TommyPROM

Some time ago, I mentioned that I had been unable to test the various ROMs in the UK101, lacking an EPROM programmer. That has now been rectified, thanks to the fine work of Tom Nisbet, creator of TommyPROM. 

In addition to an Arduino sketch, the Github linked above hosts a couple of circuit boards, and I had them made as part of my last order from Seeed Studio.

TommyPROM with a very old 2716 EPROM

Populating this PCB cost under €10, quite the bargain. This version of the programmer can read 27-series EPROMs but can't manage the voltages required to program them.

Configuring the programmer requires editing the Arduino sketch for chip series and type, and uploading the modified sketch. This wouldn't pass muster in a commercial product but is absolutely fine for this project's target market: hobbyists on a budget.

Opening a serial port to the board shows the following menu:

Main Menu

Reading that EPROM, which is nearly 40 years old, produced the following output:

Amnesia?

Unsurprisingly, given its age, it seems to have lost its memory! Luckily it is not too difficult to find copies of UK101 (and Superboard) firmware online, Mark's Lab has copies for instance.

Burning old-school 27-series EPROMs requires more advanced hardware, and the second TommyPROM board I had made can do it, TommyPROM32. I will review that iteration when I have assembled it.

What the current version of TommyPROM can do is write to 28-series EEPROMs, e.g., an AT28C16. This is pin-compatible with the 2716 so once it arrives and has been programmed, the next step in testing the UK101 can be taken: hooking the board up to a logic analyser and spying on the data bus using another fine Open Source tool: 6502Decoder.

(I have several PCBs left over from that order. If you want one at cost, PM me.)

Update (April 2024): after some further investigation, it turns out that the basic TommyPROM can only handle 28-pin chips so any data retrieved from smaller chips (as above) won't be valid. TommyPROM32 is definitely the way to go here. Stay tuned for a review of that coming soon!

Testing ICs

It seems like about two years ago, IC testers made quite a splash, judging by reviews of the Retro Chip Tester Pro (Adrian's Digital Basement) and the BackBit Chip Tester Pro (Noel's Retro Lab). Nice products though they are, they are a little out of my price range for this project.

TTL testing

Luckily for my wallet, there are several DIY projects out there which can be assembled from commonly-available parts in not too long. Nick Gammon (who's excellent interrupt and power-saving pages were inspiration for my Interrupted project) has a super-simple one which can be assembled on a breadboard in about an hour for the cost of 16 resistors, some dupont connectors and an Arduino Uno!

It ain't pretty but it works!

Nick credits an Instructable by one "JorBi" for inspiration for his work. That project is more elaborate, using an Arduino Nano and a display --- Nick's just uses the Arduino serial port, nothing wrong with that. JorBi's project also provided the data file containing the actual chip tests: JorBi stores this in an EPROM while Nick puts it in the on-chip flash.

Another tester of note is the Smart-IC-Tester: an Arduino shield with TFT display. This stores the test database on an SD card, in a format very similar to JorBi's, just adding a one-line description to each part.

So? Well out of 34 74-series TTL chips on the UK101, Nick's tester flagged 7 as bad, 15 as good, 4 were unknown and 8 not tested, due to having wires soldered to them. Not bad for a day's work!

The bad chips were as follows:

• Two 74125s
• Three 74123s (all from the same batch)
• One 7403
• One 7404 (in bad shape, one pin broke off in the socket)

Note, it could be that the 74123 and 74125 tests are incorrect. I have none which tested good.

Update: After playing with the IC-Tester for a while longer, I managed to add tests for the hitherto-unknown chips. The latest score is 9 bad, 22 good and 3 not tested. My fork of Nick's repo is here.

Update 2: I built this circuit on a piece of vero board and piggy-backed it onto my home-brew Uno!

An improvement anyway!

Memory testing

In addition to the discrete TTL chips, the UK101 also had 32 2114 static RAM for main memory and another 4 for the display. Nick's tester is unable to test this IC. Luckily somone named "skjerk" has written a sketch for the Arduino Uno which does exactly this.

The 2114 has a pretty bad rep when it comes to living into old age so I didn't hold out much hope for my set. Rather incredibly however they all came up good!

Untested so far

Chips which I haven't been able to test yet:

• The 6502 and 6851
• The Basic ROMs and Character Generator ROM
• The monitor EPROM

Given an EPROM programmer, it should be possible to read the ROMs and compute their checksums. Stay tuned for that!

Of course the two commercial testers mentioned above can test all of these chips, out-of-the-box. But where's the fun in that?

It speaks!

Porting TTS to the ESP8266 processor has been on my to-do list for a while now, prompted by a comment on an issue I'd created to remind me. To do this, I realised that I'd have to reacquaint myself with PWM on the ATmega328p.

At its most basic, a PWM waveform is a square-wave with a varying duty-cycle. Both the frequency of the square-wave and the duty-cycle are determined on an AVR by the its built-in timers. PWM on Arduino pin 9 (hardware pin 15, OC1A), for example, is driven by Timer-1, a 16-bit timer.

Several PWM modes are available; TTS chooses the "phase and frequency corrected" option.

The diagram shows how this works to change the duty-cycle of the output pin:

• the timer counts from BOTTOM to TOP and back down again
• when it crosses the OCR1 threshold going up, it sets the OC1A output high,
• when it crosses this threshold on the way down again, it sets the OC1A output low. 

Therefore by dynamically changing OCR1, we can change the "volume" on the output pin.

The final piece of the puzzle is how the frequency of the square wave is determined. Since the timer ticks on every clock cycle, it will take 2*TOP cycles to cycle from BOTTOM to TOP and back. The AVR has a clock frequency of 16MHz and PWM_TOP in the TTS library is defined as 1200/2. This gives a frequency of about 13kHz.

By default PWM on Arduino/ESP8266 has a range of 1023 and frequency of 1kHz. However it provides APIs to change these values. By setting them appropriately, the ESP8266 speaks!

Wemos Mini D1 with TTS amplifier and super-cheap speaker

The picture shows a Wemos Mini D1 connected to a modified version of the simple LM386 amplifier described at the TTS GitHub. (The modifications arose from a Hackaday article on the LM386 and are summarised in the GitHub's README.)

This amplifier sounds surprisingly good, considering its total cost was about €1. Hooking it up to a signal generator and oscilloscope produced the following pretty pictures.

At low volume, the output is a pretty faithful inversion of the input signal (a 1kHz sine wave):

Undistorted Signal

At high volume, the output is clipped by the supply rails (but still sounds pretty good to my untrained ear):

Clipped Signal

These pictures were produced using an Xprotolab Plain, an amazing combination of Oscilloscope, Signal Generator and Protocol Analyzer implemented on an AVR XMEGA. Buy one!

Xprotolab Plain

ArduTouch

One of the very few downsides of ordering PCBs from Seeed Studio's Fusion service is the shipping cost. While an individual PCB can cost less than $0.50 in multiples of 10, shipping to Western Europe can add another $20.

One way to amortize these costs is to add more PCBs to an order, either from my own "design pipeline" or by searching online for cool projects. One very cool project discovered in this way, was ArduTouch from Cornfield Electronics.

One the hardware side, this is basically a battery-powered Arduino Uno with an LM386 amplifier driving a 4-Ohm mini-speaker, or your HiFi via line-in. The keypad is very nicely integrated into the PCB, see my construction below.

ArduTouch with Batteries and Cheap Speaker

Software-wise its GitHub provides five (at the last count!) different synthesizer sketches and a library for constructing your own. As usual, sketches are compiled in the Arduino IDE and uploaded by FTDI cable. A nice touch is that they uploaded a BOM for Mouser, making ordering the parts a snip!

I have several PCBs left over from this order. If you want one, contact me through the Tip Jar on the right. However I would encourage you to support the guys who made this great project possible by buying one of their kits.