arduino
In Arduino micro/leonardo Pin HWB is not exposed as digital pin
In the Atmega32u4 Pin Mapping PE2 is documented as HWB so the expected behavior would be the user to be able to use it as a digital pin. In the official boards (leonardo & micro) is tied to ground but is totally usable if you are using it with a atmega32u4 board. The pull down on this pin is not required as the default fuses do not have the HWBE bit set, meaning that is completely ignored during bootup. So make sense to Included in the pins_arduino.h a mapping for as arduino digital pin 31 and Alias name HWB.
FYI
"HWB allows to execute the bootloader section after reset when tied to ground during external reset pulse. The HWB mode of this pin is active only when the HWBE fuse is enable. During normal operation (excluded Reset), this pin acts as a general purpose I/O." -- The 32u4 datasheet
It is tied to ground, and when pressing the reset button you enter the bootloader unlike normal power-up so my guess is this fuse is set.
I agree that breaking it out to e.g. one of the unused pins is would be preferred, but I guess they reserve these things for as of yet unplanned future changes.
I made a patch that add support for HWB pin; it works on leonardo and micro.
--- variants/leonardo/pins_arduino.h.old 2023-02-09 11:22:14.996472892 +0100
+++ variants/leonardo/pins_arduino.h 2023-02-09 11:25:36.415048394 +0100
@@ -88,7 +88,7 @@
#undef OCR2_6
#undef OCR2_7
-#define NUM_DIGITAL_PINS 31
+#define NUM_DIGITAL_PINS 32
#define NUM_ANALOG_INPUTS 12
#define TX_RX_LED_INIT DDRD |= (1<<5), DDRB |= (1<<0)
@@ -107,6 +107,8 @@
#define LED_BUILTIN_RX 17
#define LED_BUILTIN_TX 30
+#define HWB 31
+
// Map SPI port to 'new' pins D14..D17
#define PIN_SPI_SS (17)
#define PIN_SPI_MOSI (16)
@@ -270,6 +272,8 @@
PB, // D28 / D10 - A10 - PB6
PD, // D29 / D12 - A11 - PD6
PD, // D30 / TX Led - PD5
+
+ PE, // D31 / HWB - PE2
};
const uint8_t PROGMEM digital_pin_to_bit_mask_PGM[] = {
@@ -308,6 +312,8 @@
_BV(6), // D28 / D10 - A10 - PB6
_BV(6), // D29 / D12 - A11 - PD6
_BV(5), // D30 / TX Led - PD5
+
+ _BV(2), // D31 / HWB - PE2
};
const uint8_t PROGMEM digital_pin_to_timer_PGM[] = {
@@ -346,6 +352,8 @@
NOT_ON_TIMER,
NOT_ON_TIMER,
NOT_ON_TIMER,
+
+ NOT_ON_TIMER,
};
const uint8_t PROGMEM analog_pin_to_channel_PGM[] = {
