I'm trying to develop a program using C and a CCS compiler that will
calculate the duty cycle of an ADXL202E accelerometer on an 16F877 PIN_B7
using #int_RB. Here's the code of the interrupt service routine.

#int_RB

void accelerometer_isr(void)
{
reading = get_timer1();                     //Read Timer_1
current_portb = input_b();                  //Read value of PORT_B
delta_portb = current_portb ^ last_portb;   //Did PORT_B change?
last_portb = current_portb;                 //Remember PORT_B value
if (delta_portb = 0x80 || 0xc0)             //If PIN_B7 has changed
{
if (bit_test(current_portb, 7))             //If PIN_B7 is high
 {
  period = reading - old_reading;           //Calculate period
  old_reading = reading;
 }
 else
  duty = reading - old_reading;             //Calculate duty
 }
else;
}

When I run the program I consistently get values of around 5200 for the
period, while values for duty vary between 3 values; 2380, 2675 and 4950.

I've checked the output of PIN_B7 with an old analog scope and the value of
2380 seems to be the right one.

Any ideas on what's happening here?

Thanks, Harry 
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