Quote:
|
Originally Posted by ryan_ryan dear all,
(1)if using a 4mhz quartz, and using no prescalar or post scalar, and if i want to interrupt every 5ms, that wld requird 5ms/1us = 5000 instructions.. 5000 = 0x1388 and pr2 register only 8 bits so unable to hold such a big value...
(2)So with the same configurations but with a prescale of 1:16, this will give me (1/(4*prescaler)) * CLK freq = (1/(4*16)) * 4Mhz = 62500Hz or 16us per machine instruction...To interrupt every 5ms, i will need 5ms/16us = 312.5 , again too big for the pr2 register which is 8 bits..
(3)So my question is how do i use postscaler? if i am using both post and pre scal, will i be able to make the value smaller so that i can write to pr2?what is the general way to calculate the value i need to write to pr2 reg if i am using both prescalar and post scalar. Is there a maths like formula as i wrote in part (2) that explains how to get the value if i am using pre and post scaler??
I hope those who have an idea of what i am saying can sound me out..thanks |
This sets tmr2 to give 1mS interrupts, you can increase it 5mS by increasing the PR2 value to "D'125'-1", I've used it at "D'250'-1" to give 10mS interrupts for a clock display.
Code:
; Set up Timer 2 to generate interrupts every 1 ms. Since we're assuming an instruction
; cycle consumes 1 us, we need to cause an interrupt every 1000 instruction cycles.
; We'll set the prescaler to 4, the PR2 register to 25, and the postscaler to 10. This
; will generate interrupts every 4 x 25 x 10 = 1000 instruction cycles.
; ***********************************************************************************
clrf TMR2 ; Clear Timer2 register
bsf STATUS, RP0 ; Bank1
bsf INTCON,PEIE ; Enable peripheral interrupts
clrf PIE1 ; Mask all peripheral interrupts except
bsf PIE1,TMR2IE ; the timer 2 interrupts.
bcf STATUS, RP0 ; Bank0
clrf PIR1 ; Clear peripheral interrupts Flags
movlw B'01001001' ; Set Postscale = 10, Prescale = 4, Timer 2 = off.
movwf T2CON
bsf STATUS, RP0 ; Bank1
movlw D'25'-1 ; Set the PR2 register for Timer 2 to divide by 25.
movwf PR2
bcf STATUS, RP0 ; Bank0
bsf INTCON,GIE ; Global interrupt enable.
bsf T2CON,TMR2ON ; Timer2 starts to increment 
7th February 2005, 09:02 AM
question on using tmr2 to generate interrupt every 5ms 
Linear Mode
