*** SOLVED - ProMicro (a sparkfun clone) does not like LED_BUILTIN ****
Look at the code below which is just an amendment to the standard Blink example:
When using LED_BUILTIN which I understand to be portable between all Arduino I have the problem. use a different constant name and all OK
// original code from http://www.arduino.cc/en/Tutorial/Blink as follows:
//int LED_BUILTIN = 17;
int RXLED = 17;
void setup() {
// initialize digital pin LED_BUILTIN as an output.
// pinMode(LED_BUILTIN, OUTPUT);
pinMode(RXLED, OUTPUT);
}
void loop() {
// digitalWrite(LED_BUILTIN, HIGH); // turn the LED on (HIGH is the voltage level)
digitalWrite(RXLED, HIGH); // turn the LED on (HIGH is the voltage level)
delay(1000); // wait for a second
// digitalWrite(LED_BUILTIN, LOW); // turn the LED off by making the voltage LOW
digitalWrite(RXLED, LOW); // turn the LED on (HIGH is the voltage level)
delay(1000); // wait for a second
}