From b7bcd7848d46d863d83f3c472258c9591efdca89 Mon Sep 17 00:00:00 2001
From: Rodrigo Alfonso <rodri042@gmail.com>
Date: Mon, 5 Aug 2024 10:28:27 -0300
Subject: [PATCH] Adding pinout and parity check to PS/2 implementation

---
 README.md               | 28 +++++++++++++++++++++++++--
 lib/LinkPS2Keyboard.hpp | 43 ++++++++++++++++++++++++++++-------------
 2 files changed, 56 insertions(+), 15 deletions(-)

diff --git a/README.md b/README.md
index 348ddb4..bfc4dbf 100644
--- a/README.md
+++ b/README.md
@@ -24,7 +24,7 @@ A set of Game Boy Advance (GBA) C++ libraries to interact with the Serial Port.
 
 ## Usage
 
-- Include the library you want (e.g. [LinkCable.hpp](lib/LinkCable.hpp)) in your game code, and refer to its comments for instructions. Most of these libraries are provided as single header files for simplicity. The only external dependency is **libtonc**, which comes preinstalled with *devkitPro*.
+- Include the library you want (e.g. [LinkCable.hpp](lib/LinkCable.hpp)) in your game code, and refer to its comments for instructions. Most of these libraries are provided as single header files for simplicity. The only external dependency is **libtonc**, which comes preinstalled with *devkitARM*.
 - Check out the [examples](examples) folder.
 	* Builds are available in [Releases](https://github.com/afska/gba-link-connection/releases).
 	* They can be tested on real GBAs or using emulators.
@@ -406,6 +406,18 @@ Name | Return type | Description
 `deactivate()` | - | Deactivates the library.
 `report(data[3])` | - | Fills the `data` int array with a report. The first int contains _clicks_ that you can check against the bitmasks `LINK_PS2_MOUSE_LEFT_CLICK`, `LINK_PS2_MOUSE_MIDDLE_CLICK`, and `LINK_PS2_MOUSE_RIGHT_CLICK`. The second int is the _X movement_, and the third int is the _Y movement_.
 
+## Pinout
+
+```
+ ____________
+|PS/2 --- GBA|
+|------------|
+|CLOCK -> SI |
+|DATA --> SO |
+|VCC ---> VCC|
+|GND ---> GND|
+```
+
 # ⌨️ LinkPS2Keyboard
 
 A PS/2 keyboard driver for the GBA. Use it to add keyboard support to your homebrew games.
@@ -422,4 +434,16 @@ Name | Return type | Description
 --- | --- | ---
 `isActive()` | **bool** | Returns whether the library is active or not.
 `activate()` | - | Activates the library.
-`deactivate()` | - | Deactivates the library.
\ No newline at end of file
+`deactivate()` | - | Deactivates the library.
+
+## Pinout
+
+```
+ ____________
+|PS/2 --- GBA|
+|------------|
+|CLOCK -> SI |
+|DATA --> SO |
+|VCC ---> VCC|
+|GND ---> GND|
+```
\ No newline at end of file
diff --git a/lib/LinkPS2Keyboard.hpp b/lib/LinkPS2Keyboard.hpp
index 53851b7..02d287a 100644
--- a/lib/LinkPS2Keyboard.hpp
+++ b/lib/LinkPS2Keyboard.hpp
@@ -78,6 +78,7 @@ class LinkPS2Keyboard {
 
     bitcount = 0;
     incoming = 0;
+    parityBit = 0;
     prevFrame = 0;
     frameCounter = 0;
 
@@ -94,33 +95,52 @@ class LinkPS2Keyboard {
   void _onVBlank() { frameCounter++; }
 
   void _onSerial() {
+    if (!isEnabled)
+      return;
+
     u8 val = (REG_RCNT & LINK_PS2_KEYBOARD_SO_DATA) != 0;
 
     u32 nowFrame = frameCounter;
     if (nowFrame - prevFrame > LINK_PS2_KEYBOARD_TIMEOUT_FRAMES) {
       bitcount = 0;
       incoming = 0;
+      parityBit = 0;
     }
     prevFrame = nowFrame;
 
-    u8 n = bitcount - 1;
-    if (n <= 7)
-      incoming |= (val << n);
-    bitcount++;
-
-    if (bitcount == 11) {
-      onEvent(incoming);
+    if (bitcount == 0 && val == 0) {  // start bit detected
+      // start bit is always 0, so only proceed if val is 0
+      bitcount++;
+    } else if (bitcount >= 1 && bitcount <= 8) {  // data bits
+      incoming |= (val << (bitcount - 1));
+      bitcount++;
+    } else if (bitcount == 9) {  // parity bit
+      // store parity bit for later check
+      parityBit = val;
+      bitcount++;
+    } else if (bitcount == 10) {  // stop bit
+      if (val == 1) {             // stop bit should be 1
+        // calculate parity (including the stored parity bit from previous IRQ)
+        u8 parity = 0;
+        for (u8 i = 0; i < 8; i++)
+          parity += (incoming >> i) & 1;
+        parity += parityBit;
 
+        if (parity % 2 != 0)  // odd parity as expected
+          onEvent(incoming);
+      }
       bitcount = 0;
       incoming = 0;
+      parityBit = 0;
     }
   }
 
  private:
   bool isEnabled = false;
-  uint8_t bitcount = 0;
-  uint8_t incoming = 0;
-  uint32_t prevFrame = 0;
+  u8 bitcount = 0;
+  u8 incoming = 0;
+  u8 parityBit = 0;
+  u32 prevFrame = 0;
   u32 frameCounter = 0;
   std::function<void(u8 event)> onEvent;
 };
@@ -135,9 +155,6 @@ inline void LINK_PS2_KEYBOARD_ISR_VBLANK() {
 }
 
 inline void LINK_PS2_KEYBOARD_ISR_SERIAL() {
-  if (!linkPS2Keyboard->isActive())
-    return;
-
   linkPS2Keyboard->_onSerial();
 }