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diff --git a/docs/source/gpio.rst b/docs/source/gpio.rst
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--- a/docs/source/gpio.rst
+++ b/docs/source/gpio.rst
@@ -4,4 +4,91 @@
  GPIO
 ======
 
-Stub.
+The Maple features 38 ready-to-use general purpose input/output (GPIO)
+pins for digital input/output, numbered D0 through D37.  These numbers
+correspond to the numeric values next to each header on the Maple
+silkscreen.
+
+Many of these pins may additionally be used for special features or
+peripheral functions.  This page documents those capabilities, by pin.
+
+The current and voltage limitations have not been copied over from the
+STM32 datasheet (see the :ref:`Recommended Reading
+<gpio-recommended-reading>` for a link).  In particular, a number of
+GPIO pins are 5v tolerant (which means that applying 5v to a pin and
+reading it as input or allowing it to drain to ground will not damage
+that pin), while some are not.
+
+.. contents:: Contents
+   :local:
+
+Pin Mapping Mega Table
+----------------------
+
+This huge reference table shows the available functionality on every
+GPIO pin, by peripheral type. The "STM32" column refers to the port
+and number that the header is connected to on the microcontroller.
+The "5v?" column documents whether or not the pin is 5v tolerant (see
+above).
+
+.. csv-table::
+   :header: "Pin", "STM32", ":ref:`ADC <adc>`", ":ref:`Timer <timers>`", ":ref:`I2C <i2c>`", ":ref:`UART <usart>`", ":ref:`SPI <spi>`", "5v?"
+
+   "D0",  "PA3",  "ADC3", "TIM2_CH4",     "-",         "USART2_RX",  "-",         "No"
+   "D1",  "PA2",  "ADC2", "TIM2_CH3",     "-",         "USART2_TX",  "-",         "No"
+   "D2",  "PA0",  "ADC0", "TIM2_CH1_ETR", "-",         "USART2_CTS", "-",         "No"
+   "D3",  "PA1",  "ADC1", "TIM2_CH2",     "-",         "USART2_RTS", "-",         "No"
+   "D4",  "PB5",  "-",    "-",            "ISC1_SMBA", "-",          "-",         "No"
+   "D5",  "PB6",  "-",    "TIM4_CH1",     "I2C1_SCL",  "-",          "-",         "Yes"
+   "D6",  "PA8",  "-",    "TIM1_CH1",     "-",         "USART1_CK",  "-",         "Yes"
+   "D7",  "PA9",  "-",    "TIM1_CH2",     "-",         "USART1_TX",  "-",         "Yes"
+   "D8",  "PA10", "-",    "TIM1_CH3",     "-",         "USART1_RX",  "-",         "Yes"
+   "D9",  "PB7",  "-",    "TIM4_CH2",     "I2C1_SDA",  "-",          "-",         "Yes"
+   "D10", "PA4",  "ADC4", "-",            "-",         "USART2_CK",  "SPI1_NSS",  "No"
+   "D11", "PA7",  "ADC7", "TIM3_CH2",     "-",         "-",          "SPI1_MOSI", "No"
+   "D12", "PA6",  "ADC6", "TIM3_CH1",        -       -       SPI1_MISO       No
+   D13  PA5     ADC5    -       -       -       SPI1_SCK        No
+   D14  PB8     -       TIM4_CH3        -       -       -       Yes
+
+   Analog header
+
+   D15  PC0     ADC10   -       -       -       -       No
+   D16  PC1     ADC11   -       -       -       -       No
+   D17  PC2     ADC12   -       -       -       -       No
+   D18  PC3     ADC13   -       -       -       -       No
+   D19  PC4     ADC14   -       -       -       -       No
+   D20  PC5     ADC15   -       -       -       -       No
+
+   External header
+
+   D21  PC13    -       -       -       -       -       No
+   D22  PC14    -       -       -       -       -       No
+   D23  PC15    -       -       -       -       -       No
+   D24  PB9     -       TIM4_CH4        -       -       -       Yes
+   D25  PD2     -       TIM3_ETR        -       -       -       Yes
+   D26  PC10    -       -       -       -       -       Yes
+   D27  PB0     ADC8    TIM3_CH3        -       -       -       No
+   D28  PB1     ADC9    TIM3_CH4        -       -       -       No
+   D29  PB10    -       -       I2C2_SCL        USART3_TX       -       Yes
+   D30  PB11    -       -       I2C2_SDA        USART3_RX       -       Yes
+   D31  PB12    -       TIM1_BKIN       I2C2_SMBAL      USART3_CK       SPI2_NSS        Yes
+   D32  PB13    -       TIM1_CH1N       -       USART3_CTS      SPI2_SCK        Yes
+   D33  PB14    -       TIM1_CH2N       -       USART3_RTS      SPI2_MISO       Yes
+   D34  PB15    -       TIM1_CH3N       -       -       SPI2_MOSI       Yes
+   D35  PC6     -       -       -       -       -       Yes
+   D36  PC7     -       -       -       -       -       Yes
+   D37  PC8     -       -       -       -       -       
+
+
+Stub (unfinished).
+
+.. _gpio-recommended-reading:
+
+Recommended Reading
+-------------------
+
+STMicro documentation for STM32F103RB microcontroller:
+
+  * `All <http://www.st.com/mcu/devicedocs-STM32F103RB-110.html>`_
+  * `Datasheet (pdf) <http://www.st.com/stonline/products/literature/ds/13587.pdf>`_
+  * `Reference Manual (pdf) <http://www.st.com/stonline/products/literature/rm/13902.pdf>`_