I thought I would post a bit of information about using a Zarlink MT8816 crosspoint switch (datasheet) with an Arduino or similar microcontroller. The MT8816 is a 40-pin IC which allows you to route any of its 8 X pins to any of its 16 Y pins – the connections are bidirectional, so you have do 8 ins, 16 outs or 16 ins, 8 outs. What you get from this is a cool matrix signal router – a great device to have for musical or other nefarious purposes.
Here’s the pinout:
Each of the pins beginning with “A” is an address pin – they’re how you address a specific X/Y connection. To interface this with an Arduino, you need to connect 11 digital output pins from the Arduino:
- 7 address pins (AX0-AX3, AY0-AX2, these use a strange binary-ish number system – more on that later)
- DATA (High or low to indicate whether to open or close the specified switch)
- CS (Chip Select – you can just tie this high if you’re only using one)
- STROBE (Setting this high writes the DATA to the indicated address)
- RESET (I found that if I didn’t reset the chip upon powering the circuit, I’d get strange results)
Other than that, you need to connect VDD, VSS, and VEE to +, GND, and – supplies.
So, as I mentioned, the addressing scheme is a little strange – and caused me 2 – 3 lost days of head-scratching and frustration. Here’s the info from the datasheet:
As you can see, the address pins indicate an address in a parallel, binary-ish scheme. So, if we’re going to select a particular matrix point – let’s use X3/Y1 – we use all of the address pins at once to indicate the numbers we need. Pins AX0 – AX3 give us 4 X address pins – 4four bits, which lets us count from 0 – 15 in binary. AY0 – AY2 give us 3 bits, 0 – 7 in binary. The 4-bit binary representation of our X address, 3, is 0011, and our Y address, 1, is 001. Consulting the table above, we can look up the value for X3 and see that it is actually 0110, and if we jump down a bit, we see that Y1 is 001. So, to represent our X3/Y1 we just turn our Arduino pins X1, X2, and Y0 high, and leave the others low.
This is all pretty clear, and the addressing follows binary counting rules until you get to X6. Look at the data sheet – X12 and X13 are actually represented by the binary numbers 6 (0110) and 7 (0111). I found an easy fix for my application, but long story short – never assume your chip follows any logic, and always read the datasheet thoroughly. I will admit to much profanity upon discovery of this design “feature” – you can see I had fun on this one…
Here’s my code, apologies for crummy WordPress formatting. Note that I’m only using an 8 x 8 subset of the chip, so my compensation may not work for your needs.
void togglePins(int chip, uint8_t x, uint8_t y, int state){
if(x >= 6){ // compensate for strange x-axis addressing scheme
x += 2;
}
digitalWrite(chip, HIGH);
// next lines convert from integer to binary address
// bitRead returns whether a given bit position in the binary representation of a value is high or low
if(bitRead(x, 0)) digitalWrite(X0, HIGH);
if(bitRead(x, 1)) digitalWrite(X1, HIGH);
if(bitRead(x, 2)) digitalWrite(X2, HIGH);
if(bitRead(x, 3)) digitalWrite(X3, HIGH);
if(bitRead(y, 0)) digitalWrite(Y0, HIGH);
if(bitRead(y, 1)) digitalWrite(Y1, HIGH);
if(bitRead(y, 2)) digitalWrite(Y2, HIGH);
// after address pins are set, set strobe high
digitalWrite(STROBE, HIGH);
// make sure DATA pin is the correct value
digitalWrite(DATA, state);
// reset all pins to low
digitalWrite(STROBE, LOW);
digitalWrite(X0, LOW);
digitalWrite(X1, LOW);
digitalWrite(X2, LOW);
digitalWrite(X3, LOW);
digitalWrite(Y0, LOW);
digitalWrite(Y1, LOW);
digitalWrite(Y2, LOW);
digitalWrite(chip, LOW);
}
It’s a pretty simple function, and you can see the rest of the program in my bitbucket repository.
I’ll be posting more soon about the device I’m building – it’s called pucktronix.snake.corral. I hope this helps decipher the datasheet, and saves some possible head-scratching.
Musician/hacker living in San Diego, CA. Studying computer music at UCSD. 