A little help on C!

[rougie]

Hi,

I am doing the CCS exercise book and I am doing the example on interupts....

Please consider the following code:

//=====================================
#include <prototype.h>
#define PUSH_BUTTON PIN_A4

int16 overflow_count;

#int_timer1 //declaring a timer interupt function
timer1_isr()
{
overflow_count++;
}

void main()
{
int32 time; //declaring a 32 bit variable

setup_timer_1(T1_INTERNAL | T1_DIV_BY_1); //sets up an intrnl. timer
enable_interrupts(int_timer1); //enables interupts for timers

while(TRUE)
{
enable_interrupts(global); //enables official interupt scope

while(input(PUSH_BUTTON));
set_timer1(0); //resets timer
overflow_count = 0; //resets overflow count

while(!input(PUSH_BUTTON));
disable_interrupts(global); //disables official interrupt scope

time = get_timer1(); //get time value from timer1
time = time + ((int32) overflow_count<<16); //??????
time -= 15; //???????
printf("Time is %lu.%06lu seconds.\r\n",
time/5000000, (time/5)%1000000);
}
}

//============================================

I don't really understand the following lines:

time = time + ((int32) overflow_count<<16); //??????
time -= 15; //???????

Q1:
When I consider the first line:

time = time + ((int32) overflow_count<<16); //??????

I guess that we are shifting the "overflow_count" variable 16 bits to the left and then type casting it to a 32 bit type, However, right after the "disable_interrupts(global)" line, isn't overflow_count equal to 0? so why shift it 16 bits to the left?

Q2:
Why are we removing 15 from time?

All sincere feedback is appreciated!

Thanks

Regards
Roberto

[ckielstra]

[rougie]

I most thankyou for your reply, you are very kind!

Yes I agree with you also, that the 15 ticks x 2microseconds would compensate for the few lines before the prinf statement.

Allthough in all due respect, I was wondering if the program waits in a loop until you press a button? I actually thought that the program measured the time *while* the button was pressed. Hence:

while(!input(PUSH_BUTTON));

And once we let go the button, then we calculate the time! I don't know about you, but I am 4 days old in this CCSMicro PICS stuff, so I could be wrong!

All I know is that it took all afternoon to make sence of this code fragment!

Anyways, again I thankyou for your generous help!

Kind regards
Roberto

[Ttelmah]

Which way round the 'button' is, depends completely on the hardware. Assuming you have 'pull up' resistors, and the button to ground (the commonest configuration, given that the PIC has the possibility of internal 'pull up' resistors), then the button is pushed, when '!PUSH_BUTTON' is true, rather than when 'PUSH_BUTTON' is true. However the button can equally well be wired the other way round....
I must admit, this is the sort of thing where a bit of 'forethought' by the programmer, can make things a lot clearer. This is particularly important, where logic levels 'change' between parts of the circuit. For instance, a lot of my output drives, will be 'fail safe', and set to turn drivers off, and alarms on, if the power fails. Given that these will then probably drive through inverting logic to the actual signal, keeping track of all this is hard, whereas if this is all 'pre-done' in the defines, it makes things much easier to 'keep right'. Sofor example, my defines for a motor control system, moving a platform under processor control, are:

[rougie]

Thankyou Ttelmal,

I will make note of this!

I appreciate the effort in your reply and the fact that you shared some methology of programming, I will keep your advice in mind!

Kind Regards
Roberto