Watchdog reset timeout value

[Sourdev]

I am using S9KEAZN64AMLC MCU (custom board). I configured Watchdog in window mode using an internal oscillator (32KHz). 

Configured watchdog timeout value is 500 msec.

when 256 clock Prescaler is enabled then the watchdog timer will reset at 534 msec and when 256 clock Prescaler is disabled then the watchdog timer will reset at 518 msec.

why it is taking more time to reset? 

when I use an External oscillator with 16Mhz, it resets at 508msec.  With an external oscillator, I am getting correct results but the internal oscillator and LPO creating problems (Not resetting at the exact timeout).

I checked with different timeout values and both oscillators, Please see the below table.

Internal Oscillator (32Khz)		External Oscillator (16Mhz)
Msec	Actual reset time (In msec)	Difference (In Msec)		Timeout value (in msec)	Actual reset time (In msec)	Difference (In Msec)
100	108	8		100	106	6
200	211	11		200	206	6
300	313	13		300	306	6
400	415	15		400	406	6
500	517	17		500	506	6
600	619	19		600	605	5
700	722	22		700	706	6
800	824	24		800	806	6
900	927	27		900	906	6
1000	1.028	0.028		1000	1.006	0.006
2000	2.05	0.05

How I can get the exact watchdog reset time with the internal oscillator?

NOTE: Observed the Same result with normal watchdog and Window watchdog. 

[Robin_Shen]

Hi 

The frequency of internal reference clock is factory trimmed to 31.25 kHz. Did you check the value of Watchdog Timeout Value Register? 

Best Regards,
Robin
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[Sourdev]

Hi Robin Shen,

You are correct internal reference clock is factory trimmed to 31.25 kHz. Now I am getting the correct results.

Thank you for the support!!!!!!!!!