Question about Peripheral Timing Generator (PTG) Module on dsPIC33CK256MP508

Question

I am a newbie to programming in the Microchip PIC environment, so please excuse my naivete! I recently began experimenting with the dsPIC33CK Curiosity dev board (which contains the dsPIC33CK256MP508 dsp/mcu at its core), and have been exploring the on-board modules, like direct memory access (DMA) and the peripheral trigger generator (PTG).

DMA is working, but I am having trouble with a seemingly-simple PTG task, and I was wondering if anyone has any ideas -- the task involves a simple external interrupt/trigger to the PTG, which then needs to trigger another external pin to go logic-high.

In real life, an incoming logic-high signal will indicate the start of a series of analog data to be read, so the outgoing signal will (repeatedly) trigger an external ADC. I am using an external ADC (an ADS831, which I have working well) since the internal ADC in the dsPIC33 does not sample at the rates I need.

I am unsure if my mistake lies with my PTG commands, or if maybe my Peripheral Pin Select (PPS) module setup is flawed. Unfortunately, it appears that the builtin IO pin unlocking function ("__builtin_write_RPCON()") is undefined, at least with my included libraries. So, fell back on in-line assembler to perform the unlock sequence (detailed in datasheet DS70005349J-page 132).

Also strange: you'll notice I start my PTG sequence on "STEP3", though the PTG documentation (e.g., DS70000669B-page 33) advises starting at "STEP0". The reason I've done this is because "STEP1" and "STEP2" are apparently used by more than one Special Function Register (SFR). I discovered this in a support file included in my compiler directory (specifically, lines 26278 and 26281 of file "p33CK256MP508.h").

Anyway, here is my simple PTG-testbed code:

 /* Testbed to explore the PTG module.
 * 
 * File:   newmain.c
 * Author: benjamin sadler
 *
 * Created on January 26, 2023, 11:43 AM
 */

#include <xc.h>

/*
#define PTGCTRL(x)   ((0x0 << 4) | ((x) & 0x0F))  // PTG command definitions
#define PTGWHI(x)    ((0x4 << 4) | ((x) & 0x0F))  // from DS70000669B-page 15
#define PTGTRIG(x)   ((0x4 << 5) | ((x) & 0x1F))  // (currently commented to 
#define PTGJMPC0(x)  ((0x6 << 5) | ((x) & 0x1F))  // try alternate commands 
*/                                                // (see lines 58-60))


void init_PTG(void);                              // init function primitives
void init_PPS(void);


int main(int argc, char** argv) {                   

    ANSELB = 0x0;                                 // configure PORTB and PORTC
    ANSELC = 0x0;                                 // as digital, and set 
    TRISBbits.TRISB15 = 0;                        // direction bits:
    TRISCbits.TRISC0  = 1;                        // B15->out, C0->in
    
    init_PPS();                                   // initialize PPS
    init_PTG();                                   // initialize PTG
    
    PTGCSTbits.PTGSTRT = 1;                       // start PTG ...
    
    while(1);                                     // .. and wait for INT2->high.
    return (1);
}


void init_PPS( void )
{
    INTCON2bits.GIE = 1;                          // enable global interrupts
    
    asm ("mov #0x55, w0");                        // "__builtin_write_RPCON()"
    asm ("mov w0, _NVMKEY");                      // function doesn't appear 
    asm ("mov #0xAA, w0");                        // to be defined, so in-line  
    asm ("mov w0, _NVMKEY");                      // assembly needed unlock IOLOCK
    RPCONbits.IOLOCK = 0;                         // (that is, set IOLOCK = 0)
    
    _INT2R = 48;                                  // PPS connect INT2 with RP48/PORTC0
      
    asm ("mov #0x55, w0");                        // more in-line assembly to
    asm ("mov w0, _NVMKEY");                      // re-lock IOLOCK ...
    asm ("mov #0xAA, w0");
    asm ("mov w0, _NVMKEY");
    RPCONbits.IOLOCK = 1;                         // re-lock IOLOCK
}


void init_PTG( void )
{
    PTGCON = 0;                                   // most bits on PTGCST and
    PTGCST = 0;                                   // PTGCST registers should be
    PTGCSTbits.PTGEN = 1;                         // zero, except PTGEN=1 which
                                                  // enables module
        
    PTGQPTR = 3;                                  // start at STEP3 (STEP1, STEP2
                                                  // aren't available??)
    PTGC0LIM = 5;                                 // loop 5 times back to STEP3
    
  /*                                              // set STEP commands:
    _STEP3 = PTGWHI(15);                          // wait for INT2 interrupt
    _STEP4 = PTGTRIG(25);                         // then trigger RP47/B14 high
    _STEP5 = PTGJMPC0(3);                         // and jump back to STEP3
   */
    
    _STEP3 = 0b01001111;                          // alternate try at setting 
    _STEP4 = 0b10011001;                          // PTG step commands, using 
    _STEP5 = 0b11000011;                          // data from Table 24-1 in 
}                                                 // datasheet DS70005349J-page 478

And here are the results I'm getting:

Using the simulator (MPLAB X IDE v6.05, with xc16 compiler v2.00) with the correct device (dsPIC33CK256MP508) selected, this code will compile and run, but when I feed in a simulated RC0-> high with the Stimulus tool, I can see that PORTC0 goes high in the variable watch window, but the PTG-triggered output (on RP47/PORTB15) is never observed.

B15 stays low :(

You can also see I've tried programming the PTG steps both using the suggested bit-wise operators (currently commented out), as well as manually loading the bit fields.

I have worked on this for several days without any success, and now I humbly ask for help!