Trying to use the PID controller

[JCLemme]

I want to use the PID controller/MMU in a project and I'm at my wits end trying to understand the damn thing. I realize that the PID system isn't officially supported [?] but if someone smarter than me wants to debug my train of thought I would much appreciate.

To my understanding, the MMU maps processes 0 and 1 identically, so I figured I'd try switching from process 0 to 1. I wrote some assembly that sets up the PID registers (section 27.3.3) and called it from a C function:

Code: Select all

  
    // Step 1 - set NMI_MASK_ENABLE_REG to 1
    movi    a6, 0x3FF1F000
    
    movi    a7, 0x1
    s32i    a7, a6, 0x54
    
    // Step 2 - set PID_DELAY_REG et al to some values
    movi    a7, 0x44
    s32i    a7, a6, 0x20
    
    movi    a7, 0x55
    s32i    a7, a6, 0x24
    
    // Step 3 - Set PID_NEW_REG
    movi    a7, 0x1
    s32i    a7, a6, 0x48
    
    // Step 4 - Set LEVEL_REG - not sure how or why
    //movi    a7, 0xF
    //s32i    a7, a6, 0x28
    
    // Step 5 - Set PID_CONFIRM_REG and NMI_MASK_DISABLE to 1
    movi    a7, 0x1
    s32i    a7, a6, 0x4C
    s32i    a7, a6, 0x58 

The chip seems to do something when I switch processes - if I try switching to process 0 it continues running fine - but when I switch to process 1 it throws up illegal instruction exceptions which eventually devolve into gibberish, and the chip hangs. Occasionally it gets restarted by the watchdog.

Code: Select all

Triggering syscall "pid_switch"...
Guru Meditation Error: Core  0 panic'ed (IllegalInstruction). Exception was unhandled.
Memory dump at 0x4000bfd8: 00000000 00000000 00000000
Guru Meditation Error: Core  0 panic'ed (IllegalInstruction). Exception was unhandled.
Memory dump at 0x4000cff4: 00000000 00000000 00000000
Guru Meditation Error: Core  0 panic'ed (IllegalInstruction). Exception was unhandled.
Memory dump at 0x4000cff4: 00000000 00000000 00000000
...

I don't know where to go from here honestly. I've tried playing with the delay values, the PID I'm switching to (this results in a hang without any exceptions), where I call the process switching code from, setting the levels register, etc. Nothing works. What am I missing?