Support slower baud rates

AltSoftSerial.cpp

@@ -21,6 +21,9 @@
  * THE SOFTWARE.
  */
 
+// Revisions are now tracked on GitHub
+// https://github.com/PaulStoffregen/AltSoftSerial
+//
 // Version 1.2: Support Teensy 3.x
 //
 // Version 1.1: Improve performance in receiver code
@@ -62,16 +65,38 @@ static volatile uint8_t tx_buffer[TX_BUFFER_SIZE];
 #define INPUT_PULLUP INPUT
 #endif
 
+#define MAX_COUNTS_PER_BIT  6241  // 65536 / 10.5
+
 void AltSoftSerial::init(uint32_t cycles_per_bit)
 {
-	if (cycles_per_bit < 7085) {
+	//Serial.printf("cycles_per_bit = %d\n", cycles_per_bit);
+	if (cycles_per_bit < MAX_COUNTS_PER_BIT) {
 		CONFIG_TIMER_NOPRESCALE();
 	} else {
 		cycles_per_bit /= 8;
-		if (cycles_per_bit < 7085) {
+		//Serial.printf("cycles_per_bit/8 = %d\n", cycles_per_bit);
+		if (cycles_per_bit < MAX_COUNTS_PER_BIT) {
 			CONFIG_TIMER_PRESCALE_8();
 		} else {
-			return; // minimum 283 baud at 16 MHz clock
+#if defined(CONFIG_TIMER_PRESCALE_256)
+			cycles_per_bit /= 32;
+			//Serial.printf("cycles_per_bit/256 = %d\n", cycles_per_bit);
+			if (cycles_per_bit < MAX_COUNTS_PER_BIT) {
+				CONFIG_TIMER_PRESCALE_256();
+			} else {
+				return; // baud rate too low for AltSoftSerial
+			}
+#elif defined(CONFIG_TIMER_PRESCALE_128)
+			cycles_per_bit /= 16;
+			//Serial.printf("cycles_per_bit/128 = %d\n", cycles_per_bit);
+			if (cycles_per_bit < MAX_COUNTS_PER_BIT) {
+				CONFIG_TIMER_PRESCALE_128();
+			} else {
+				return; // baud rate too low for AltSoftSerial
+			}
+#else
+			return; // baud rate too low for AltSoftSerial
+#endif
 		}
 	}
 	ticks_per_bit = cycles_per_bit;
@@ -194,7 +219,6 @@ void AltSoftSerial::flushOutput(void)
 /**            Reception               **/
 /****************************************/
 
-
 ISR(CAPTURE_INTERRUPT)
 {
 	uint8_t state, bit, head;
@@ -213,7 +237,8 @@ ISR(CAPTURE_INTERRUPT)
 	state = rx_state;
 	if (state == 0) {
 		if (!bit) {
-			SET_COMPARE_B(capture + rx_stop_ticks);
+			uint16_t end = capture + rx_stop_ticks;
+			SET_COMPARE_B(end);
 			ENABLE_INT_COMPARE_B();
 			rx_target = capture + ticks_per_bit + ticks_per_bit/2;
 			rx_state = 1;

config/AltSoftSerial_Timers.h

@@ -24,6 +24,7 @@
 #if defined(ALTSS_USE_TIMER1)
   #define CONFIG_TIMER_NOPRESCALE()	(TIMSK1 = 0, TCCR1A = 0, TCCR1B = (1<<ICNC1) | (1<<CS10))
   #define CONFIG_TIMER_PRESCALE_8()	(TIMSK1 = 0, TCCR1A = 0, TCCR1B = (1<<ICNC1) | (1<<CS11))
+  #define CONFIG_TIMER_PRESCALE_256()	(TIMSK1 = 0, TCCR1A = 0, TCCR1B = (1<<ICNC1) | (1<<CS12))
   #define CONFIG_MATCH_NORMAL()		(TCCR1A = TCCR1A & ~((1<<COM1A1) | (1<<COM1A0)))
   #define CONFIG_MATCH_TOGGLE()		(TCCR1A = (TCCR1A & ~(1<<COM1A1)) | (1<<COM1A0))
   #define CONFIG_MATCH_CLEAR()		(TCCR1A = (TCCR1A | (1<<COM1A1)) & ~(1<<COM1A0))
@@ -50,6 +51,7 @@
 #elif defined(ALTSS_USE_TIMER3)
   #define CONFIG_TIMER_NOPRESCALE()	(TIMSK3 = 0, TCCR3A = 0, TCCR3B = (1<<ICNC3) | (1<<CS30))
   #define CONFIG_TIMER_PRESCALE_8()	(TIMSK3 = 0, TCCR3A = 0, TCCR3B = (1<<ICNC3) | (1<<CS31))
+  #define CONFIG_TIMER_PRESCALE_256()	(TIMSK3 = 0, TCCR3A = 0, TCCR3B = (1<<ICNC3) | (1<<CS32))
   #define CONFIG_MATCH_NORMAL()		(TCCR3A = TCCR3A & ~((1<<COM3A1) | (1<<COM3A0)))
   #define CONFIG_MATCH_TOGGLE()		(TCCR3A = (TCCR3A & ~(1<<COM3A1)) | (1<<COM3A0))
   #define CONFIG_MATCH_CLEAR()		(TCCR3A = (TCCR3A | (1<<COM3A1)) & ~(1<<COM3A0))
@@ -76,6 +78,7 @@
 #elif defined(ALTSS_USE_TIMER4)
   #define CONFIG_TIMER_NOPRESCALE()	(TIMSK4 = 0, TCCR4A = 0, TCCR4B = (1<<ICNC4) | (1<<CS40))
   #define CONFIG_TIMER_PRESCALE_8()	(TIMSK4 = 0, TCCR4A = 0, TCCR4B = (1<<ICNC4) | (1<<CS41))
+  #define CONFIG_TIMER_PRESCALE_256()	(TIMSK4 = 0, TCCR4A = 0, TCCR4B = (1<<ICNC4) | (1<<CS42))
   #define CONFIG_MATCH_NORMAL()		(TCCR4A = TCCR4A & ~((1<<COM4A1) | (1<<COM4A0)))
   #define CONFIG_MATCH_TOGGLE()		(TCCR4A = (TCCR4A & ~(1<<COM4A1)) | (1<<COM4A0))
   #define CONFIG_MATCH_CLEAR()		(TCCR4A = (TCCR4A | (1<<COM4A1)) & ~(1<<COM4A0))
@@ -102,6 +105,7 @@
 #elif defined(ALTSS_USE_TIMER5)
   #define CONFIG_TIMER_NOPRESCALE()	(TIMSK5 = 0, TCCR5A = 0, TCCR5B = (1<<ICNC5) | (1<<CS50))
   #define CONFIG_TIMER_PRESCALE_8()	(TIMSK5 = 0, TCCR5A = 0, TCCR5B = (1<<ICNC5) | (1<<CS51))
+  #define CONFIG_TIMER_PRESCALE_256()	(TIMSK5 = 0, TCCR5A = 0, TCCR5B = (1<<ICNC5) | (1<<CS52))
   #define CONFIG_MATCH_NORMAL()		(TCCR5A = TCCR5A & ~((1<<COM5A1) | (1<<COM5A0)))
   #define CONFIG_MATCH_TOGGLE()		(TCCR5A = (TCCR5A & ~(1<<COM5A1)) | (1<<COM5A0))
   #define CONFIG_MATCH_CLEAR()		(TCCR5A = (TCCR5A | (1<<COM5A1)) & ~(1<<COM5A0))
@@ -141,6 +145,12 @@
 					NVIC_SET_PRIORITY(IRQ_FTM0, 48); \
 					FTM0_C0SC = 0x18; \
 					NVIC_ENABLE_IRQ(IRQ_FTM0);
+  #define CONFIG_TIMER_PRESCALE_128()	FTM0_SC = 0; FTM0_CNT = 0; FTM0_MOD = 0xFFFF; \
+					FTM0_SC = FTM_SC_CLKS(1) | FTM_SC_PS(7); \
+					digitalWriteFast(21, HIGH); \
+					NVIC_SET_PRIORITY(IRQ_FTM0, 48); \
+					FTM0_C0SC = 0x18; \
+					NVIC_ENABLE_IRQ(IRQ_FTM0);
   #define CONFIG_MATCH_NORMAL()		(FTM0_C6SC = 0)
   #define CONFIG_MATCH_TOGGLE()		(FTM0_C6SC = (FTM0_C6SC & 0xC3) | 0x14)
   #define CONFIG_MATCH_CLEAR()		(FTM0_C6SC = (FTM0_C6SC & 0xC3) | 0x18)

examples/ReceiveTest/ReceiveTest.ino

@@ -8,7 +8,7 @@
 
 AltSoftSerial altser;
 const int mybaud = 9600;
-            
+
 // Board            Serial1 TX   AltSoftSerial RX
 // -----            ----------   ----------------
 // Teensy 3.x            1              20
@@ -17,6 +17,9 @@ const int mybaud = 9600;
 // Arduino Leonardo      1              13
 // Arduino Mega         18              48
 
+// Serial1 on AVR @ 16 MHz minimum baud is 245
+// Serial1 on Teensy 3.2 @ 96 MHz minimum baud is 733
+
 byte sentbyte;
 unsigned long prevmillis;
 byte testbyte=0xF0;