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.
Button Interface
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).
Front Board Ribbon Pins
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 | ๏ปฟ |
11 | Speaker + | ๏ปฟ |
12 | GND (Speaker -) | ๏ปฟ |
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Button Presses
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๏ปฟ 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.
Toggle Sliders
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.
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