first re-commit.

pigpio-master/EXAMPLES/C/POT_CAP_RECHARGE/README

@@ -0,0 +1,3 @@
+Function to time capacitor charging (through a resistance).
+
+The time can be used to estimate the resistance.

pigpio-master/EXAMPLES/C/POT_CAP_RECHARGE/pot_cap_charge.c

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+#include <stdio.h>
+
+#include <pigpio.h>
+
+/*
+   Measure how long a capacitor takes to charge through a resistance.
+
+   A potentimeter is used to vary the resistance.
+
+   The time taken will be proportional to the resistance.
+
+   3V3 ----- Potentiometer --+-- Capacitor ----- Ground
+                             |
+                             +-- gpio
+
+
+  gcc -o pot_cap_charge pot_cap_charge.c -lpigpio -lpthread -lrt
+  sudo ./pot_cap_charge
+
+*/
+
+#define GPIO 25
+
+#define MAX_READING 1000
+
+static uint32_t rechargeTick = 0;
+
+void callback(int gpio, int level, uint32_t tick)
+{
+   static uint32_t smooth = 0;
+   static int reading = 0;
+
+   uint32_t raw;
+
+   if (level == 1) /* measure recharge time */
+   {
+      ++reading;
+
+      if (rechargeTick)
+      {
+         raw = tick - rechargeTick; /* set in main */
+
+         if (raw < MAX_READING) /* ignore outliers */
+         {
+            /* smooth using 0.8 * smooth + 0.2 * raw */
+
+            smooth = (raw + (4 * smooth)) / 5;
+
+            printf("%d %d %d\n", reading, raw, smooth);
+         }
+         else
+         {
+            /* ignore outlier, set dot at fixed position */
+            printf("%d %d %d\n", reading, 40, smooth);
+         }
+      }
+      else
+      {
+         /* ignore reschedule, set dot at fixed position */
+         printf("%d %d %d\n", reading, 20, smooth);
+      }
+   }
+}
+
+int main (int argc, char *argv[])
+{
+   uint32_t t1, t2;
+   int tDiff;
+
+   if (gpioInitialise()<0) return 1;
+
+   gpioSetAlertFunc(GPIO, callback); /* callback when GPIO changes state */
+    
+   while (1)
+   {
+      gpioWrite(GPIO, PI_OFF); /* drain capacitor */
+
+      gpioDelay(200); /* microseconds */
+
+      t1 = gpioTick();
+
+      gpioSetMode(GPIO, PI_INPUT); /* start capacitor recharge */
+
+      t2 = gpioTick();
+
+      /* dump reading if rechargeTick not accurate to 3 micros */
+
+      if ((t2 - t1) < 3) rechargeTick = t1; else rechargeTick = 0;
+
+      gpioDelay(5000); /* microseconds, nominal 200 readings per second */
+   }
+
+   gpioTerminate();
+}