when, why and how much delay_ms and delay_us

[freedom]

Dear all.
I have searched in this forum and even ccs compiler manual to find out and understand a basic things on delay_ms and delay_us function.

I just know this functions are used to delay as on given parameter.

But I don't know when it should be used ? If used then how much to delay ?

But I don't know when it MUST be used ? If used then how much to delay ?

I have seen many example program where this function is used. To do same thing some one use delay_ms(5) , some one use delay_ms(2), some one use delay_us(50), some one use delay_us(10) etc etc...

[temtronic]

delay_ms(xxx) is a set of instructions that CCS cut to give a specific amount of delay.As such this is 'inline' code, meaning that once called, the PIC will stay there and do NOTHING else until the time is up.
one great example is...
...
delay_ms(500);
lcd_init();
...

This delay is required to allow the LCD module time to 'organize' itself BEFORE the PIC accesses it. All peripherals need a finite time(specified in the datasheets) before accessing them. The ADC within the PIC is no different.

Delays can be used in 'main' or 'functions' to control overall timing of the program but NEVER,EVER use delays inside an ISR(Interrupt Service Routine) ! ISRs are to be short, fast code. Set bits(flags), simple tests, etc. but NO delays !!

Jay

[ckielstra]

What ????????????????????????
Is this a serious question?
Are you browsing the CCS manual and are you trying to understand each command?

Short answer: if you don't know if you need a delay, then you don't need one.

For all other situations the specifications of your project or hardware will tell you when a delay is required and how long it should be.

[freedom]

[RF_Developer]

[Ttelmah]

RF Developer's penultimate line, really is the critical bit:

"Even so, using delays is wasting processor time, and for anything longer than a few us, there's often better, i.e. more productive, ways of using time. For example, its possible to be processing the result of the last ADC conversion while taking a new one. Longer delays, of ms, are better done using timers to generate a clock tick. "

I almost never use delays. I'll have a few 'delay_cycles' instructions, possibly in interrupts, when doing some very fast hardware operation. Longer delays will always be done using a 'tick' interrupt, because this way the processor can be doing other jobs while waiting.
Structure throughout, is:

1) Do all jobs that _must_ be done. Handle hardware events, serial, etc., using buffers etc..
2) Do any minute delays where needed for exact timings.
3) Do everything else based upon times, using a 'tick' or counter, and while the time is not yet 'up', keep looping doing the other things.

Sitting 'delaying', is a single threaded way of working.
Instead:

[asmboy]

System timers are your friend when you need delays.
here is an example.
Keep in mind - you can test other registers and pins in the PIC for a desired state change to terminate these delays early- which you can not do with the compiler generated delays.

http://www.ccsinfo.com/forum/viewtopic.php?t=53857