Somewhat more linear slide

firmware/main.c

@@ -53,44 +53,41 @@ uint32_t sys_time_seconds = 0;
  * longest cycle. Here too the above caveats apply.
  *
  * This value is in TIM1/TIM3 timer counts. */
-#define ADC_PRETRIGGER 150 /* trigger with about 12us margin to TIM1 CC IRQ */
-
-/* Bit timing base value. This is the lowes bit interval used */
-#define PERIOD_BASE 4
+#define ADC_PRETRIGGER 300 /* trigger with about 12us margin to TIM1 CC IRQ */
 
 /* This value is a constant offset added to every bit period to allow for the timer IRQ handler to execute. This is set
  * empirically using a debugger and a logic analyzer. */
-#define TIMER_CYCLES_FOR_SPI_TRANSMISSIONS 120
+#define TIMER_CYCLES_FOR_SPI_TRANSMISSIONS 240
 
 /* This is the same as above, but for the reset cycle of the bit period. */
-#define RESET_PERIOD_LENGTH 40
+#define RESET_PERIOD_LENGTH 80
 
 /* Defines for brevity */
 #define A TIMER_CYCLES_FOR_SPI_TRANSMISSIONS
-#define B PERIOD_BASE
+#define B 40
 
 /* This is a constant offset containing some empirically determined correction values */
-#define C (1 /* reset pulse comp */ - 3 /* analog snafu comp */)
+#define C 0
 
 /* This lookup table maps bit positions to timer period values. This is a lookup table to allow for the compensation for
  * non-linear effects of ringing at lower bit durations.
  */
 static uint16_t timer_period_lookup[NBITS] = {
     /* LSB here */
-    A + (B<< 0) - C,
-    A + (B<< 1) - C,
-    A + (B<< 2) - C,
-    A + (B<< 3) - C,
-    A + (B<< 4) - C,
-    A + (B<< 5) - C,
-    A + (B<< 6) - C,
-    A + (B<< 7) - C,
-    A + (B<< 8) - C,
-    A + (B<< 9) - C,
-    A + (B<<10) - C,
-    A + (B<<11) - C,
-    A + (B<<12) - C,
-    A + (B<<13) - C,
+    A + 1,
+    A + 3,
+    A + 10,
+    A + 29,
+    A + 70,
+    A - C + (B<< 2),
+    A - C + (B<< 3),
+    A - C + (B<< 4),
+    A - C + (B<< 5),
+    A - C + (B<< 6),
+    A - C + (B<< 7),
+    A - C + (B<< 8),
+    A - C + (B<< 9),
+    A - C + (B<<10),
     /* MSB here */
 };
 
@@ -182,7 +179,7 @@ int main(void) {
     /* Configure TIM1 for display strobe generation */
     TIM1->CR1 = TIM_CR1_ARPE;
 
-    TIM1->PSC = 1; /* Prescale by 2, resulting in a 15MHz timer frequency and 66.7ns timer step size. */
+    TIM1->PSC = 0; /* Prescale by 2, resulting in a 15MHz timer frequency and 66.7ns timer step size. */
     /* CH2 - clear/!MR, CH3 - strobe/STCP */
     TIM1->CCMR2 = (6<<TIM_CCMR2_OC3M_Pos) | TIM_CCMR2_OC3PE | (6<<TIM_CCMR2_OC4M_Pos);
     TIM1->CCER |= TIM_CCER_CC3E | TIM_CCER_CC3NE | TIM_CCER_CC3P | TIM_CCER_CC3NP | TIM_CCER_CC4E;
@@ -306,7 +303,7 @@ void TIM1_BRK_UP_TRG_COM_IRQHandler(void) {
         /* Set up the following reset pulse cycle. This cycle is short as it only needs to be long enough for the below
          * part of this ISR handler routine to run. */
         TIM1->ARR = RESET_PERIOD_LENGTH;
-        TIM1->CCR3 = 1; /* This value is fixed to produce a very short reset pulse. IOs, PCB and shift registers all can
+        TIM1->CCR3 = 2; /* This value is fixed to produce a very short reset pulse. IOs, PCB and shift registers all can
                            easily handle this. */
     } else {
         /* Set up everything for the data cycle of the *next* period. The timer is set to count from 0 to ARR. ARR and

firmware/offset_test.py

@@ -92,13 +92,13 @@ if __name__ == '__main__':
     uut.send_framebuf([0]*uut.nchannels)
     time.sleep(args.wait)
     readings = [ bp.adc_value for _ in range(args.oversample) ]
-    mean, stdev = statistics.mean(readings), statistics.stdev(readings)
+    zero_mean, stdev = statistics.mean(readings), statistics.stdev(readings)
     cur.execute('''
         INSERT INTO measurements (
                 run_id, channel, duty_cycle, voltage, voltage_stdev, timestamp
             ) VALUES (?, -1, 0, ?, ?, ?)''',
-            (run_id, mean, stdev, time.time()))
-    print('Zero cal: {:5.4f}V stdev={:5.4f}V'.format(mean, stdev))
+            (run_id, zero_mean, stdev, time.time()))
+    print('Zero cal: {:5.4f}V stdev={:5.4f}V'.format(zero_mean, stdev))
 
     for ch in channels:
         for i in range(nbits):
@@ -123,7 +123,7 @@ if __name__ == '__main__':
                         ) VALUES (?, ?, ?, ?, ?, ?)''',
                         (run_id, ch, duty_cycle, mean, stdev, time.time()))
                 print('{:08d} ch={} {:04x}({:6.5f}): {:5.4f} stdev {:5.4f}'.format(
-                    cur.lastrowid, ch, val, duty_cycle, mean, stdev))
+                    cur.lastrowid, ch, val, duty_cycle, mean-zero_mean, stdev))
 
     uut.send_framebuf([0]*uut.nchannels)
     bp.power_on = False