory can be found here.

Building it

First of all, all of the needed resources can be found in Victors GitLab here:

Source description

Once you download the project and unzip it, you’ll get the following files:

Unzipped project.

CPLD contains the source and files to program the CPLD (main chip),

gerbers contain the PCB design ,

photos contains the photos of the prototypes,

PI_OS is the raspberryPi firmware,

PI_SD contains the source code for TK-Pie, as well as the precompiled version for Pi-Zero, if you want to use other raspberries, you need to recompile this code for your version,

schematics contains the schema helping you to assemble everything.

PCB and bill of materials

I will describe the bill of materials we used, since there are two variants that can be built. The difference is the power source, one takes it’s power from 5v line and is a little bit easier to build; however I had problems with stability, so I suggest building the 9v one.

  • PCB, 1 piece, 2 layer pcb, we ordered ours from: https://jlcpcb.com/
    -RaspberryPi Zero,1 piece, we used RaspberryPi Zero, other types can be used with slight software modification (recompile the source code for the desired model)
    -CPLD, 1 piece, we used EPM240T100C5N
    -ZX Spectrum connector, 1 piece, edge connector 28x2P (double sided) Pin pitch : 2.54x5.08mm, 0.1"x0.2", you have to make some modifications.
    -RPI connector-for board, 1 piece, 2x20 Pin 40 Pin header female, pitch 2.54
    -RPI connector-for PiZero, 1 piece, 2x20 Pin 40 Pin header male, pitch 2.54
    -JTAG connector-for programmator, 1 piece, 2x5 DC3–10P (2x 5 pins),pitch 2.54
    -R1-R30, 30 pieces, 330 Ohm SMD resistor, size 1206
    -R31, 1 piece (OPTIONAL for version 1), 0 Ohm SMD resistor size 1206, you can just short the via with solder, not needed for 9V version
    -C1-C7, C10-C12, 6 pieces, 100 nF capacitor 50V ceramic THT
    -C8, C9,2 piece, 100 uF capacitor (10V or 16V)
    -U2,1 piece, LM1117, 3.3V LDO regulator (SOP or SOT223–4)
    -U3,1 piece, L7805C2T, 5V positive voltage regulator
    -CPLD Programmer
    -Mini HDMI(Type C) Male to HDMI(Type A) Female

ZX Spectrum connector

Since every ZX hardware project needs a way to connect to the beloved speccy, the most common way is through the edge connector.

The connector is 28x2P with pin pitch : 2.54x5.08mm, 0.1"x0.2" (double sided, 28 pined, very standard pitch (distance between pins)). However some modifications are needed. The edges must be trimmed (you can use more than 28 pinned connector as you’ll have to trim it anyway) and a key needs to be inserted to the 5th pin from the right. The key is there to prevent shortages between power and data/address lines, a 3d printed version can be found here.

Trim the edges and insert the key. Don’t forget to glue the key, if you pull it out and don’t notice it, shorting the power and data lines will have a catastrophic consequences.
Approximate size of trimmed connector.

Assemble everything

This is not the only way on how to assemble this, it might not even be the easiest, but we started with soldering the EPM240T100C5N to the board. This is the hardest part in this assembly and the only reason this might not be a suitable project for a beginner. However if you’re doing this for the first time, be careful for chip orientation and that all of the legs are aligned with the pads.

Soldering on the main CPLD
Just to get an idea, how tiny the legs on this thing are, we had to use an microscope that helped us find the shortage.

After we soldered the CPLD, we also checked all of the consecutive pins with a multimeter in continuity mode. At first we found quite some that needed to be fixed. Please mind that there are 2 of them that are shorted by design in the gerber.

Shorted pins.
We continued with 330 Ohm SMD resistors, size 1206 on both sides of the board.
Next step was to solder on the R0 resistor, which can be omitted to simplify the bill of materials (just bridge the pad with solder). THIS IS ONLY FOR VERSION 1 USING INTERNAL 5V!
Mount 6, 100 nF ceramic capacitors orientation does not matter)
Add the LM1117, 3.3V LDO regulator
Add a 100uF capacitor (only one in version 1)
Finish off by soldering the connectors
Version 2 has an additional 100 uF capacitor, L7805C2T voltage regulator and removed 0ohm resistor.

Program the CPLD

CPLD proggraming resources that helped us a lot, since this was our first project with CPLD

If you’re using EPM240 or EMP7128 CPLD, you have already precompiled .pof files that you just need to upload to CPLD. Just don’t forget to power up the chip when doing so. If you’re using different CPLDs the project also contains the sources, so you may compile it on your own.

Powering the chip during the flashing.
Firstly click on identify, than add the file, check the “Program/Configure” and than click Start, the green progress bar at top right corner and the the logs indicate the upload was successful.

There is also available an alternative firmware and it can be found here. However this did not work at all for me. I have tried it on 2 different boards.

Add software to micro SD card

The micro SD card should contain the following software from:

TK-Pie-master\PI_SD

TK-Pie-master\PI_OS

folders so the at the end it should look like:

Root of the micro SD card.

Final setup and usage

Now you only need to add your RaspberryPi Zero to the Pi connector, add a mini HDMI adapter (RaspberryPi Zero has a mini HDMI connector) connect it to your TV/monitors HDMI and you’re good to go. TK-Pie will be powered from your Spectrum.

Simple enhancement

Later on we also added a reset button.

To add a reset functionality, just connect the RESET signal from the edge connector and GND. It’s a bit of a hack job, but it works.

Comparison between Tk-Pie and Composite

All of the following pictures, except the last one, are taken from my Samsung Smart TV, which has a weird post processing of composite input, so the results are not optimal to begin with and the difference is much bigger than the one taken on my friend’s monitor.

The picture from TK-Pie is obviously much more vivid and cleaner than the composite input, also no interference is present. There was no noticeable lag and even the games can be played quite well (I’m sure the purists will disagree).

However the improvement will also depend on TV/monitor you’re using. The composite picture on my friends monitor is much better than on my TV, so the difference is not as obvious.

This comparison is from my friends computer monitor/TV

Final thoughts

Assembling the device is a bit challenging for a beginner and even a seasoned electronic enthusiast will have some challenge with it, without the proper equipment, because of the 100 pin CPLD and SMD components.

Otherwise the device is quite straight-forward. However one thing that I would like to mention is that the TK-Pie is not quite consistent. There are some bugs in the firmware that sometimes do some erratic things .

But the biggest issue I personally had was that it did not work on its own, it only worked when connected via DivMMC EnJOY! PRO, why is something I was not able to figure out. It could be that my edge connector is damaged or some components might be slowly failing in my ZX 48k. If I find out what the reason is, I’ll update the article.

Second con that I noticed is that the TK-Pie could only be used for around 20 minutes before the ZX stopped to cooperate.

The screen i got after 20 min of using TK-Pie. The artifacts appear both on the HDMI and composite. Usually this kind of screen indicates the lower RAM failure.

Due to the time delay I think that the TK-Pie is pushing the ZX to its limit and something might be overheating (maybe a voltage regulator or something else). This could be fixed by using an external power, however it would require a slight update to the design. Or maybe a simple firmware update and under-clocking the Raspberry Pi would do(I plan to test this theory and update the article).

Overall it was a fun and interesting little project and the design is working; however I think the project would benefit greatly from a bit of refining. In its current state I would not recommend it for everyday use.

Basic debug

This image suggests that the kernel.img (on Raspberry Pi) file isn’t booting, which means there’s something wrong with the image on your SD card. I got this image when i used the Pi Zero W with image compiled for Pi Zero, for example.
The image on composite/RF was fine. The Raspberry Pi boots and works, however it’s not getting proper data from the TK-Pie.
I got this image after extensive use of Tk-Pie, both on HDMI and composite. I presume something is overloading the Spectrum.

Resources and reviews:

Some videos to help you get the idea.

In Czech, use google translate:

In Russian, use google translate:

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