docs/esp32/tutorial: Add example for pin access via registers.

Synchronous access to pins directly via registers.

Signed-off-by: Ihor Nehrutsa <Ihor.Nehrutsa@gmail.com>

docs/esp32/tutorial/peripheral_access.rst

@@ -42,3 +42,83 @@ for this:
 
 The MCPWM0 peripheral is in bit position 17 of the above two registers, hence
 the value of ``DPORT_PWM0_CLK_EN``.
+
+Synchronous access to pins directly via registers
+-------------------------------------------------
+
+The following code shows how to access pins directly via registers.  It has been
+tested on a generic ESP32 board.  It configures pins 16, 17, 32 and 33 in output
+mode via registers, and switches pin output values via registers.  Pins 16 and
+17 are switched simultaneously.
+
+.. code-block:: python3
+
+    from micropython import const
+    from machine import mem32, Pin
+
+    GPIO_OUT_REG = const(0x3FF44004)  # GPIO 0-31 output register
+    GPIO_OUT1_REG = const(0x3FF44010)  # GPIO 32-39 output register
+
+    GPIO_ENABLE_REG = const(0x3FF44020)  # GPIO 0-31 output enable register
+    GPIO_ENABLE1_REG = const(0x3FF4402C)  # GPIO 32-39 output enable register
+
+    M16 = 1 << 16  # Pin(16) bit mask
+    M17 = 1 << 17  # Pin(17) bit mask
+
+    M32 = 1 << (32-32)  # Pin(32) bit mask
+    M33 = 1 << (33-32)  # Pin(33) bit mask
+
+    # Enable pin output mode like
+    # p16 = Pin(16, mode=Pin.OUT)
+    # p17 = Pin(17, mode=Pin.OUT)
+    # p32 = Pin(32, mode=Pin.OUT)
+    # p33 = Pin(33, mode=Pin.OUT)
+    mem32[GPIO_ENABLE_REG] = mem32[GPIO_ENABLE_REG] | M16 | M17
+    mem32[GPIO_ENABLE1_REG] = mem32[GPIO_ENABLE1_REG] | M32 | M33
+
+    print(hex(mem32[GPIO_OUT_REG]), hex(mem32[GPIO_OUT1_REG]))
+
+    # Set outputs to 1 like
+    # p16(1)
+    # p17(1)
+    # p32(1)
+    # p33(1)
+    mem32[GPIO_OUT_REG] = mem32[GPIO_OUT_REG] | M16 | M17
+    mem32[GPIO_OUT1_REG] = mem32[GPIO_OUT1_REG] | M32 | M33
+
+    print(hex(mem32[GPIO_OUT_REG]), hex(mem32[GPIO_OUT1_REG]))
+
+    # Set outputs to 0 like
+    # p16(0)
+    # p17(0)
+    # p32(0)
+    # p33(0)
+    mem32[GPIO_OUT_REG] = mem32[GPIO_OUT_REG] & ~(M16 | M17)
+    mem32[GPIO_OUT1_REG] = mem32[GPIO_OUT1_REG] & ~(M32 | M33)
+
+    print(hex(mem32[GPIO_OUT_REG]), hex(mem32[GPIO_OUT1_REG]))
+
+    while True:
+        # Set outputs to 1
+        mem32[GPIO_OUT_REG] = mem32[GPIO_OUT_REG] | M16 | M17
+        mem32[GPIO_OUT1_REG] = mem32[GPIO_OUT1_REG] | M32 | M33
+
+        # Set outputs to 0
+        mem32[GPIO_OUT_REG] = mem32[GPIO_OUT_REG] & ~(M16 | M17)
+        mem32[GPIO_OUT1_REG] = mem32[GPIO_OUT1_REG] & ~(M32 | M33)
+
+
+Output is::
+
+    0x0 0x0
+    0x30000 0x3
+    0x0 0x0
+
+Pins 16 and 17 are switched synchronously:
+
+.. image:: img/mem32_gpio_output.jpg
+
+Same image on pins 32 and 33.
+
+Note that pins 34-36 and 39 are inputs only. Also pins 1 and 3 are Tx, Rx of the REPL UART,
+pins 6-11 are connected to the built-in SPI flash.