Add an option on Uno boards to enable a 4 MHz clock signal on pin 3

This clock signal can be used to reprogram the fuses of an AVR that
has been set wrong (ie. when an external clock has been configured but
the board/chip does not have one).

This is disabled by default. We check that we compile for known chips
before enabling the clock generation. Maybe there is other chips that
support this code.

Author: pr1csynth

build/shared/examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino

@@ -53,6 +53,22 @@
 #define SPI_CLOCK 		(1000000/6)
 
 
+// With an Genuino/Uno board, uncomment following line to generate a 4 MHz clock
+// signal on pin 3 (only available on Uno / ATmega*8)
+// 
+// Can be useful to recover an AVR chip with fuses setup for an external clock
+
+// #define ENABLE_CLOCK_GEN
+
+#ifdef ENABLE_CLOCK_GEN
+
+#if defined(__AVR_ATmega328__) || defined(__AVR_ATmega328P__) || defined(__AVR_ATmega168__) || defined(__AVR_ATmega88__)
+#define CLOCK_GEN_ENABLED
+#endif
+
+#endif
+
+
 // Select hardware or software SPI, depending on SPI clock.
 // Currently only for AVR, for other architectures (Due, Zero,...), hardware SPI
 // is probably too fast anyway.
@@ -216,6 +232,19 @@ static BitBangedSPI SPI;
 
 #endif
 
+#ifdef CLOCK_GEN_ENABLED
+
+// ouput a 4MHz clock on pin 3 (Uno) using fast PWM
+void setup_clock_gen(){
+  pinMode(3, OUTPUT);
+  TCCR2A = 0x23;
+  TCCR2B = 0x09;
+  OCR2A = 3;
+  OCR2B = 1;
+}
+
+#endif
+
 void setup() {
   SERIAL.begin(BAUDRATE);
 
@@ -226,6 +255,10 @@ void setup() {
   pinMode(LED_HB, OUTPUT);
   pulse(LED_HB, 2);
 
+#ifdef CLOCK_GEN_ENABLED
+  setup_clock_gen();
+#endif
+
 }
 
 int error = 0;