How can I implement a software reset in my source code?
We know that the 51 MCU allows data to be stored in the program memory, meaning there's no fundamental difference between program code and data — they're just specific bits of information. If we store a program as an array of data in the program memory, can it be executed? The answer is yes.
To perform a software reset, you can use a function pointer that points to address 0x0000, which is the reset vector for the 8051 microcontroller. This approach effectively jumps back to the start of the program. Here's how you can do it:
((void (code *) (void)) 0x0000)();
This line of code creates a function pointer that points to the address 0x0000 and then calls it, simulating a hardware reset. You can wrap this into a function like this:
void reset(void)
{
((void (code *) (void)) 0x0000)();
}
Then, in your main function, you can call reset() to trigger the soft reset.
void main(void)
{
reset();
}
However, there’s a catch. The above method does not clear the 8051 interrupt system or some peripheral registers. If you call this function from within an interrupt service routine, the interrupt may not behave as expected. In such cases, the interrupt might not be re-enabled properly, leading to unpredictable behavior.
To handle resets inside interrupt routines, you can use a small assembly snippet. This function pushes the address 0x0000 onto the stack and then uses RETI (return from interrupt) to pop it, effectively resetting the interrupt context and restarting the program from address 0x0000.
Here's an example of such an assembly function:
asm
mov r0, #0x00
mov r1, #0x00
push acc
push b
push dpl
push dph
push psw
push at2
push at3
mov sp, #0x07
ljmp 0x0000
endasm
This approach ensures that all necessary registers are saved and restored before jumping to the reset vector. However, it works best when using the bank 0 register set. If another register bank is active, the reset may not function correctly. To avoid this, make sure to select bank 0 by adding `mov psw, #0` at the beginning of your code or in the startup file.
In summary, implementing a software reset on the 8051 requires careful handling of the program counter and the interrupt system. While a simple function pointer jump can work in most cases, using an assembly routine provides more reliable results, especially when called from interrupt service routines. Always ensure that the correct register bank is selected to prevent unexpected behavior.
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