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Prevent low pulse on TX initialization in SoftwareSerial 

Previously, the TX pin would be set to output first and then written
high (assuming non-inverted logic). When the pin was previously
configured for input without pullup (which is normal reset state), this
results in driving the pin low for a short when initializing. This could
accidenttally be seen as a stop bit by the receiving side.

By first writing HIGH and then setting the mode to OUTPUT, the pin will
have its pullup enabled for a short while, which is harmless.
This commit is contained in:
Matthijs Kooijman 2015-01-26 17:04:26 +01:00
parent 9cf3740a03
commit 90ca3934f2
1 changed files with 5 additions and 1 deletions
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6
hardware/arduino/avr/libraries/SoftwareSerial/SoftwareSerial.cpp
View File

@ -266,8 +266,12 @@ SoftwareSerial::~SoftwareSerial()
void SoftwareSerial::setTX(uint8_t tx)
{
pinMode(tx, OUTPUT);
// First write, then set output. If we do this the other way around,
// the pin would be output low for a short while before switching to
// output hihg. Now, it is input with pullup for a short while, which
// is fine. With inverse logic, either order is fine.
digitalWrite(tx, _inverse_logic ? LOW : HIGH);
pinMode(tx, OUTPUT);
_transmitBitMask = digitalPinToBitMask(tx);
uint8_t port = digitalPinToPort(tx);
_transmitPortRegister = portOutputRegister(port);