ATmega64 watchdog issue

Compiled by: Win AVR

Debugging: AVR Studio 4

Problem: The watchdog is set to interrupt for 2 seconds, but the actual interruption is obviously less than 2 seconds

Problem discovery process:

When using the on-chip eeprom, it is found that if more than 4 write statements are not fed to the watchdog, a watchdog reset will occur. However, the watchdog is set to reset in 2 seconds, and 4 write statements will not take 2 seconds anyway.

Using AVR Studio 4 to debug, I checked the value of WDTCR and it was 0x08, which was wrong. According to the initialization setting, it should be 0x0f. I suspected that the watchdog setting was wrong, so I checked the code:

WDTCR = 0x18;

WDTCR = 0x0f;

It seems like there are only two lines of code, written according to the manual, but after trying several times, the value of AWDTCR is always 0x08... ...

Finally, I checked the compiled assembly code in the Disassembler window and found that there were multiple codes after writing "1" to WDCE and WDS in the gray part of the assembly code, which made the writing of WDP exceed the 4 clock cycles required by the manual:

This is what the manual says, and here is security level 1:

I thought it would be reliable to use assembly language to initialize the watchdog, so I embedded the assembly language in C language:

This is the modified assembly code. You can see that the gray-font assembly code is consistent with the manual:

The actual test results are consistent with the expected ones.

Conclusion: When using C language to write code related to the microcontroller instruction cycle, if there is a problem, you can check whether the compiled assembly code meets the timing. Writing this part of the code in embedded assembly is a way to solve the problem!