We disassemble the chip from the musical card

Ken Shirriff, a computer historian and rebuilding enthusiast, tweeted a new project to reverse engineer a Christmas chip

Festive showdown of a tiny chip playing a Christmas melody. The UM66T chip looks like a transistor, but inside it has a 64-note ROM and a speaker control circuit. It was specially designed for musical cards and toys. Let’s see what’s inside.

I dissolved the body of the chip in boiling sulfuric acid to get to the 1.8 x 1.8 mm silicon crystal inside. This was the first time I opened the chip with acid, and everything went well, although there were some scratches on the crystal. Under the microscope, you can see the CMOS structure of the chip.


UM66T01A 9249


Coin diameter 19 mm


Pink crystal with greenish and bluish areas. From above, all are connected together by metal conductors.

64 notes are stored in 64×6 ROM. Each note stores 4 bits of frequency and 2 bits of duration. By changing the metal layer, you can program the chip to perform different songs, and sell different versions. Above the ROM is a binary decoder. Note the sequence 8,4,2,1.


ROM, a grid of transistors. The activation of specific transistors is controlled by metal conductors.

The next ROM determines the frequencies of 16 notes. The 32 kHz timer is divided to generate pulses. Instead of a counter, division is performed by a shift register with linear feedback. The ROM contains pseudo-random values ​​that initiate the shift register.


Frequency setting ROM


The shift register consists of 7 similar steps

Each stage of the shift register is a flip-flop consisting of 26 transistors. The inverter loop holds the bit until it passes it to the next stage. By using shift registers instead of conventional counters, several transistors have been saved.


On-chip shift register stage. A bunch of reddish transistors connected by metal wires.


Scheme of the corresponding stage

I marked the functional blocks in the photo of the crystal. The music chip circuit is minimalistic. It uses only triggers and gates, and no microcontroller. The chip has 3 pins, but the die has 8 pads. The rest are for checking the chip.

To generate a clock frequency of 32 kHz, the chip has a simple resistor-capacitor-inverter generator. The white rectangle is the capacitor. Green zigzag – resistor. The resistance can be adjusted by short-circuiting part of the resistor in the metal layer. White zigzags are transistor bases.

An RC oscillator on a chip is cheap and imprecise, unlike a quartz oscillator. When the voltage changes, the frequency also changes. In the next video, I distort the sound of the melodies by raising and lowering tension.

Cool, they managed to cram everything at once on one music chip. All that remains is to add the battery and speaker. For a postcard, of course, a compact piezo emitter is more suitable. After turning on the power, the chip plays the melody from start to finish and stops. I think that such a chip first appeared in 1990.

The chip is based on the most advanced integrated circuits – CMOS. But the inverter is p-channel MOS on the left, n-channel on the right. The p-channel MOS turns on when a 0 is applied to the input, pulling the output up. n-channel turns on when 1 arrives, pulling the output down. It turns out an inverter.

Much larger transistors are used for the high current output contacts. The zig-zag structure on the gate of the transistor allows all the elements to be positioned more efficiently.

Hope this thread gives you a festive mood. 🙂

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