ChatGPT answer on the counter interrupt logic that needs to be implemented

[amichai-bd]

@roman012285
Your goal to have a timer interrupt every 1024 cycles in your RISC-V system can be achieved by setting up the mtime and mtimecmp registers appropriately and handling the interrupt in your machine-level interrupt service routine.
Here's a step-by-step guide on how to do this:

Initialize mtime and mtimecmp:

• mtime is a read-only register that increments automatically. You don't need to initialize it; just read its value when needed.
• Set mtimecmp to the initial value of mtime plus 1024. This means the first timer interrupt will occur 1024 cycles after this initialization.

Enable Timer Interrupts:

• Set the Machine Timer Interrupt Enable (MTIE) bit in the Machine Interrupt Enable register (mie). This enables timer interrupts at the machine level.
• Ensure that global interrupts are enabled in the Machine Status register (mstatus).

Handling the Timer Interrupt:

• When mtime equals or exceeds mtimecmp, a timer interrupt is generated.
• The CPU, upon recognizing this interrupt (if enabled and not masked), will save the current program counter to mepc, set the appropriate cause in mcause, and jump to the interrupt handler specified in mtvec.

In the Interrupt Service Routine (ISR):

• First, determine the cause of the interrupt by checking mcause.
• If it's a timer interrupt, handle it accordingly. This usually involves running your specific timer interrupt handling code.
• After handling the interrupt, update mtimecmp to the current mtime value plus 1024. This sets up the next timer interrupt 1024 cycles from the current mtime.
• Use the MRET instruction at the end of your ISR. This will return control to the location where the interrupt occurred (the value stored in mepc).

Return to Main Execution:

• After the MRET instruction is executed, the CPU will return to where it was interrupted and continue executing the main program.
• This cycle will repeat, with the timer interrupt occurring every 1024 cycles.

It's important to handle the setting of mtimecmp carefully, especially on RV32 systems, to avoid spurious interrupts. The provided code sequence in the specification is a safe way to update mtimecmp without accidentally triggering an interrupt.

By following these steps, your system should correctly handle timer interrupts every 1024 cycles, allowing for periodic execution of a routine in the context of a longer-running main program. Remember that precise timing might be influenced by the time it takes to handle the interrupt and the exact frequency at which mtime increments.

[roman012285]

Thanks