Hello,
my son made a programming mistake by confusing digitalWrite with pinMode: he wrote things like
pinMode(pin, LOW); and pinMode(pin, HIGH); which I thought should be meaningless, however, it does compile, and when applied to the hardware it turns out to look like this:
pinMode(pin, LOW); might to do the same as digitalWrite(pin, HIGH); (not proven in the application, however outcome is the same)
pinMode(pin, HIGH);seems to do the same as digitalWrite(pin, LOW); (brings the pin to ground)
isn't that weird?
any idea of the reason(s) why it is doing that?
Cheers,
Pascal.
pinMode(pin, INPUT_PULLUP); turns on the pullup resistors, which often confuses people because the pin outputs about 5V when measured by a multimeter (and it will light up an LED weakly).
It may be that misuse of LOW in that function call also turns on the pullup resistor.
Edit: Here is the source code for pinMode(). If the second parameter's value is not INPUT (0) or INPUT_PULLUP (2), the pin becomes an output.
void pinMode(uint8_t pin, uint8_t mode)
{
uint8_t bit = digitalPinToBitMask(pin);
uint8_t port = digitalPinToPort(pin);
volatile uint8_t *reg, *out;
if (port == NOT_A_PIN) return;
// JWS: can I let the optimizer do this?
reg = portModeRegister(port);
out = portOutputRegister(port);
if (mode == INPUT) {
uint8_t oldSREG = SREG;
cli();
*reg &= ~bit;
*out &= ~bit;
SREG = oldSREG;
} else if (mode == INPUT_PULLUP) {
uint8_t oldSREG = SREG;
cli();
*reg &= ~bit;
*out |= bit;
SREG = oldSREG;
} else {
uint8_t oldSREG = SREG;
cli();
*reg |= bit;
SREG = oldSREG;
}
}
INPUT, OUTPUT, HIGH and LOW are just different names for 0 and 1. pinMode() doesn't know which one was used.
Arduino.h
#define HIGH 0x1
#define LOW 0x0
#define INPUT 0x0
#define OUTPUT 0x1
#define INPUT_PULLUP 0x2
pinMode(pin, LOW) is the same as pinMode(pin, INPUT)
pinMode(pin, HIGH) is the same as pinMode(pin, OUTPUT)
pinMode and digitalWrite actually take integers as the second argument. In the core LOW is defined as 0 and HIGH is defined as 1. INPUT is also defined as 0 and OUTPUT is defined as 1 and I think INPUT_PULLUP is defined as 2. So when you use LOW in pinMode it sees the 0 and it's the same as using INPUT.