# use delay function

[Guest]

HI
Can somebody please tell me why mplab doesnt want to compile when i use DELAY_MS(20) function but when i use #USE DELAY(CLOCK = 2000) it works. Iwould prefer adressing the pic in seconds rather than cycles.

[Douglas Kennedy]

The compiler needs to know the clock frequency . CCS uses
#use delay(clock=xxxxxxx) to inform the compiler of the application hardwares frequency. As you would expect built in functions delay_us(x) delay_ms(x) also require knowledge of the clock frequency to compute the delays. You have to work with the compiler you have not the compiler you want. The compiler you have needs to see the #use delay before it can generate the timing you want.

[Ttelmah]

Understand first of all, that #use_delay, is a 'defintion', not a 'function'. It tells a wole lot of other parts of the code what values to use, to do things. The value in it, _must_ be the clock rate that the CPU is using, or timing delays won't work, nor will the serial interface (which is dependant on the clock rate), etc etc..
You need to set the delay to 20000000, assuming this is the rate you are using, or a whole lot of other parts will give problems.
Now, if you want a delay in seconds, then just write one.

[Ttelmah]

Simultaneous replies I see. :-)
Should be 'delay_ms', not 'delay_m' in my code.

Best Wishes

[Guest]

Thanks.

So just to make sure i understand corectly: The #use delay is not necesarily a delay before the next instruction, it is more involved with overall clock frequency. Is the delay_ms function one that actualy puts a delay before the next function??

[Douglas Kennedy]

Yes #use delay(clock=xxxxx) is to inform the compiler of your target device frequency thats all. Since frequency is essential to the delay_ms(x) delay_us(x) that may have been the reason for the choice of the syntax #use delay(clock=xxxx) but as others have mentioned any built in function dependent on the clock timing will make use of it also. The example given was a software UART . I include #use delay(clock=xxxx) it in all code since it at a minimum indicates the internal frequency of the device. The external clock can be different when using PLL capabilities of certain devices. If the external Xtal was 10mhz and a 4x increase is achieved by a devices PLL then code #use delay(clock=40000000) PICs are designed to complete almost all instructions in 4 clocks ( goto takes 8 clocks) so if you are running at an internal clock of 40000000 then 10 million instructions will absorb one second of time. For delay-ms and delay-us the compiler generates instructions to have the device execute enough NOP instructions to absorb the time and create the delay.