Hey everyone, I’m officially out of ideas and need a sanity check.

# The Problem

I’m building an analog patchbay using an MT8812 (40-pin DIP) controlled by an Arduino Mega. The goal is simple: programmatically route audio signals between inputs and outputs.

I saw someone do something similar with an MT8816 and thought, “How hard can this be?”

Well… three days later I can’t even get a single crosspoint to close.

# Hardware

* MT8812AE (DIP-40) – bought two just in case
* Arduino Mega 2560 (powered via USB, 5V logic)
* External 5V supply for test signals
* All grounds tied together (Arduino + MT8812 VSS + external supply)

# The Simplest Test in the World

I’m literally trying to do the most basic thing possible:

* Apply +5V from the external supply to X0 (pin 33)
* Y0 (pin 35) has a 10kΩ resistor to GND
* Activate crosspoint X0 → Y0 via Arduino
* Measure Y0 with a multimeter

**Expected result:** \~4.9V on Y0 when the switch closes  
**Actual result:** 0V at all times, as if the switch never closes

# Connections (Arduino Mega → MT8812)

    AX0–AX3: Mega pins 22–25
    AY0–AY2: Mega pins 26–28
    DATA:    pin 29
    STROBE:  pin 30
    RESET:   pin 31
    VDD (pin 40): connected to Arduino 5V
    VSS (pin 20): connected to GND
    

# What I Have Already Verified

Because I know this is what people will ask:

✓ VDD is 5V at pin 40  
✓ VSS is correctly tied to GND  
✓ All control signals reach the MT8812 (measured with multimeter)  
✓ When DATA is HIGH, I measure 5V at pin 38  
✓ STROBE pulse is visible at pin 18  
✓ The +5V test signal reaches X0 (pin 33)  
✓ Chip orientation is correct (pin 1 notch verified)  
✓ Tried the second MT8812 – same behavior

# Things I’ve Tried

* Delays from microseconds to milliseconds
* Repeating the latch sequence 100 times just in case
* Different address combinations (X0Y0, X1Y1, X2Y0, etc.)
* Long STROBE pulses (up to 10ms)
* Following datasheet timing exactly: ADDRESS stable → DATA stable → STROBE HIGH → STROBE LOW
* Issuing RESET before configuration
* Praying (didn’t help)

# Simplified Code

    // Initial setup
    digitalWrite(RESET_PIN, HIGH);
    delay(1);
    digitalWrite(RESET_PIN, LOW);
    delay(10);
    
    // Set address X0,Y0 (all bits LOW)
    digitalWrite(AX0, LOW); digitalWrite(AX1, LOW);
    digitalWrite(AX2, LOW); digitalWrite(AX3, LOW);
    digitalWrite(AY0, LOW); digitalWrite(AY1, LOW);
    digitalWrite(AY2, LOW);
    
    // DATA = HIGH
    digitalWrite(DATA_PIN, HIGH);
    delay(10);
    
    // STROBE pulse
    digitalWrite(STROBE, HIGH);
    delay(10);
    digitalWrite(STROBE, LOW);  // Latch occurs here per datasheet
    

After this, Y0 should show \~5V. Instead: 0V, like the switch is open.

# My Theories (Probably All Wrong)

1. **Fake chips** – The MT8812s I got are old / remarked / dead
2. **Missing external components** – Maybe I need decoupling caps or something not obvious in the datasheet
3. **Timing issue** – Some subtle timing detail I’m missing
4. **Physical problem** – DIP pin not making good contact in the breadboard
5. **I’m an idiot** – There’s something obvious I’m overlooking

# Question for Anyone Who Has Used This

Has anyone successfully used the MT8812?  
Am I missing something fundamental?  
Is there some “gotcha” not clearly stated in the datasheet?

If I can’t get this working, I’m considering switching to a **CH446Q** (SPI-controlled instead of parallel addressing — fewer wires, theoretically fewer problems).

Has anyone used that?  
Or can you recommend a better DIY analog switch matrix alternative?

I’ll take any idea, conspiracy theory, or black magic ritual at this point.

Thanks in advance — and sorry for the wall of text. This thing is driving me insane.

[https://www.alldatasheet.es/datasheet-pdf/view/77069/MITEL/MT8812AP.html](https://www.alldatasheet.es/datasheet-pdf/view/77069/MITEL/MT8812AP.html)



MT8812 Not Closing Switches – Has Anyone Actually Used This Thing?