PIC12F675 wait issue

[sfyris]

In the following code all is going well except of three things.
a. After compile I have a warning: "Inaccurate microsecond delay due to use of variable at the current clock speed"
b. Checking in oscilloscope the outgoing pulse is 3ms intead of 1ms it should be
c. When using a constant value like wait 1000 us the timing is ok
How can I sort this (the use of a word variable is necessary in my application)
Using "pulseout outputpin, variable us" results in same issues
These issues don't happen if using "wait byte_variable 10us" but then we can't use large values (255 is the limit)

;Chip Settings

-#chip 12F675,4

;Defines (Constants)
-#define GLOW_LED GPIO.2

;Variables
Dim flag As integer
Dim pulsew As word
Dim GPIO As byte
Dim gen_counter As byte

;Interrupt Handlers
On Interrupt Timer0Overflow Call mypulseout

Dir GLOW_LED Out
pulsew = 1000
gen_counter = 20
flag = 0
InitTimer0 Osc, PS0_1/4

redo:
Goto redo

Sub mypulseout
gen_counter = gen_counter + 1
if gen_counter > 19 then
Set GLOW_LED On
wait pulsew us
Set GLOW_LED Off
gen_counter = 0
end if
End Sub

[David Stephenson]

When working at a clock speed of 4MHz it should take about 1us to execute one ASM instruction.
Trying to get accurate us delays when you are pulling a value from RAM is difficult, then you have the overheads in the delay loop.
If it does not matter about accuracy no problem, but if it does you need a high clock rate which on a 12F675 means a crystal oscillator.
Even so there will be some overheads in the compiled delay loop and even more so if the delay is a word rather than a byte.

[sfyris]

A ok I understood. So I adapted the value of my variable to have the proper pulse timing. After few experiments I found that a value of 1000 is good enough to have an output of 3ms and a value of 230 to have 1ms. Thanks for help David.