diff --git a/docs/library/index.rst b/docs/library/index.rst
index 2b9af5b930..2b1d6b9651 100644
--- a/docs/library/index.rst
+++ b/docs/library/index.rst
@@ -120,6 +120,7 @@ The following libraries are specific to the pyboard.
   :maxdepth: 2
 
   pyb.rst
+  stm.rst
   lcd160cr.rst
 
 
diff --git a/docs/library/stm.rst b/docs/library/stm.rst
new file mode 100644
index 0000000000..a181d6044c
--- /dev/null
+++ b/docs/library/stm.rst
@@ -0,0 +1,104 @@
+.. currentmodule:: stm
+
+:mod:`stm` --- functionality specific to STM32 MCUs
+===================================================
+
+.. module:: stm
+    :synopsis: functionality specific to STM32 MCUs
+
+This module provides functionality specific to STM32 microcontrollers, including
+direct access to peripheral registers.
+
+Memory access
+-------------
+
+The module exposes three objects used for raw memory access.
+
+.. data:: mem8
+
+    Read/write 8 bits of memory.
+
+.. data:: mem16
+
+    Read/write 16 bits of memory.
+
+.. data:: mem32
+
+    Read/write 32 bits of memory.
+
+Use subscript notation ``[...]`` to index these objects with the address of
+interest.
+
+These memory objects can be used in combination with the peripheral register
+constants to read and write registers of the MCU hardware peripherals, as well
+as all other areas of address space.
+
+
+Peripheral register constants
+-----------------------------
+
+The module defines constants for registers which are generated from CMSIS header
+files, and the constants available depend on the microcontroller series that is
+being compiled for.  Examples of some constants include:
+
+.. data:: GPIOA
+
+    Base address of the GPIOA peripheral.
+
+.. data:: GPIOB
+
+    Base address of the GPIOB peripheral.
+
+.. data:: GPIO_BSRR
+
+    Offset of the GPIO bit set/reset register.
+
+.. data:: GPIO_IDR
+
+    Offset of the GPIO input data register.
+
+.. data:: GPIO_ODR
+
+    Offset of the GPIO output data register.
+
+Constants that are named after a peripheral, like ``GPIOA``, are the absolute
+address of that peripheral.  Constants that have a prefix which is the name of a
+peripheral, like ``GPIO_BSRR``, are relative offsets of the register.  Accessing
+peripheral registers requires adding the absolute base address of the peripheral
+and the relative register offset.  For example ``GPIOA + GPIO_BSRR`` is the
+full, absolute address of the ``GPIOA->BSRR`` register.
+
+Example use:
+
+.. code-block:: python3
+
+    # set PA2 high
+    stm.mem32[stm.GPIOA + stm.GPIO_BSRR] = 1 << 2
+
+    # read PA3
+    value = (stm.mem32[stm.GPIOA + stm.GPIO_IDR] >> 3) & 1
+
+
+Functions specific to STM32WBxx MCUs
+------------------------------------
+
+These functions are available on STM32WBxx microcontrollers, and interact with
+the second CPU, the RF core.
+
+.. function:: rfcore_status()
+
+    Returns the status of the second CPU as an integer (the first word of device
+    info table).
+
+.. function:: rfcore_fw_version(id)
+
+    Get the version of the firmware running on the second CPU.  Pass in 0 for
+    *id* to get the FUS version, and 1 to get the WS version.
+
+    Returns a 5-tuple with the full version number.
+
+.. function:: rfcore_sys_hci(ogf, ocf, data, timeout_ms=0)
+
+    Execute a HCI command on the SYS channel.  The execution is synchronous.
+
+    Returns a bytes object with the result of the SYS command.