fixes for STM32 (#1011)

resolves #1010
* compute `F_CPU` at runtime for STM32 in `startWrite()`
* fix scanner examples `min()`/`max()` calls

RF24.cpp

@@ -1368,6 +1368,11 @@ bool RF24::startWrite(const void* buf, uint8_t len, const bool multicast)
     ce(HIGH);
 #if !defined(F_CPU) || F_CPU > 20000000
     delayMicroseconds(10);
+#endif
+#ifdef ARDUINO_ARCH_STM32
+    if (F_CPU > 20000000) {
+        delayMicroseconds(10);
+    }
 #endif
     ce(LOW);
     return !(status & _BV(TX_FULL));

examples/scanner/scanner.ino

@@ -141,7 +141,7 @@ void loop(void) {
   if (Serial.available()) {
     int8_t c = Serial.parseInt();
     if (c >= 0) {
-      c = min(125, max(0, c));  // clamp channel to supported range
+      c = min((int8_t)125, c);  // clamp channel to supported range
       constCarrierMode = 1;
       radio.stopListening();
       delay(2);
@@ -199,7 +199,7 @@ void loop(void) {
     // Print out channel measurements, clamped to a single hex digit
     for (int i = 0; i < num_channels; ++i) {
       if (values[i])
-        Serial.print(min(0xf, values[i]), HEX);
+        Serial.print(min((uint8_t)0xf, values[i]), HEX);
       else
         Serial.print(F("-"));
     }

examples/scannerGraphic/scannerGraphic.ino

@@ -89,7 +89,7 @@ struct ChannelHistory {
       sum += history[i];
     }
     history[cacheMax - 1] = value;
-    maxPeak = max(sum * 2, maxPeak);  // sum * 2 to allow half-step decay
+    maxPeak = max((uint8_t)(sum * 2), maxPeak);  // sum * 2 to allow half-step decay
     return sum;
   }