[avr-chat] Reading Ports?

[Chris Spencer]

I've written a simple test program to blink an LED according to the 
status of two input bits on a port. The bits represent the status of an 
actuator. One bit is high when the actuator is fully contracted, and the 
other is high when the actuator is fully extended. These lines also have 
pull-down resistors. There's also a control bit for making the actuator 
extend and contract, and I've successfully tested that bit, confirming 
that the actuator responds to the chip's signal.

However, my program doesn't cause the LED to blink, leaving me to 
believe it's having a problem reading the two status bits. I've manually 
confirmed that the hardware is setting the correct signals based on 
actuator state, but the AVR acts like the they're always low, even when 
I measure them high. The following is my short test program. It should 
make the actuator extend and contract indefinitely, lighting the LED 
while it's extending.

#include <avr/io.h>

int main(void) {

   // initialize IO ports
   DDRB |= _BV(PB3); // Output, actuator control line "ctrl", make = 1 
to contract
   DDRB &= ~(_BV(PB4)); // Input, actuator contracted signal "cn", 1 = 
contracted
   DDRB &= ~(_BV(PB5)); // Input, actuator extended signal "ex", 1 = 
extended
   DDRD |= _BV(PD1); // Output, LED

   while(1) {
       // Perform extention cycle.
       PORTD &= ~(_BV(PD1)); // LED off
       PORTB |= _BV(PB3); // Set control high to start contracting.
       loop_until_bit_is_set(PORTB, PB4); // Wait until fully contracted.

       // Perform extention cycle.
       PORTD |= _BV(PD1); // LED on
       PORTB &= ~(_BV(PB3)); // Set control low to start extending.
       loop_until_bit_is_set(PORTB, PB5); // Wait until fully extended.
   }

   return 0;
}

Am I using DDRB/PORTB correctly? I've been trying to debug this for 
hours and I've gotten no where, so any help is greatly appreciated.

Thanks,
Chris