Powering a Cambridge Z88 with a MT3068 module

keyboard of a small computer with a USB cabled gadget on top of it
my Z88 → USB power cable, with slightly stoory Z88 underneath

I got a Cambridge Computer Z88 again, after perhaps 30 years of not having one. They are very nice little portable computers, remarkable for packing a multi-tasking OS into an 8-bit machine that runs for weeks on 4× AA batteries.

But the Z88 is a Clive Sinclair joint, so there’s going to be at least one deeply weird and/or ill-advised detail about it. For me, it’s the power adapter. The original had a 6 V centre-positive barrel connector, but in true Sinclair fashion, that wasn’t quite ideal.

The DC power comes through a diode, which drops it down to ~5.2 V. The batteries supply up to 6 V and there’s no protection circuit, so the DC adapter won’t do anything until the internal batteries discharge down to that level. This is far from ideal.

The clever way of dealing with this was Rakewell’s Z88 PSU. This was a variable voltage PSU set to deliver 7 V, so that even when dropped through the power diode, it wouldn’t deplete the batteries.

Unfortunately, Rakewell stopped selling these a while ago. But I remembered I had a some MT3608 (datasheet) boards bought at a hamfest some years back. MT3608s are boost DC-DC converters, so they’ll convert from a lower DC voltage to a higher one, typically at least +2  V higher than the input. With a hacked-up USB cable, a small length of DC cable with a connector and a 3d printed case, I built one that works quite well, at least in the short time I’ve tested it.

  • I used the MT3608 step up DC boost box design from Thingiverse
  • Newer MT3608 boards sometimes feature a micro-USB connector, which is much more convenient than hacking up a USB cable
  • It’s really important to know how to adjust a MT3608 DC-DC Booster before connecting the final voltage, as it’s very easy to burn the board out
  • You can’t use a computer USB port to power an MT3608, as the cable’s not smart enough to negotiate a decent amount of current. A good phone charger should be enough.
  • I set the voltage output to be 7.05 V, with no load

PROTODOME’s wonderful chiptunes: how to play them on your own ATtiny85 chips

electronics breadbord with battery, speaker and sound generated by an 8-ping ATtiny85 mincrocontroller. Additional chips on the board are spares holding other tunes
Six whole tunes ready to play on this tiny chiptune player; a couple are included at the end of this article!

I love the ingenuity that goes into making very tiny projects do very big things. I also love chiptunes. So when I read the metafilter post about PROTODOME’s compositions for the ATtiny85, I was very much there for it.

The circuit to play this is no more than a $2 microcontroller, a lithium coin cell and a speaker or piezo buzzer. The microcontroller has 8 KB of program space and 512 bytes of RAM. The output is a single pin, but with very clever pulse width modulation tricks, sounds like three channels plus percussion.

The album is cool enough on its own, but Blake ‘PROTODOME’ Troise has not only published the source code, but also written an academic article on 1-bit music: “The 1-Bit Instrument: The Fundamentals of 1-Bit Synthesis, Their Implementational Implications, and Instrumental Possibilities.Journal of Sound and Music in Games 1.1 (2020): 44-74.

I remembered I had bought a tube of ATtiny microcontrollers a while back. I knew I had a coin cell and tiny speaker. “I can do this!”, I thought.

So what follows is tutorial on compiling embedded code for an ATtiny85 microcontroller on Linux. There are larger tutorials out there, there are better tutorials: but there are also many out-of-date and misleading tutorials. This isn’t a general ATtiny development tutorial, but one specialized on getting PROTODOME’s tunes playing on your microcontroller.


The very minimum you will need to play the music is:

But that’s not all: you’ll need much more kit to program these tiny chips:

  • a computer running Linux. Yes, you can do this under Windows and Mac OS, but I don’t know how and there are search engines that care about that more than I do. I tested all of this on a Raspberry Pi 4. Tablets and phones are out, sorry
  • an AVR programmer. You can use an Arduino for this (either an official one or a cheaper clone) but you’ll need some additional fiddling and a 10 µF capacitor to get that going. I used a dedicated USBtinyISP programmer just because I had one, but it’s not really necessary. Whatever you use, you’ll need a USB cable for it
  • probably more jumper wires.


There are two separate toolchains involved — one to build the mmml-compiler to convert PROTODOME’s compositions to µc embedded C code, and another to compile that to ATtiny85 instructions. We can install it all in one go:

sudo apt install avrdude gcc-avr binutils-avr avr-libc build-essential git

Building mmml-compiler is easy enough:

git clone https://github.com/protodomemusic/mmml.git
cd mmml/mmml-compiler
gcc -o mmml-compiler mmml-compiler.c

You can then run the compiler on each of the songs; the album title track, for example:

cd ../demo-songs/4000ad/
../../mmml-compiler/mmml-compiler 4000ad.mmml

⚠️ If you get [ERROR 14] Too few channels stated! instead of Successfully compiled! it seems that the compiler isn’t too happy running on some 64-bit systems. I did all my compilation on a Raspberry Pi 4 running Raspbian and all was well. If you can’t get them to compile, I’ve pre-compiled them for you and they’re at the end of this article.

You should now have a musicdata.h file that contains all the tune data. Copy it to the same folder as the mmml-player C code:

cp musicdata.h ../../mmml-player/
cd ../../mmml-player/

That folder now contains the player and one tune data file. Now you need to compile it into AVR instruction to write to your chip:

avr-gcc -g -Os -mmcu=attiny85 -DF_CPU=8000000 -o mmml.bin mmml.c
avr-objcopy -j .text -j .data -O ihex mmml.bin mmml.hex
rm mmml.bin

The end result of what that just did is create a single small file mmml.hex containing the ATtiny85 program instructions for the 8+ minute track 4000AD. If you’re compiling for a different µc, you’ll need a different avr-gcc line:

  • -mmcu=attiny85 will need to be changed for your µc. avr-gcc –target-help lists the supported targets in the ‘Known MCU names’ section way up at the top of its too-copious output. If you’re using the ATmega32P chip made popular by Arduinos, that option should be -mmcu=atmega328p
  • -DF_CPU=8000000 tells the compiler that the CPU frequency should be 8 MHz. The AVR µcs can run at a huge range of speeds, but PROTODOME’s music is timed to work at 8 MHz only.

→→→ aside

If you find yourself compiling a few simple AVR projects but want to stop short of a fine-but-overly-complex Makefile project for AVR development, this script to create a hex file from a single embedded C source file might be useful:

# avrbuild.sh - build a simple AVR project - scruss, 2020-04
# usage: avrbuild.sh file.c mcutype freq
# eg: avrbuild.sh mmml.c attiny85 8000000

rm -f "$b.bin" "$b.hex"
avr-gcc -g -Os -mmcu="$2" -DF_CPU="$3" -o "$b.bin" "$b.c"
avr-objcopy -j .text -j .data -O ihex "$b.bin" "$b.hex"
avr-size --format=avr --mcu="$2" "$b.bin"
rm -f "$b.bin"

In addition to creating a hex file, it also runs the avr-size tool to show you much memory your program uses. The 4000AD tune uses 98% of the ATtiny85’s 8192 byte program space — not quite enough to include that 14 minute extra bass solo, sorry …

←←← end aside

Flashing the chip

So now we do some wiring. If you’re using a dedicated programmer, use jumpers to connect its ICSP port to the ATtiny 85 like this:

                       |o   A   |             
               Reset  -+ 1  T  8+-  VCC       
                       |    t   |             
                      -+ 2  i  7+-  SCK       
                       |    n   |             
                      -+ 3  y  6+-  MISO      
                       |    8   |             
               GND    -+ 4  5  5+-  MOSI      

                 MISO    o1 2o   VCC   
                 SCK     o3 4o   MOSI     
                 Reset   o5 6o   GND 


Wire VCC to VCC, MISO to MISO, MOSI to MOSI, SCK to SCK, Reset to Reset and GND to GND. If you’re using an Arduino, you want to do this:

This is ‘OLD_STYLE_WIRING’ for using ArduinoISP, apparently. But it works!

The wiring for that is:

  • Arduino D10 → ATtiny Pin 1 (Reset)
  • Arduino GND → ATtiny Pin 4 (GND)
  • Arduino D11 → ATtiny Pin 5 (MOSI)
  • Arduino D12 → ATtiny Pin 6 (MISO)
  • Arduino D13 → ATtiny Pin 7 (SCK)
  • Arduino 5V → ATtiny Pin 8 (VCC)
  • You’ll also need to put a 1-10 µF electrolytic capacitor between the Arduino’s Reset and GND pins, but only after you’ve programmed it with the ArduinoISP sketch.

You’re almost there!

Setting up the programmer: USBtinyISP

If you haven’t used one with your computer before, you need to do a little bit of prep so your computer recognizes it. These are modified from a gist:

  • do sudo vi /etc/udev/rules.d/41-usbtiny.rules
  • add the line SUBSYSTEM=="usb", ATTR{idVendor}=="1781", ATTR{idProduct}=="0c9f", GROUP="plugdev", MODE="0666"
  • save and exit
  • do sudo udevadm control --reload then sudo udevadm trigger

Your system should automatically recognize the device and give you permission to use it without sudo privileges.

Setting up the programmer: ArduinoISP

  • Load the ArduinoISP sketch (it’s in FileExamples)
  • Add (or find and uncomment) the line #define USE_OLD_STYLE_WIRING
  • Upload the code to your Arduino
  • Connect the 1-10 µF electrolytic capacitor between the Arduino’s Reset and GND pins

To program the mmml.hex you created earlier, you’ll need one of these avrdude commands:


avrdude -c usbtiny -p attiny85 -U lfuse:w:0xe2:m -U hfuse:w:0xdf:m -U efuse:w:0xff:m -U flash:w:mmml.hex:i

For ArduinoISP:

avrdude -c arduino -P /dev/ttyUSB0 -b 19200 -p attiny85 -U lfuse:w:0xe2:m -U hfuse:w:0xdf:m -U efuse:w:0xff:m -U flash:w:mmml.hex:i

What all that means:

  • -c usbtiny or -c arduino: programmer type. In addition, the arduino programmer takes additional parameters -P /dev/ttyUSB0 -b 19200 which specify the port (usually /dev/ttyUSB0 or /dev/ttyACM0) and the baud rate (always 19200, unless you changed it in the source of ArduinoISP)
  • -p attiny85: the chip type, as used in the avr-gcc compiler call way up the top
  • -U lfuse:w:0xe2:m -U hfuse:w:0xdf:m -U efuse:w:0xff:m: fuses are AVR’s confusing name for configuration bits. You might just have to take my word that this sets an ATtiny85 to use the internal 8 MHz oscillator (as opposed to an external crystal) we told the compiler to use further back. A guide to fuse settings is available at the Engbedded AVR Fuse Calculator
  • -U flash:w:mmml.hex:i: the hex file we prepared, mmml.hex.

If everything went right with your flashing process, you should see lots of “avrdude: verifying … done. Thank you”. If you don’t, likely you missed a connection somewhere.

♫ Playing the tunes! ♫

This circuit’s a lot simpler than it looks!

I already described all of the bits in the bill of materials in the Hardware section. If you want it in ASCII art, here’s all there is to it:

                       |o   A   |             
          VCC--(10kΩ)--+ 1  T  8+--VCC        
                       |    t   |             
                      -+ 2  i  7+-            
                       |    n   |             
                      -+ 3  y  6+-      (     
                       |    8   |      ((     
                  GND--+ 4  5  5+--(SPKR(--GND
                       |________|      ((     

          Pin 1: RST - held high through pull-up to prevent reset
          Pin 4: GND
          Pin 5: PB0 - through speaker/buzzer to GND
          Pin 8: VCC - can be a CR2032 Lithium coin cell

          Not shown: 100 nF decoupling capacitor between VCC and GND
          Short Pin 1 to GND to restart song

If you weren’t able to compile the tunes, I’ve included (with Blake’s permission) source for any AVR µc plus hex files for ATtiny85s here: protodome-mmml-examples.zip

Last but not least, there are a couple of tracks included in the source that aren’t on the 4000AD album. Blake gave me permission to include them here, too:

Fly Me to the Moon by Bart Howard, arranged for ATtiny85 microcontroller by PROTODOME, 2020.
Download: fly_me_to_the_moon.mp3
Till There was You by Meredith Willson (from the musical ‘The Music Man’), arranged for ATtiny85 microcontroller by PROTODOME, 2020.
Download: till_there_was_you.mp3

These weren’t recorded from a tiny speaker (that went badly), but directly to a Marantz solid state recorder. The rig’s the same as the playback one, with the speaker replaced by a potentiometer (for level control), a 100 µF capacitor (to take off some of the DC bias and also to cut some of the very high frequencies) and a headphone socket. Have fun!

it’s the most awkward walkman!

Lovely automata: bbcbasicbot

bbcbasicbot rendering of my one-liner

BBC BASIC bot [beta2] on Twitter is lovely. You tweet a BBC BASIC program to it and it replies with an animation rendering of what your program would look like on a BBC Micro.

I sent it this:


which readers might recognize as 10 PRINT, the endless random maze one-liner for the C64. This program even inspired its own book – also called 10 PRINT CHR$(205.5+RND(1)); : GOTO 10 – about simple generative art.

You can run it in your browser thanks to the amazing JSBeeb.

The Sharpie Liquid Pencil – massive letdown

In Primary Six to about First Year [so about 1979 to 1981], the thing to have was a Papermate Replay, the first real* erasable ballpoint. Despite their waxy purplish-blue ink (which had a strong piney aroma) it was the one thing all the cool kids had. The Replay erasers were gritty and smudgy, and left black crumbs on the page. I remember the gummy click of the ball on the paper, and the rising fug of Replay ink from thirty desks. When it eventually dried, Replay ink could stick pages lightly together, a bit like paste-up wax.

With the Liquid Pencil, Sharpie probably hopes to repeat the (at least initial) success of the Replay. The technology feels similar — slightly less sticky, and the smell of the ink is different, but there’s still an unusual high note to it. The ink looks curiously as if it’s been photocopied and is of an uneven weight, just like the Replay used to be. Leaning on a freshly-written page from the Liquid Pencil smudges the ink on your hand and partially erases the text — just like the old Papermate Replay.

While the Replay really didn’t like regular erasers, the Liquid Pencil is better with them. If the LP were a real pencil, a heavy trace would conduct:

███ Liquid Pencil: ∞Ω  ███ Faber-Castell 9000 HB: 400kΩ

It doesn’t, so it’s no pencil.

I’m pretty sure the Sharpie Liquid Pencil is just the naff old Replay, repackaged for a new generation. After all, Newell Rubbermaid owns both the Sharpie and Papermate brands. I bet the old news stories about Replays being used for cheque-fraud will resurface. Even writing this has given me the old Replay ink smell headache — déjà pew!
* there were the chemically erasable kind available before, which had a yellowish felt tip on one end that bleached the ink and prevented you writing over it.

Unscrewing the barrel revealed the familiar old Papermate Replay refill. I think we’ve been had.

Update: a bunch of reviews. The ones that actually tried it came to pretty much the same conclusion:

got it bad

That retrocomputing itch … sometimes a guy just gotta rediscover the Commodore Amiga of his youth.