sigcapture: add decimation for capturing slow phenomena

sigcapture_set_decimation(sc, n) stores 1 of every n samples and reports
base_frequency/n in the preamble, trading time resolution for a longer
capture window (e.g. a multi-second quasi-static current sweep). The ADC
hot path skips n-1 of every n sigcapture_wrptr() calls; trigger countdown
and the readout are unchanged.

Author: andoma

include/mios/sigcapture.h

@@ -36,6 +36,10 @@ sigcapture_t *sigcapture_create_vllp(size_t depth_power_of_2, size_t channels,
                                      const sigcapture_desc_t channel_descriptors[],
                                      uint32_t nominal_frequency);
 
+// Store only 1 of every `n` samples (n>=1, 1 = full rate), to capture slow
+// phenomena over a longer time window at reduced time resolution.
+void sigcapture_set_decimation(sigcapture_t *sc, unsigned int n);
+
 int16_t *sigcapture_wrptr(sigcapture_t *sc);
 
 void sigcapture_trig(sigcapture_t *sc, size_t leading_samples);

src/lib/diag/sigcapture.c

@@ -21,6 +21,12 @@ sigcapture_wrptr(sigcapture_t *sc)
   if(sc->state != SIGCAPTURE_STATE_RUN)
     return NULL;
 
+  if(sc->decimation > 1) {
+    if(++sc->decim_phase < sc->decimation)
+      return NULL;  // skip this sample
+    sc->decim_phase = 0;
+  }
+
   if(sc->trig_countdown) {
     sc->trig_countdown--;
     if(sc->trig_countdown == 0) {
@@ -62,6 +68,17 @@ sigcapture_reset(sigcapture_t *sc)
 }
 
 
+void
+sigcapture_set_decimation(sigcapture_t *sc, unsigned int n)
+{
+  if(n < 1)
+    n = 1;
+  sc->decimation = n;
+  sc->decim_phase = 0;
+  sc->pkt_preamble.nominal_frequency = sc->base_frequency / n;
+}
+
+
 // Transport-neutral readout state machine. Emits, in order: preamble (0xff),
 // one descriptor per channel, the captured data columns, then the trailer
 // (0xfe). After the trailer it hands the buffer back to the sampler and reports
@@ -172,6 +189,8 @@ sigcapture_alloc(size_t depth_power_of_2, size_t channels,
   sc->channels = channels;
   sc->storage = storage;
   sc->columns_per_xfer = 32 / channels;
+  sc->decimation = 1;
+  sc->base_frequency = nominal_frequency;
 
   sc->pkt_preamble.pkt_type = 0xff;
   sc->pkt_preamble.channels = channels;

src/lib/diag/sigcapture_internal.h

@@ -45,6 +45,13 @@ struct sigcapture {
   size_t channels;
   uint8_t columns_per_xfer;
 
+  // Decimation: store 1 of every `decimation` samples (>=1), to capture slow
+  // phenomena over a longer window. base_frequency is the full-rate frequency;
+  // the preamble reports base_frequency / decimation.
+  uint16_t decimation;
+  uint16_t decim_phase;
+  uint32_t base_frequency;
+
   // Readout cursor + frame staging (only touched during READOUT).
   uint32_t send_index;
   uint32_t xfer_rows;