Reading RP2040 BOOTSEL pin from code

0000code.py

import microcontroller, digitalio, time
d = digitalio.DigitalInOut(microcontroller.pin.GPIO33)
while True:
    print(d.value)
    time.sleep(.1)

0000README.md

It turns out you CAN read the pico BOOTSEL pin from software.

I became aware of this possibility after pimoroni tweeted that you could use the BOOTSEL button as a MIDI trigger on their upcoming "Tiny2040". We speculated whether there was a small HW circuit to accomplish this, or if it was SW only. Now I have the answer.

I was also heavily informed by the bootloader source, the body of the reading code is taken directly from there with adjusted timings.

The general idea:

• Pressing the button applies a strong pull-down.
• Normally after start-up, this pin is always being driven by the Pico's QSPI peripheral, so the button has no effect
• We can read it by jumping to code in RAM which does all these steps:
  • disconnect the peripheral from the pin
  • turn off the output driver
  • waits a bit for the BOOTSEL pull-down to pull the voltage down if pressed
  • take some readings
  • reconnect the QSPI peripheral
  • return the result of the reading

The reading is a bit weird. Rather than enabling an internal pull (is there one?) it just disables driving and takes a "majority of 10" vote to decide the pin's input value.

The new pin also needs special handling in CircuitPython; these changes are "make it work"-quality, not pull-request quality.

Does this deserve to actually be enabled in CircuitPython, or is it too nasty a hack to consider?

0001-WIP-allow-reading-bootsel.patch

From 348917565bd17f2941e3626cd944ff6e55d053cc Mon Sep 17 00:00:00 2001
From: Jeff Epler <[email protected]>
Date: Fri, 12 Feb 2021 12:07:47 -0600
Subject: [PATCH] WIP allow reading bootsel

!!! it's probably broekn !!!
---
 .../common-hal/digitalio/DigitalInOut.c       | 52 ++++++++++++++++++-
 .../common-hal/microcontroller/Pin.c          | 16 +++++-
 .../common-hal/microcontroller/__init__.c     |  1 +
 .../common-hal/microcontroller/__init__.h     |  2 +-
 ports/raspberrypi/peripherals/pins.c          |  1 +
 ports/raspberrypi/peripherals/pins.h          |  1 +
 6 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/ports/raspberrypi/common-hal/digitalio/DigitalInOut.c b/ports/raspberrypi/common-hal/digitalio/DigitalInOut.c
index b0bc1b96e..07d49083c 100644
--- a/ports/raspberrypi/common-hal/digitalio/DigitalInOut.c
+++ b/ports/raspberrypi/common-hal/digitalio/DigitalInOut.c
@@ -32,9 +32,12 @@
 
 #include "common-hal/microcontroller/Pin.h"
 #include "shared-bindings/digitalio/DigitalInOut.h"
+#include "shared-bindings/microcontroller/__init__.h"
 #include "supervisor/shared/translate.h"
 
 #include "src/rp2_common/hardware_gpio/include/hardware/gpio.h"
+#include "src/rp2_common/hardware_clocks/include/hardware/clocks.h"
+#include "hardware/regs/io_qspi.h"
 
 digitalinout_result_t common_hal_digitalio_digitalinout_construct(
         digitalio_digitalinout_obj_t* self, const mcu_pin_obj_t* pin) {
@@ -66,6 +69,13 @@ void common_hal_digitalio_digitalinout_deinit(digitalio_digitalinout_obj_t* self
 
 void common_hal_digitalio_digitalinout_switch_to_input(
         digitalio_digitalinout_obj_t* self, digitalio_pull_t pull) {
+    const uint8_t pin = self->pin->number;
+    if(pin == 33) {
+        if (pull != PULL_UP) {
+            mp_raise_RuntimeError(translate("Pin only usable in input mode with pull up"));
+        }
+        return;
+    }
     self->output = false;
     // This also sets direction to input.
     common_hal_digitalio_digitalinout_set_pull(self, pull);
@@ -75,6 +85,9 @@ digitalinout_result_t common_hal_digitalio_digitalinout_switch_to_output(
         digitalio_digitalinout_obj_t* self, bool value,
         digitalio_drive_mode_t drive_mode) {
     const uint8_t pin = self->pin->number;
+    if(pin == 33) {
+        mp_raise_RuntimeError(translate("Pin only usable in input mode"));
+    }
     gpio_set_dir(pin, GPIO_OUT);
     // TODO: Turn on "strong" pin driving (more current available).
 
@@ -92,6 +105,9 @@ digitalio_direction_t common_hal_digitalio_digitalinout_get_direction(
 void common_hal_digitalio_digitalinout_set_value(
         digitalio_digitalinout_obj_t* self, bool value) {
     const uint8_t pin = self->pin->number;
+    if(pin == 33) {
+        mp_raise_RuntimeError(translate("Pin only usable in input mode"));
+    }
     if (self->open_drain) {
         gpio_set_dir(pin, value ? GPIO_IN : GPIO_OUT);
     } else {
@@ -99,9 +115,43 @@ void common_hal_digitalio_digitalinout_set_value(
     }
 }
 
+static inline void inline_delay_loop(uint32_t count) {
+    asm volatile (
+    "1: \n\t"
+    "sub %0, %0, #1 \n\t"
+    "bne 1b"
+    : "+r" (count)
+    );
+}
+
+static bool __attribute__((noinline)) __not_in_flash_func(read_bootsel)(int clock_mhz) {
+    // Check CSn strap: delay for pullups to charge trace, then take a majority vote.
+
+    io_rw_32 *reg = (io_rw_32 *) (IO_QSPI_BASE + IO_QSPI_GPIO_QSPI_SS_CTRL_OFFSET);
+    io_rw_32 oreg = *reg;
+    *reg = (*reg & ~0x1f) | 5; // return pin to sio
+    sio_hw->gpio_hi_oe_clr = 1u << 1;      // Output disable
+    inline_delay_loop(200 * clock_mhz / 3);
+    uint32_t sum = 0;
+    for (int i = 0; i < 9; ++i) {
+        inline_delay_loop(5 * clock_mhz / 3);
+        sum += (sio_hw->gpio_hi_in >> 1) & 1u;
+    }
+    sio_hw->gpio_hi_oe_set = 1u << 1;      // Output enable
+    *reg = oreg;
+    return (sum >= 8);
+}
+
 bool common_hal_digitalio_digitalinout_get_value(
         digitalio_digitalinout_obj_t* self) {
-    return gpio_get(self->pin->number);
+    const uint8_t pin = self->pin->number;
+    if(pin == 33) {
+        common_hal_mcu_disable_interrupts();
+        bool result = read_bootsel((clock_get_hz(clk_sys) + 999999) / 1000000);
+        common_hal_mcu_enable_interrupts();
+        return result;
+    }
+    return gpio_get(pin);
 }
 
 digitalinout_result_t common_hal_digitalio_digitalinout_set_drive_mode(
diff --git a/ports/raspberrypi/common-hal/microcontroller/Pin.c b/ports/raspberrypi/common-hal/microcontroller/Pin.c
index ca5cf5a04..1dbff37a3 100644
--- a/ports/raspberrypi/common-hal/microcontroller/Pin.c
+++ b/ports/raspberrypi/common-hal/microcontroller/Pin.c
@@ -33,6 +33,7 @@
 
 #include "src/rp2_common/hardware_gpio/include/hardware/gpio.h"
 
+bool bootsel_in_use;
 #ifdef MICROPY_HW_NEOPIXEL
 bool neopixel_in_use;
 #endif
@@ -47,8 +48,9 @@ bool speaker_enable_in_use;
 STATIC uint32_t never_reset_pins;
 
 void reset_all_pins(void) {
+    bootsel_in_use = false;
     for (size_t i = 0; i < TOTAL_GPIO_COUNT; i++) {
-        if ((never_reset_pins & (1 << i)) != 0) {
+        if (i < TOTAL_GPIO_COUNT && (never_reset_pins & (1 << i)) != 0) {
             continue;
         }
         reset_pin_number(i);
@@ -64,6 +66,11 @@ void never_reset_pin_number(uint8_t pin_number) {
 }
 
 void reset_pin_number(uint8_t pin_number) {
+    if (pin_number == 33) {
+        bootsel_in_use = false;
+        return;
+    }
+
     if (pin_number >= TOTAL_GPIO_COUNT) {
         return;
     }
@@ -113,6 +120,9 @@ void common_hal_reset_pin(const mcu_pin_obj_t* pin) {
 }
 
 void claim_pin(const mcu_pin_obj_t* pin) {
+    if (pin == &pin_GPIO33) {
+        bootsel_in_use = true;
+    }
     #ifdef MICROPY_HW_NEOPIXEL
     if (pin == MICROPY_HW_NEOPIXEL) {
         neopixel_in_use = true;
@@ -135,6 +145,7 @@ void claim_pin(const mcu_pin_obj_t* pin) {
 }
 
 bool pin_number_is_free(uint8_t pin_number) {
+
     if (pin_number >= TOTAL_GPIO_COUNT) {
         return false;
     }
@@ -145,6 +156,9 @@ bool pin_number_is_free(uint8_t pin_number) {
 }
 
 bool common_hal_mcu_pin_is_free(const mcu_pin_obj_t* pin) {
+    if (pin == &pin_GPIO33) {
+        return !bootsel_in_use;
+    }
     #ifdef MICROPY_HW_NEOPIXEL
     if (pin == MICROPY_HW_NEOPIXEL) {
         return !neopixel_in_use;
diff --git a/ports/raspberrypi/common-hal/microcontroller/__init__.c b/ports/raspberrypi/common-hal/microcontroller/__init__.c
index bfea8b2d9..440d8111e 100644
--- a/ports/raspberrypi/common-hal/microcontroller/__init__.c
+++ b/ports/raspberrypi/common-hal/microcontroller/__init__.c
@@ -176,5 +176,6 @@ const mp_rom_map_elem_t mcu_pin_global_dict_table[TOTAL_GPIO_COUNT] = {
     { MP_ROM_QSTR(MP_QSTR_GPIO27), MP_ROM_PTR(&pin_GPIO27) },
     { MP_ROM_QSTR(MP_QSTR_GPIO28), MP_ROM_PTR(&pin_GPIO28) },
     { MP_ROM_QSTR(MP_QSTR_GPIO29), MP_ROM_PTR(&pin_GPIO29) },
+    { MP_ROM_QSTR(MP_QSTR_GPIO33), MP_ROM_PTR(&pin_GPIO33) },
 };
 MP_DEFINE_CONST_DICT(mcu_pin_globals, mcu_pin_global_dict_table);
diff --git a/ports/raspberrypi/common-hal/microcontroller/__init__.h b/ports/raspberrypi/common-hal/microcontroller/__init__.h
index cc509b6b1..6bcf9bd4f 100644
--- a/ports/raspberrypi/common-hal/microcontroller/__init__.h
+++ b/ports/raspberrypi/common-hal/microcontroller/__init__.h
@@ -29,7 +29,7 @@
 
 #include "src/rp2040/hardware_regs/include/hardware/platform_defs.h"
 
-#define TOTAL_GPIO_COUNT NUM_BANK0_GPIOS
+#define TOTAL_GPIO_COUNT (1+NUM_BANK0_GPIOS)
 
 extern const mp_rom_map_elem_t mcu_pin_global_dict_table[TOTAL_GPIO_COUNT];
 
diff --git a/ports/raspberrypi/peripherals/pins.c b/ports/raspberrypi/peripherals/pins.c
index a2a7b85bd..be23e6013 100644
--- a/ports/raspberrypi/peripherals/pins.c
+++ b/ports/raspberrypi/peripherals/pins.c
@@ -65,3 +65,4 @@ PIN(26);
 PIN(27);
 PIN(28);
 PIN(29);
+PIN(33);
diff --git a/ports/raspberrypi/peripherals/pins.h b/ports/raspberrypi/peripherals/pins.h
index 99ab9cfe4..0db9b79c3 100644
--- a/ports/raspberrypi/peripherals/pins.h
+++ b/ports/raspberrypi/peripherals/pins.h
@@ -67,5 +67,6 @@ extern const mcu_pin_obj_t pin_GPIO26;
 extern const mcu_pin_obj_t pin_GPIO27;
 extern const mcu_pin_obj_t pin_GPIO28;
 extern const mcu_pin_obj_t pin_GPIO29;
+extern const mcu_pin_obj_t pin_GPIO33;
 
 #endif  // MICROPY_INCLUDED_RASPBERRYPI_PERIPHERALS_PINS_H
-- 
2.20.1

Hi Jeff,
I'd like to port my "old" Yubikey simulator from Arduino to CircuitPython on RasPi-Pico.
The HID features of CircuitPython should make this easy. But I would need to solder a button to the RasPi-Pico.
This could be avoided, if the BOOTSEL button of the RasPi-Pico could be used easily in CircuitPython.
So my question is: Has your demo code for MicroPython already been ported to CircuitPython?
Anf if yes, in which version?
Thanks, Mike

No, I did not push for inclusion of this functionality, nor do I plan to. See eg the schematic of the qtpy rp2040 for how to design a board that can use the same switch as a regular gpios (costs one gpio and one diode): https://learn.adafruit.com/adafruit-qt-py-2040/downloads

Thanks, for the quick reply. I'll go the solder route. 😃
Adding a tiny button switch between GND and GP15 (or GND and GP12) is easy enough on the RasPi-Pico.