Where a pin was last used with a PWM comman[d], a digital read of the pin without first performing a mode change statement will just read the last value written to the pin's data register. The voltage at the pin will remain to be the last value written by the pwm command
So far so good.
And here's the thing:
The PWM timer mode and prescale values for all timers are set up in the Arduino init()
function (in wiring.c
A call to analogWrite()
for a PWM pin sets the duty cycle and makes the PWM signal connection to that pin. (See Footnote.)
The PWM signal itself is generated by the timer circuitry and does not go through the pin's output register.
If the value is zero the analogWrite()
function writes a zero to the PWM pin's bit in that port's output register. (The timer signal is disconnected from the pin at this point.)
If the value is 255, the analogWrite()
function writes a one to the PWM pin's bit in that port's output register. (The timer signal is disconnected from the pin at this point.)
Otherwise, the analogWrite()
function connects the timer signal to the PWM pin and does not write anything to that pin's bit in the output register.
Bottom line: The output register bit values for all I/O pins are initialized to zero. If you have executed an analogWrite()
function for a particular PWM pin and you have not subsequently executed a pinMode()
function for that that pin to make it an input, digitalRead()
will read a value of zero for that pin unless and until you have executed either a digitalWrite()
with a value of one to that pin or an analogWrite()
value of 255.
Post-bottom-line note: The digitalRead()
function disconnects the timer from the pin, so PWM is no longer applied to the output. It definitely does not
automatically set the pin mode to input, and the result is that the pin's bit value in the output register is applied to the pin. Subsequent analogWrite()
function calls can be used to connect the timer signal to the PWM pin again.
function sets a pin to output mode. If the pin is a PWM pin it applies a signal from a timer. If you later perform a digitalWrite()
to that pin, it disconnects the timer signal and makes the pin act as a normal output pin. Of course you can call the pinMode()
function if you want to use the pin as a digital input. (After changing to input mode, executing digitalRead()
for that pin turns off the timer.)
Finally, note that if you execute an analogWrite()
function to a pin that is not one of the PWM pins, the function writes a zero if the byte value is less than 128, and writes a one if the byte value is greater than or equal to 128.