WDT WatchDog Questions?

[Guest]

Hi,

I would like to know, How I can calculate the time-out period? my pic is a PIC16F688,I do not understand that the time-out period for each Prescale Rate.

I think:
Bit Value = Prescale Rate -->c
0000 = 1:32 -->equals 1ms
0001 = 1:64 -->equals ???????
0010 = 1:128 -->equals ???????
0011 = 1:256 -->equals ???????
0100 = 1:512 (Reset value) -->equals ???????
0101 = 1:1024 -->equals ???????
0110 = 1:2048 -->equals ???????
0111 = 1:4096 -->equals ???????
1000 = 1:8192 -->equals ???????
1001 = 1:16384 -->equals ???????
1010 = 1:32768 -->equals ???????
1011 = 1:65536 -->equals 268 seconds


Please me a good answer to this question.

Best Regars

[Guest_7068]

You must note that this version of the chip has 2 scalers.
Option_Reg : This is a standard pre-scaler that allows scaling of the WDT from 1:1 to 1:128

WDTCON : This register allows additional pre-scaling of the WDT from 1:32 to 1:65536

The WDT runs off a 31 Khz Internal Oscillator, so the lowest WDT period value you get is 31 Khz /32 ~= 1ms

The default value (RESET value) is
Option_Reg = 1:1
WDTCON = 1:512
This will give you a rate: 31 Khz / 512 = 16ms

Now, understanding the CCS options is a little tricky as they have to keep the old chip and new chip options compatible. so they use the following method:

[Guest]

Hi, thanks for your response,

I configure the WDTCON = 0x10, for the pre-escaler 1/8192 this watchdog time period is big aprox. 8seg or more. I think that the time period for 1/65536 correspond the 268sec, and 1/4096 the 1sec. I would like to know how calculate these values.