docs/esp32/tutorial: Add an example of peripheral control via regs.

docs/esp32/tutorial/index.rst

@@ -20,3 +20,4 @@ to `<https://www.python.org>`__.
 
    intro.rst
    pwm.rst
+   peripheral_access.rst

docs/esp32/tutorial/peripheral_access.rst

+Accessing peripherals directly via registers
+============================================
+
+The ESP32's peripherals can be controlled via direct register reads and writes.
+This requires reading the datasheet to know what registers to use and what
+values to write to them.  The following example shows how to turn on and change
+the prescaler of the MCPWM0 peripheral.
+
+.. code-block:: python3
+
+    from micropython import const
+    from machine import mem32
+
+    # Define the register addresses that will be used.
+    DR_REG_DPORT_BASE = const(0x3FF00000)
+    DPORT_PERIP_CLK_EN_REG = const(DR_REG_DPORT_BASE + 0x0C0)
+    DPORT_PERIP_RST_EN_REG = const(DR_REG_DPORT_BASE + 0x0C4)
+    DPORT_PWM0_CLK_EN = const(1 << 17)
+    MCPWM0 = const(0x3FF5E000)
+    MCPWM1 = const(0x3FF6C000)
+
+    # Enable CLK and disable RST.
+    print(hex(mem32[DPORT_PERIP_CLK_EN_REG] & 0xffffffff))
+    print(hex(mem32[DPORT_PERIP_RST_EN_REG] & 0xffffffff))
+    mem32[DPORT_PERIP_CLK_EN_REG] |= DPORT_PWM0_CLK_EN
+    mem32[DPORT_PERIP_RST_EN_REG] &= ~DPORT_PWM0_CLK_EN
+    print(hex(mem32[DPORT_PERIP_CLK_EN_REG] & 0xffffffff))
+    print(hex(mem32[DPORT_PERIP_RST_EN_REG] & 0xffffffff))
+
+    # Change the MCPWM0 prescaler.
+    print(hex(mem32[MCPWM0])) # read PWM_CLK_CFG_REG (reset value = 0)
+    mem32[MCPWM0] = 0x55      # change PWM_CLK_PRESCALE
+    print(hex(mem32[MCPWM0])) # read PWM_CLK_CFG_REG
+
+Note that before a peripheral can be used its clock must be enabled and it must
+be taken out of reset.  In the above example the following registers are used
+for this:
+
+- ``DPORT_PERI_CLK_EN_REG``: used to enable a peripheral clock
+
+- ``DPORT_PERI_RST_EN_REG``: used to reset (or take out of reset) a peripheral
+
+The MCPWM0 peripheral is in bit position 17 of the above two registers, hence
+the value of ``DPORT_PWM0_CLK_EN``.