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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.

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();

#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

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.

#define pwm_record_factor ((float)F_CPU*0.0005/256.0)	//500us
#define lower_pwm_bound (u08)(pwm_record_factor*4.25)	//valid pwm limits
#define upper_pwm_bound (u08)(pwm_record_factor*1.75)
#define GROUND
#define check_input PORTC & 0x01
#define copy_pwm PORTB=(PORTB & ~0x03)|(PORTC & 0x03);	//move PORTC 0,1 to PORTB 0,1
 
void enable_ground_control();
void disable_ground_control();

#include "main.h"
#include "ground.h"
 
volatile u08 pwm_counter=0x00;
volatile u08 pwm_period=0x00;
 
ISR(PCINT1_vect)
{
	if(pwm_counter==0x0A)									//a stream of valid pwm
	{
		cbi(TCCR1A,COM1A1);									//turn off PWM output to servo
		copy_pwm;
	}
	static u08 pulse;
	if(check_input)										//start of pulse
	{
		TCNT0=0x00;										//reset timer at start of C.0 pulse
		pulse=TRUE;
	}
	else if(pulse)												//end of C.0 pulse
	{
		pulse=FALSE;
		if(TCNT0>upper_pwm_bound && TCNT0<lower_pwm_bound)	//valid pwm
		{
			if(pwm_counter<0x0A)
			{
				pwm_counter++;
			}
			else
			{
				pwm_period=TCNT0;	//store the pwm period, the kalman filter will like it!
			}
		}
		else
		{
			sbi(TCCR1A,COM1A1);								//reenable the PWM
			pwm_counter=0x00;
		}
	}
}
 
ISR(TIMER0_OVF_vect)
{
	sbi(TCCR1A,COM1A1);								//reenable the PWM
	pwm_counter=0x00;									//with valid pwm, there will be no overflows
}
 
void enable_ground_control()
{
	sbi(TIMSK0, TOIE0);									//Timer0 interrupts on
	sbi(PCICR, PCIE1);									//Pin change interrupts on
}
 
void disable_ground_control()
{
	cbi(TIMSK0, TOIE0);									//Timer0 interrupts on
	cbi(PCICR, PCIE1);
	pwm_counter=0x00;									//Pin change interrupts on
}