Front Button Board (PC Engine GT)
The front PCB that screws into the front shell and has the button pads on the board has a BU5782K ASIC that handles headphone audio and button presses.
Here is a quick TikTok explaining the button interface.
Buttons are polled every 33 milliseconds.
The read trigger is a 1us pulse that happens on pin 7 every 33ms.
Each pin has four states.
- No Buttons Pressed = 5V DC
- Button A Pressed = 5x Low Pulses 13us apart
- Button B Pressed = 5x High Pulses 13us apart
- A + B Pressed = 0V DC
The pulses happen as soon as the Trigger (pin 7) pulses high.
Here is an example of the pins 9 down to 2 from top to bottom. with Left and Run currently pressed.
As you can see pin 7 is the main trigger pin when buttons are read. Pin 6 is a pulse map (the times at which the button pins toggle into the same pattern or inverse pattern.
Then pins 2 to 5 are dual buttons (2 buttons per pin).
Pin | Function | Notes |
|---|---|---|
1 | 5V | With bulk cap CC800 connected to it |
2 | Up / I | Up (Low Pulse), I (High Pulse) |
3 | Right / II | Right (Low Pulse), II (High Pulse) |
4 | Down / Select | Down (Low Pulse), Select (High Pulse) |
5 | Left / Run | Left (Low Pulse), Run (High Pulse) |
6 | Pulse Map | Button Pressed Wave Form |
7 | Trigger | 1us 5V pulse every 133ms |
8 | !Backlight On | Front board shorts this pin to ground to turn on backlight |
9 | ??? Always 5V | Tied to 5V and C702 (470uF) is on that rail |
10 | GND | īģŋ |
11 | Speaker + | īģŋ |
12 | GND (Speaker -) | īģŋ |
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The trigger pin 7 goes high, and then the four button data pins (2 to 5) are read in right at that moment.
Take pin 2 for example which is Up and I buttons.
Without anything pressed the blue pin is 5V.
If Up is pressed, then pin will output the pulse map but inverted.
If I is pressed, then the pin will output the pulse map non-inverted.
When both Up and I are pressed, the pin will go to ground.
The I and II toggle sliders on the front are turbo buttons. They have 3 positions, left, middle and right.
The left position is off, the middle position turbo presses (repeat press/release cycle) while the respective button (I or II) is held down, 4 presses on, 4 presses off, so 132ms on, 132ms off or approximately 4 times a second.
The right position is twice as fast so 2 presses on, 2 presses off, (66mm + 66ms = 132ms) and approximately 7 to 8 times a second.
NOTE: If your physical Turbo button springs do not make good contact with the PCB there will be no connection to ground or the turbo pulse pins to buttons I and II, as they pass through them.
You can instead just short the top bar and left pad of the turbo contacts on the PCB (so grounding one side of each I and II button, to let them act normally and disable the turbo action.
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