Hello,

I'm trying to use the Analog Devices ADXL202E with a PIC16F877 running at
20 MHz. I'm
writing the code in CCS PIC C PCW Compiler version 3.068. I'm trying to use
the PIC to
measure the PWM signal coming out of the accelerometer.  I was trying to
use the PWM
capture port on the PIC but didn't fully understand the data being returned
and I also didn't
want to be limited on using a PIC with a capture port. I would like to use
one of the small
8 pin PICs eventually.

Basically all I want is to capture the acceleration data and send it out
the serial port to a PC
running a terminal program. I've read everything I could find on how to get
this thing to work
but I'm still missing something.

Here's my code -

#include <16F877.h>
#device *=16
#use delay(clock=20000000)
#fuses HS,NOWDT,NOPUT,NOLVP,BROWNOUT,NOCPD,WRT
#use rs232(baud=9600,parity=N,xmit=PIN_C6,rcv=PIN_C7)

//-----< Initialize all RAM >-----
#ZERO_RAM

#byte TRIS_A = 0x85
#byte TRIS_B = 0x86
#byte TRIS_C = 0x87
#byte TRIS_D = 0x88
#byte TRIS_E = 0x89

#define  bit_scale_factor  256

long Xrise_a, Xrise_b, Yrise_a, Yrise_b, T1, T2, calx, caly,calt1;
long time_ta = 0, time_tb, time_tc=0, time_td, time_te, T1X, T1Y, Zcal,
Zactual, T2cal;
long T2actual, T1Xcal, T1Ycal, Acceleration, K;
int x;

void micro_init()
{
    port_b_pullups(TRUE);
    setup_adc_ports(NO_ANALOGS);
    setup_adc(ADC_CLOCK_DIV_2);
    setup_spi(FALSE);
    setup_psp(PSP_DISABLED);
    setup_timer_1(T1_INTERNAL);
    setup_timer_2(T2_DISABLED,0,1);
    setup_ccp1(CCP_OFF);
    setup_ccp2(CCP_OFF);

    return;
}

//********************************************************
//
//               _____________                      _____________
//                |                    |                      |
//  Xout        |                    |                      |
//  ________|                    |_____________|
//              ta                    tb                  te
//
//    T1 = tb - ta    ta should be zero since it is the start of the timer
//    T2 = te - ta     this is the period
//
//
//
//                            ______________                     ____________
//                            |                       |
  |                  |
//  Yout                   |                       |
|                 |
//   ______________|                       |_____________|
|_____
//                                                   tc                    td
//  T2 = td - (td-tc)/2 - (tb/2)
//*************************************************************

void Measure_time_tb()
{
    while (input(PIN_C1)==1)
    {
       ;
    }

    if (input(PIN_C1)==0)      //wait for the pulse to go low
    {
       set_timer1(0);

       while (input(PIN_C1)==0)   //wait again for the pulse to go low
       {
          ;
       }

       enable_interrupts(global);    //start timer

       while (input(PIN_C1)==1)      //wait for the pulse to transition
from high to low
       {
          ;
       }
       disable_interrupts(global);   // stop timer
   }

    time_tb = get_timer1();       //pulse width of Xout is the value from
the timer

    T1X = time_tb;

    return;
}

void Measure_time_td()
{
//Yout
    if (input(PIN_C2)==0)      //wait for the pulse to be low
    {
       set_timer1(0);    //clear and set the timer value to 0

       while (input(PIN_C2)==0)  //wait again for the pulst to be low
       {
          ;
       }

       enable_interrupts(global);    //start timer

       while (input(PIN_C2)==1)    //wait for the pulse to go high
       {
          ;
       }
       disable_interrupts(global);  //stop timer

   }

    time_td = get_timer1();  //pulse width of Yout is the value from the timer

    T1Y = time_td;

    return;
}

void Measure_time_te()
{
//Xout - this will measure the time between Tb and Te (low)
    if (input(PIN_C1)==1)  //wait for pulse to be high
    {
       set_timer1(0);    //clear and set the timer to 0

       while (input(PIN_C1)==1)  //now wait for the pulse to go high again
       {
          ;
       }

       enable_interrupts(global);  //start timer

       while (input(PIN_C2)==0)   //wait for the pulse to go low
       {
          ;              //counting
       }
       disable_interrupts(global);   //stop timer
   }

    time_te = get_timer1();   //pulse width of Xout is the value from the timer

    return;
}

void measure_T2()
{

    T2 = ((time_td - (time_td/2))) - (time_tb/2);   //should be per the
data sheets

    return;
}

void calibrate()
{  //an attempt to calibrate
    measure_time_tb();
    T1Xcal = T1X;

    measure_time_td();
    T1Ycal = T1Y;

    measure_T2();
    T2cal = T2;

    K = 4 * ((T2cal * bit_scale_factor)/T2cal);

    return;
}

void Find_Zactual()
{
    //should be per the data sheets
    measure_T2();
    T2actual = T2;

    Zactual = (Zcal*T2actual)/T2cal;

    return;
}

void main()
{
    char  cmd;

    micro_init();

    printf(__DATE__);  //it might work
    printf("\nReady\n\n");

    //calibrate();
    //Find_Zactual();

    while(TRUE)
    {
       //all calculations should be per the data sheets
       //acceleration = (K*(T1-Zactual))/T2actual;

       if (kbhit())
       {
          cmd = getc();

          If (cmd == 'c')
          {
             printf("\n\n   *******************************\n");
             printf("    Xout                Yout       \n");
             printf("   ********************************\n");
             printf("   Time_Tb: %lu          Time_Td: %lu  \n", time_tb,
time_td);
             printf("   Time_Te: %lu                        \n", time_te);
             printf("  -----------------------------------\n");
             printf("   T2: %lu                            \n", T2);
          }
       }
//      calt1 = T1 - calx;
//      printf("\nacceleration = %lu", acceleration);

       delay_ms(700);   //so I can see the data
    }
}

What I was trying to do was send the raw data back to the PC, then I would
try the
acceleration calculation as well as calibrating it. Right now I only get
zeros or numbers
that make no sense to me. Hopefully someone can point me in the right
direction as
to what I'm doing wrong.

Thanks and Best Regards,

-Lee

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