start prototype sketch for SD card interface

sdcard/apple1_sdcard/apple1_sdcard.ino

@@ -0,0 +1,133 @@
+// pin definitions
+
+// VIA 6522 connections
+
+#define D0 1   /* I/O data bit 0, connects to PA0 on the VIA */
+#define D1 2   /* I/O data bit 1, connects to PA1 on the VIA */
+#define D2 3   /* I/O data bit 2, connects to PA2 on the VIA */
+#define D3 4   /* I/O data bit 3, connects to PA3 on the VIA */
+#define D4 5   /* I/O data bit 4, connects to PA4 on the VIA */
+#define D5 6   /* I/O data bit 5, connects to PA5 on the VIA */
+#define D6 7   /* I/O data bit 6, connects to PA6 on the VIA */
+#define D7 8   /* I/O data bit 7, connects to PA7 on the VIA */
+
+#define CPU_CLOCK   9   /* cpu send data clock, VIA.PB0 => MCU */  
+#define MCU_CLOCK  10   /* mcu send data clock, VIA.PB1 <= MCU */
+
+// utility function for read a bit 
+#define BIT(data,n) (((data) >> (n)) & 1)
+
+void setup() {
+  // debug on serial
+  Serial.begin(9600);  
+  
+  pinMode(CPU_CLOCK, INPUT);
+  pinMode(MCU_CLOCK, OUTPUT);
+}
+
+void loop() {
+  // applicazione di esempio: manda un messaggio quando riceve il comando 42
+  int data = receive_byte_from_cpu();
+
+  if(data == 42) {
+    Serial.println("command 42 received from CPU");
+    char *msg = "HELLO WORLD!\r\n";
+    for(int t=0; t<strlen(msg); t++) {
+      send_byte_to_cpu(msg[t]);  
+    }      
+    send_byte_to_cpu(0);  // terminatore stringa
+    Serial.println("message sento to CPU");
+  }  
+}
+
+void wait(int pin, int value) {
+  // TODO prevedere un meccanismo di timeout?
+  while(digitalRead(pin) != value);  
+}
+
+void receive_byte_from_cpu() {
+  
+  // when here, both CPU_CLOCK and MCU_CLOCK are LOW
+  
+  // set data port as input
+  pinMode(D0, INPUT);
+  pinMode(D1, INPUT);
+  pinMode(D2, INPUT);
+  pinMode(D3, INPUT);
+  pinMode(D4, INPUT);
+  pinMode(D5, INPUT);
+  pinMode(D6, INPUT);
+  pinMode(D7, INPUT);
+  
+  // tells CPU we are ready to receive
+  digitalWrite(MCU_CLOCK, HIGH);   
+  
+  // wait data to be signaled as "ready" on the CPU  
+  wait(CPU_CLOCK, HIGH);           
+
+  // read the data byte
+  int data = 
+    (digitalRead(D0) << 0) |
+    (digitalRead(D1) << 1) |
+    (digitalRead(D2) << 2) |
+    (digitalRead(D3) << 3) |
+    (digitalRead(D4) << 4) |
+    (digitalRead(D5) << 5) |
+    (digitalRead(D6) << 6) |
+    (digitalRead(D7) << 7);
+
+  // tells CPU data was received
+  digitalWrite(MCU_CLOCK, HIGH);
+  
+  // wait the CPU to end operation
+  wait(CPU_CLOCK, LOW);           
+  
+  // we also terminate
+  digitalWrite(MCU_CLOCK, LOW);
+  
+  // when here, both CPU_CLOCK and MCU_CLOCK are LOW
+  
+  return byte;
+}
+
+void send_byte_to_cpu(int data) {
+
+  // when here, both CPU_CLOCK and MCU_CLOCK are LOW
+  
+  // set data port as output
+  pinMode(D0, OUTPUT);
+  pinMode(D1, OUTPUT);
+  pinMode(D2, OUTPUT);
+  pinMode(D3, OUTPUT);
+  pinMode(D4, OUTPUT);
+  pinMode(D5, OUTPUT);
+  pinMode(D6, OUTPUT);
+  pinMode(D7, OUTPUT);
+  
+  // wait until CPU is ready to receive (should be already)
+  wait(CPU_CLOCK, LOW);
+
+  // put byte on D0-D7
+  digitalWrite(D0, BIT(data,0) );
+  digitalWrite(D1, BIT(data,1) );
+  digitalWrite(D2, BIT(data,2) );
+  digitalWrite(D3, BIT(data,3) );
+  digitalWrite(D4, BIT(data,4) );
+  digitalWrite(D5, BIT(data,5) );
+  digitalWrite(D6, BIT(data,6) );
+  digitalWrite(D7, BIT(data,7) );
+  
+  // tells CPU byte is ready on the data port
+  digitalWrite(MCU_CLOCK, HIGH);
+  
+  // wait for CPU to read the data
+  wait(CPU_CLOCK, HIGH);
+  
+  // tells CPU byte we are finished
+  digitalWrite(MCU_CLOCK, LOW);
+  
+  // wait for CPU end as well
+  wait(CPU_CLOCK, LOW);
+  
+  // when here, both CPU_CLOCK and MCU_CLOCK are LOW    
+}