docs/esp32: Add UART to quickref.
@@ -171,6 +171,33 @@ Notes:
* The pull value of some pins can be set to ``Pin.PULL_HOLD`` to reduce power
consumption during deepsleep.
+UART (serial bus)
+-----------------
+
+See :ref:`machine.UART <machine.UART>`. ::
+
+ from machine import UART
+
+ uart1 = UART(1, baudrate=9600, tx=33, rx=32)
+ uart1.write('hello') # write 5 bytes
+ uart1.read(5) # read up to 5 bytes
+
+The ESP32 has three hardware UARTs: UART0, UART1 and UART2.
+They each have default GPIO assigned to them, however depending on your
+ESP32 variant and board, these pins may conflict with embedded flash,
+onboard PSRAM or peripherals.
+
+Any GPIO can be used for hardware UARTs using the GPIO matrix, so to avoid
+conflicts simply provide ``tx`` and ``rx`` pins when constructing. The default
+pins listed below.
+
+===== ===== ===== =====
+\ UART0 UART1 UART2
+===== ===== ===== =====
+tx 1 10 17
+rx 3 9 16
+===== ===== ===== =====
+
PWM (pulse width modulation)
----------------------------
