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esp32hwencoder: document that the pulse and ctrl pins are passed crossed #103

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esp32hwencoder: document that the pulse and ctrl pins are passed crossed #103

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16 changes: 16 additions & 0 deletions src/encoders/esp32hwencoder/ESP32HWEncoder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,10 @@ ESP32HWEncoder::ESP32HWEncoder(int pinA, int pinB, int32_t ppr, int pinI)

cpr = ppr * 4; // 4x for quadrature

// NOTE: despite their names, these two fields are passed CROSSED into
// pcnt_set_pin() in init(). Pin A ends up on the PULSE input, not the
// control input. See the comment in init() before concluding that A and B
// have been given the wrong roles here.
pcnt_config.ctrl_gpio_num = _pinA;
pcnt_config.pulse_gpio_num = _pinB;
pcnt_config.counter_l_lim = INT16_MIN;
Expand Down Expand Up @@ -90,6 +94,18 @@ void ESP32HWEncoder::init()
if (initialized)
{
// Set up the PCNT peripheral
//
// pcnt_set_pin() takes (unit, channel, pulse_io, ctrl_io), so the two
// config fields are passed CROSSED with respect to their names:
// channel 0 gets pin A on PULSE and pin B on CONTROL, channel 1 the
// reverse. That is the standard quadrature arrangement, and combined
// with the pcnt_set_mode() calls below it gives the standard direction
// convention: A leading B counts up.
//
// Spelled out because the constructor assigns ctrl_gpio_num = _pinA,
// which reads like the roles are swapped. They are not -- the crossing
// here cancels it, and swapping the fields in the constructor would
// invert the counting direction rather than correct it. See issue #101.
pcnt_set_pin(pcnt_config.unit, PCNT_CHANNEL_0, pcnt_config.ctrl_gpio_num, pcnt_config.pulse_gpio_num);
pcnt_set_pin(pcnt_config.unit, PCNT_CHANNEL_1, pcnt_config.pulse_gpio_num, pcnt_config.ctrl_gpio_num);
pcnt_set_mode(pcnt_config.unit, PCNT_CHANNEL_0, PCNT_COUNT_INC, PCNT_COUNT_DEC, PCNT_MODE_REVERSE, PCNT_MODE_KEEP);
Expand Down