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--- code:ground_control [2008/07/19 23:33] – external edit 127.0.0.1
+++ code:ground_control [2008/11/13 09:50] (current) – laurenceb
@@ Line 1: / Line 1: @@
- This AVR code constantly checks the PWM input for pulse lenght. If a stream of 10 pulses are detected that match the criterion, the input PWM in routed to the output. A single failing PWM pulse will cause control to revert back to internal guidance. Useful for UAVs, allowing control to be regained from the ground in the event of a faulty control algorthym.
+===== Input capture based version =====
+This uses the input capture function on timer1 (atmega168). The ICNC1 bit in the TCCR1B register need to be set to enable oversampling to give more noise tolerance. A second function running approx every 20ms is needed to decrement counter by two and check if counter>=30. If this is the case there is a valid pwm stream and the diff variable can be used to set the pwm output.
+==== Header ====
+<code c>
+void enable_ground_control();						//enables the input capture interrupt
+void disable_ground_control();						//disables input capture interrupt
+#define PULSE_MIN (s16)(0.001*(float)F_CPU/16.0)			//pwm must be in the range 1 to 2 ms
+#define PULSE_MAX (s16)(0.002*(float)F_CPU/16.0)			//due to the bit shift we divide by 16
+#define GROUND_LED_ON PORTD|=(1<<3)
+#define GROUND_LED_OFF PORTD&=~(1<<3)
+#define GROUND_LED_SET PORTD&(1<<3)
+void run_ground_control();
+</code>
+====Main code===
+<code c>
+#ifdef GROUND
+#include "main.h"
+extern volatile u08 counter;					//globals
+extern volatile u16 diff;
+ISR(TIMER1_CAPT_vect)
+{
+	static u16 start;
+	if(TCCR1B&0x40)						//start of a pulse
+	{
+		TCCR1B&=~0x40;					//set to trigger on falling edge
+		start=ICR1;						//record start of the pulse
+	}
+	else
+	{
+		TCCR1B|=0x40;					//set to trigger on rising edge
+		if(!(counter&0x80))				//the lock bit isn't set, so we are free to run
+		{
+			diff=ICR1;					//set the global volatile s16 diff
+			if(start>diff)				//our pwm pulse spans a timer overflow
+			{
+				diff+=TOP_COUNT;
+			}
+			diff-=start;
+			if(diff>PULSE_MIN && diff<PULSE_MAX)	//test against criterion
+			{
+				if((counter&(~0x80))<30)			//30 consecutive  pulses
+				{
+					counter+=3;			//the 20ms timer1 overflow subtracts 2 from counter
+				}
+			}
+			else
+			{
+				counter&=0x80;			//set the last 7 bits to zero
+			}
+		}
+	}
+}
+void run_ground_control()
+{
+	if((~0x80&counter)>=28)
+	{
+		counter|=0x80;								//lock the diff variable using the counter flag bit
+		OCR1B=diff;
+		counter&=~0x80;
+		GROUND_LED_ON;
+	}
+	else
+	{
+		GROUND_LED_OFF;
+	}
+	if(counter&~0x80)								//if the counter variable >0, decrement by 2
+		counter-=2;
+}
+void enable_ground_control()
+{
+	TIMSK1|=(1<<ICIE1);
+	counter=0;
+}
+void disable_ground_control()
+{
+	TIMSK1&=~(1<<ICIE1);
+	GROUND_LED_OFF;
+	counter=0;
+	diff=0;
+}
+#endif
+</code>
+===== Pin change based version =====
+ This AVR code constantly checks the PWM input for pulse lenght. If a stream of 10 pulses are detected that match the criterion, the input PWM is routed to the output. A single failing PWM pulse will cause control to revert back to internal guidance. Useful for UAVs, allowing control to be regained from the ground in the event of a faulty control algorthym.
 ===== ground.h =====