From e79ed4fa8642502ecb0bd2a95b8cfb7a12ff1fca Mon Sep 17 00:00:00 2001
From: Daniele Verducci su MatissePenguin <daniele.verducci@gmail.com>
Date: Sun, 29 Nov 2020 12:19:55 +0100
Subject: [PATCH] Trying optimizing terminal to stay under 600nS per clock
 cycle (not working ATM)

---
 arduino/arduino_terminal/arduino_terminal.ino | 65 +++++++++----------
 1 file changed, 31 insertions(+), 34 deletions(-)

diff --git a/arduino/arduino_terminal/arduino_terminal.ino b/arduino/arduino_terminal/arduino_terminal.ino
index d73bf0f..acee5c5 100644
--- a/arduino/arduino_terminal/arduino_terminal.ino
+++ b/arduino/arduino_terminal/arduino_terminal.ino
@@ -5,58 +5,55 @@
  * The Arduino IDE serial monitor is used to send and receive commands to the Z80.
  */
 
-#define EN 2   // Active low
-#define WR 11   // Active low
+// EN 2   // Active low
+// WR 11   // Active low
 // DATA BUS IS: 3, 4, 5, 6, 7, 8, 9, 10;
 
+byte incomingBuffer = 0;  // Incoming from computer, to the Pat80
+byte outgoingBuffer = 0;  // Outgoing to computer, from the Pat80
+
 void setup() {
-  Serial.begin(115200);
+  Serial.begin(2000000);
   Serial.println("Pat80 terminal");
   
-  pinMode(EN, INPUT);
-  pinMode(WR, INPUT);
-  DDRD = B00000010; // Port D (used arduino pins 2 to 7) is input. Avoid changing serial pins.
-  DDRB = B00000000; // Port B (used arduino pins 8 and 9) is input
-  attachInterrupt(digitalPinToInterrupt(EN), onClk, CHANGE);
+  DDRD = B00000010; // Port D (used arduino pins 2 (EN) and 3 to 7 (DATA)) is input. Avoid changing serial pins.
+  DDRB = B00000000; // Port B (used arduino pins 8 to 10 (DATA) and 11 (WR)) is input
+  attachInterrupt(digitalPinToInterrupt(2), onClk, CHANGE);
 }
 
-void loop() {}
+void loop() {
+  if (Serial.available() > 0) {
+    incomingBuffer = Serial.read();
+  }
+  if (outgoingBuffer != 0) {
+    if ((outgoingBuffer >= 8 && outgoingBuffer <= 13) || (outgoingBuffer >= 32 && outgoingBuffer <= 127)) {
+      // Printable character
+      Serial.print((char)outgoingBuffer);
+    } else {
+      // Non-printable character
+      Serial.print("[0x");
+      Serial.print(outgoingBuffer, HEX);
+      Serial.print("]");
+    }
+  }
+}
 
 void onClk() {
-  if (digitalRead(EN)) {
+  if (PINB & B00000100 == B00000100) { // If EN is HIGH (clock pulse finished)
     // Clock pulse finished, return to high impedance state
     DDRD = B00000010;
     DDRB = B00000000;
   } else {
-    // Clock pulse started
-    if (digitalRead(WR)) {      
-      // Pat80 wants to Read (we send data)
+     // EN is LOW: Clock pulse started
+    if (PIND & B00001000 == B00001000) {  // WR is HIGH (Pat80 wants to Read (we send data))
       DDRD = DDRD | B11111000; // Port D (arduino pins 3 to 7) is output. In or to preserve serial pins and interrupt pin
       DDRB = B00000111; // Port B (0,1,2) = pins 8,9,10 output
-      byte incomingByte = 0;  // Defaults to NULL
-      // Check if serial data available
-      if (Serial.available() > 0) {
-        incomingByte = Serial.read();
-      }
       // Split byte to two parts
-      byte wbPortD = incomingByte << 3;
-      byte wbPortB = incomingByte >> 5;
-      PORTD = wbPortD;
-      PORTB = wbPortB;
+      PORTD = incomingBuffer << 3;
+      PORTB = incomingBuffer >> 5;
     } else {
       // Pat80 wants to Write (we receive data)
-      byte rb = (PIND >> 3) | (PINB << 5); // Compose the final byte from the two parts
-      if (rb == 0)
-        return; // NULL
-      if ((rb >= 8 && rb <= 13) || (rb >= 32 && rb <= 127)) {
-        // Printable character
-        Serial.print((char)rb);
-      } else {
-        // Non-printable character
-        Serial.print("[0x");
-        Serial.print(rb, HEX);
-        Serial.print("]");
-      }
+      outgoingBuffer = (PIND >> 3) | (PINB << 5); // Compose the final byte from the two parts
     }
   }
 }