ADC Sampling Synchronization using Timer and PPI

Question

I have code set up to configure Timer3 to generate a Compare event at 100 KHz, using the PPI so that triggers an ADC Sample event for the two enabled ADC channels. EasyDMA is configured to capture a run of 4K samples each alternating between 2 ADC channels. Some of the time it works perfectly, but AFAICT at random, it appears to swap the two channels in memory. I can't tell if it's caused by the ADC triggering prematurely so it's one sample out of phase (which would effectively swap the channels in memory), or if there's something else going on.

Please don't tell me to call XYZ function. I don't need yet another layer between me and the hardware, both for more learning curve and more places for errors to creep in. The hardware should work exactly as documented, whether set up by my function or yours.

Here's the raw code that sets up the ADC, Timer, and PPI, and starts the whole ball rolling. The ping-pong buffering is a recent addition to attempt to resolve this issue, but hasn't made any detectable difference. The variable PingPong is toggled by the code that calls this function.

#define Channels 2
typedef int16_t AdcValT;
typedef struct
{
AdcValT Ch [Channels];
}
SnapT;

typedef enum
{
ppPing,
ppPong,
PingPongs
}
PingPongT;
PingPongT PingPong;

SnapT Wave [PingPongs] [4096];

// Make sure we don't get any premature triggers
#define AdcPpiChan 7
#define AdcTimer NRF_TIMER3
NRF_PPI -> CHENCLR = 1 << AdcPpiChan;
AdcTimer -> TASKS_STOP = 1;
// Build the parts of WaveInfo that only we know about
WaveInfo.uSecPerPoint = 10;
WaveInfo.uVPerCount [0] = 1000;
WaveInfo.uVPerCount [1] = 1000;
// Set ADC inputs back to reset defaults
NRF_SAADC -> TASKS_STOP = 1;
for (int i = 1000000; i; --i) if (NRF_SAADC -> EVENTS_STOPPED) break;
NRF_SAADC -> ENABLE = 0;
NRF_GPIO -> DIRCLR = 0b10100000000000000000000000010100; // All ADC pins to inputs
for (int i = 0; i < 8; ++i)
{
NRF_SAADC -> CH [i].PSELP = adcNoConnect;
NRF_SAADC -> CH [i].PSELN = adcNoConnect;
}
// Set up 2 channels to capture samples at 100 KSPS
NRF_SAADC -> CH [0].PSELP = adcCh1P;
NRF_SAADC -> CH [0].PSELN = adcCh1M;
NRF_SAADC -> CH [0].CONFIG = (Range1 << 8) | (1 << 20); // nonburst mode, diffierential, 3 uSec, internal 600 mV ref, specified gain, no pullups
NRF_SAADC -> CH [1].PSELP = adcCh2P;
NRF_SAADC -> CH [1].PSELN = adcCh2M;
NRF_SAADC -> CH [1].CONFIG = (Range2 << 8) | (1 << 20); // nonburst mode, diffierential, 3 uSec, internal 600 mV ref, specified gain, no pullups
NRF_SAADC -> OVERSAMPLE = 0;
NRF_SAADC -> RESOLUTION = 2; // 12 bits
NRF_SAADC -> SAMPLERATE = 0; // Cannot use internal timer for multiple channels 0x1000 + 40; // 100 KSPS per channel
NRF_SAADC -> EVENTS_DONE = 0;
NRF_SAADC -> EVENTS_STARTED = 0;
NRF_SAADC -> EVENTS_END = 0;
NRF_SAADC -> EVENTS_STOPPED = 0;
NRF_SAADC -> EVENTS_RESULTDONE = 0;
// Connect timer output to ADC sampler via PPI
//AdcTimer -> EVENTS_COMPARE [0] = 0;
AdcTimer -> TASKS_CLEAR;
NRF_PPI -> CH [AdcPpiChan].EEP = (uint32_t) &(AdcTimer -> EVENTS_COMPARE [0]);
NRF_PPI -> CH [AdcPpiChan].TEP = (uint32_t) &(NRF_SAADC -> TASKS_SAMPLE);
NRF_PPI -> CHENSET = 1 << AdcPpiChan;
// Set up TIMERn to trigger sampling (multiple channel sampling cannot use internal timer)
AdcTimer -> SHORTS = 1; // auto restart on CC[0]
AdcTimer -> MODE = 0; // Timer
AdcTimer -> BITMODE = 3; // 32 bit timer
AdcTimer -> PRESCALER = 0; // no prescale
AdcTimer -> CC [0] = 16000000 / 100000; //FIXME

NRF_SAADC -> RESULT.PTR = (uint32_t) &(Wave [Which]);
NRF_SAADC -> RESULT.MAXCNT = sizeof Wave [Which] / sizeof (uint16_t);
NRF_SAADC -> ENABLE = 1;
NRF_SAADC -> TASKS_START = 1;
for (int i = 1000000; i; --i) if (NRF_SAADC -> EVENTS_STARTED) break;
AdcTimer -> TASKS_CLEAR;
AdcTimer -> TASKS_START = 1;