setup_timer2 function of dsPIC30f3013

[aroonkriss]

Hi,

I need to set up a 3.5 character silent space between message frames for modbus protocol. I am using dspic30f3013. 20MHz clock.

baud rate=9600
each character is 16 bit wide

I calculated the 3.5 character time as (16/9600)*3.5 = 5.8mSec

I prescaled the frequency 20 MHz into F=20,000,000/(4*256) i.e divide_by_256.

F=19531.25
1/F =.0512 mSec, timer 2 increment rate.

So in order to get 5.8 msec, I set the count as 5.8/.0512 = 113

I set the post scale as 1.

I have used the following function

[Ttelmah]

Start with three things:
Manual
The header file for the PIC
The data sheet

Key thing is that on these later PIC's, the timers are much more flexible, which is why 90% of the configuration values are no longer specific to one timer.

All the timers support /1,/8,/64, and /256 prescalers.
All _even numbered_ timers support 16 or 32bit operation

From the manual:
"setup_timerX(mode,period)"

"Period is an optional 16 bit integer parameter that specifies the timer period. The default value is 0xFFFF"

Now from data sheet, identify that the clock for the timer prescaler is Fosc/2 (different from the older PIC's where it is Fosc/4).

So for 5.8mSec, you need a count of (20000000*0.0058)/2 = 58000

So:

setup_timer2(TMR_INTERNAL | TMR_DIV_BY_1,57999);

Remember the period value is one less than the count the timer will generate. It counts from 0.

However a lot more comments.

If you are using MODBUS serial, it does not send 16bit data as one character. The 'character' is _11bits_ long, One start bit, 8 data bits, one parity bit, and one stop bit. So your timing is 'wrong'. A sixteen bit transmission is two characters, not one.

Then the pause required won't start till the hardware transmit buffer is empty. If you have sent a packet, you are going to have to test the TBE bit, and then the TRMT bit, to verify both the buffer, and the shift register have emptied, before starting your delay.

Finally, the 3.5byte time delay, is a _minimum_. You can exceed it a little, provided you don't mind loosing a little speed.

Best Wishes