This page is a general resource for information specific to the Maple.

Technical Specifications

  • MCU: STM32F103RBT6, a 32-bit ARM Cortex M3 microprocessor
  • Clock Speed: 72 MHz
  • 128 KB Flash and 20 KB SRAM
  • 43 digital I/O pins (GPIOs)
  • 15 PWM pins at 16 bit resolution
  • 15 analog input (ADC) pins at 12-bit resolution
  • 2 SPI peripherals
  • 2 I2C peripherals
  • 7 Channels of Direct Memory Access (DMA) (dma.h)
  • 3 USART (serial port) peripherals
  • One advanced and three general-purpose timers
  • Dedicated USB port for programming and communications
  • JTAG
  • Nested Vectored Interrupt Controller (NVIC) (including external interrupt on GPIOs)
  • Supplies up to 500 mA at 3.3 V, with separate 250 mA digital and analog regulators for low-noise analog performance
  • Open source, four layer design
  • Support for low power, sleep, and standby modes (<500 μA)
  • Operating Voltage: 3.3 V
  • Input Voltage (recommended): 4 V — 12 V
  • Dimensions: 2.05″ × 2.1″

Identifying your Rev

We went through three versions (“Revs”) of the Maple hardware: Rev 1, Rev 3, and Rev 5 [1]; Rev 5, the final design, is currently on sale. The following sections will help you to help you identify your Rev.

Rev 5

These boards went on sale in November 2010. They have white buttons and “r5” in small print near the “LeafLabs Maple” text next to the “infinity leaf” logo. The Maple Rev 5 repositioned the double header on the right hand side to better fit 0.1 inch pitch breadboard. This necessitated the removal of pins 21 and 22 from the double header; they are still available, but don’t have any headers installed on them.

Maple Rev 5

Rev 3

This batch of boards went on sale beginning in May 2010. They have a darker red silkscreen and the “infinity leaf” logo. The Maple Rev 3 was the first version which includes the built-in button, labeled BUT.

Maple Rev 3

Rev 1

A small number of Maple Rev 1 boards went on sale in late 2009. They have a light red silkscreen and a single pixelated leaf as a logo.

Maple Rev 1

Powering the Maple

The Maple’s power source is determined by the header to the left of the “LeafLabs” label on the silkscreen. All versions of the Maple can be powered from the barrel jack connector, USB, or a LiPo battery. We ship the Maple with a jumper on the USB selector. In order to power it off of an alternative source, unplug the Maple, then move the jumper to the desired selector before reconnecting power.

You can also power the Maple via the pin labeled “Vin” on the lower header. This pin feeds into both the digital and analog voltage regulators. However, don’t do this while simultaneously powering the board from another source, or you could damage it.

When powering the board from a barrel jack, double check the polarity of the barrel. The appropriate polarity is noted on the silkscreen right next to the connector.


Silkscreens on Maples up through Rev 5s manufactured in Spring 2011 falsely indicated that the barrel jack could be supplied by up to 18V. (Rev5s manufactured after Spring 2011 may still have this error on the silk, but it has been marked over.) We recommend a barrel jack input voltage no greater than 12V, and potentially even lower depending upon the current draw requirements of the application. The same goes for powering off LiPo batteries.

Please see Power Regulation on the Maple for more information.

Power Regulation on the Maple

Power regulation on the Maple is provided by two low dropout linear voltage regulators. (The part is the MCP1703 from Microchip, in the SOT-23A package. You can download the datasheet here ). One of the regulators supplies power to the digital voltage plane; the other supplies power to the analog voltage plane.

These voltage regulators nominally take an input of up to 16V. In addition, while the maximum continuous output current for the board is 250mA, if you are powering the board off higher voltages the amount off current it can supply goes down, due to the regulators needing to dissipate the extra power. So if you are powering the board off 12V, the max current is about 40mA at room temperature. In general (again, at room temperature) the max power dissipation (PD) for the chip is about .37W, and output current = PD/(Vin-Vout). For exact max current calculations, please refer to the datasheet linked above.

If you are planning to draw a lot of current from the Maple board, it is necessary to provide input power as close to 3.3V as possible. Powering the microcontroller circuitry and LEDs on the board alone takes approximately 30mA, so if you are powering the board with 12V that leaves only 10mA (at best) available for powering any user circuitry. Attempting to draw more than 10mA runs the risk of shorting out the power regulators and bricking your board.

Using the Built-in Battery Charger

Maples Rev 3 and Rev 5 also have a built-in LiPo battery charger. In order to use it, put a jumper across the CHRG header on the power selection header and across the USB, or EXT selectors, depending on whether you’re charging the battery via USB cable or barrel jack connector. The LED labeled CHRG will light up while the battery is being charged. When the battery is finished charging, the LED labeled DONE will light up.

GPIO Information

The Maple features 38 ready-to-use general purpose input/output (see 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.

Pin D38 is the board’s button pin. It is thus mainly useful as an input. The pin will read HIGH when the button is pressed.

More GPIOs (numbered D39D42 on the back of the Maple’s silkscreen) are available if you use the disableDebugPorts() function; see the board-specific debug pin constants for more information. (See this erratum for information about the pin numbered 43 on the silkscreen).

Master Pin Map

This table shows a summary of the available functionality on every GPIO pin, by peripheral type. The “5 V?” column documents whether or not the pin is 5 volt tolerant.

Note that this table is not exhaustive; on some pins, more peripherals are available than are listed here.

D0 PA3 CH3 2_CH4
D1 PA2 CH2 2_CH3
D2 PA0 CH0 2_CH1_ETR
D3 PA1 CH1 2_CH2
D4 PB5
D5 PB6
4_CH1 1_SCL
D6 PA8
D7 PA9
D8 PA10
D9 PB7
4_CH2 1_SDA
D10 PA4 CH4
2_CK 1_NSS
D11 PA7 CH7 3_CH2
D12 PA6 CH6 3_CH1
D13 PA5 CH5
D14 PB8
D15 PC0 CH10
D16 PC1 CH11
D17 PC2 CH12
D18 PC3 CH13
D19 PC4 CH14
D20 PC5 CH15
D21 PC13
D22 PC14
D23 PC15
D24 PB9
D25 PD2
D26 PC10
D27 PB0 CH8 3_CH3
D28 PB1 CH9 3_CH4
D29 PB10
2_SCL 3_TX
D30 PB11
2_SDA 3_RX
D31 PB12
2_SMBA 3_CK 2_NSS Yes
D32 PB13
3_CTS 2_SCK Yes
D33 PB14
3_RTS 2_MISO Yes
D34 PB15
2_MOSI Yes
D35 PC6
D36 PC7
D37 PC8
D38 PC9
D39 PA13
D40 PA14
D41 PA15
D42 PB3

GPIO Port Pin Map

The following table shows what pins are associated with each GPIO port.

PA0: D2 PB0: D27 PC0: D15
PA1: D3 PB1: D28 PC1: D16
PA2: D1 PB2: - PC2: D17
PA3: D0 PB3: D42 PC3: D18
PA4: D10 PB4: D43 PC4: D19
PA5: D13 PB5: D4 PC5: D20
PA6: D12 PB6: D5 PC6: D35
PA7: D11 PB7: D9 PC7: D36
PA8: D6 PB8: D14 PC8: D37
PA9: D7 PB9: D24 PC9: D38
PA10: D8 PB10: D29 PC10: D26
PA11: - PB11: D30 PC11: -
PA12: - PB12: D31 PC12: -
PA13: D39 PB13: D32 PC13: D21
PA14: D40 PB14: D33 PC14: D22
PA15: D41 PB15: D34 PC15: D23

Timer Pin Map

The following table shows what pins are associated with a particular timer’s capture/compare channels.

Timer Ch. 1 Ch. 2 Ch. 3 Ch. 4
1 D6 D7 D8
2 D2 D3 D1 D0
3 D12 D11 D27 D28
4 D5 D9 D14 D24

EXTI Line Pin Map

The following table shows which pins connect to which EXTI lines on the Maple.

EXTI Line Pins
EXTI0 D2, D15, D27
EXTI1 D3, D16, D28
EXTI2 D1, D17, D25
EXTI3 D0, D18, D42
EXTI4 D10, D19
EXTI5 D4, D13, D20
EXTI6 D5, D12, D35
EXTI7 D9, D11, D36
EXTI8 D6, D14, D37
EXTI9 D7, D24, D38
EXTI10 D8, D26, D29
EXTI11 D30
EXTI12 D31
EXTI13 D21, D32, D39
EXTI14 D22, D33, D40
EXTI15 D23, D34, D41


The Maple has three serial ports (also known as USARTs): Serial1, Serial2, and Serial3. They communicate using the pins given in the following table.

Serial Port TX RX CK CTS RTS
Serial1 D7 D8 D6    
Serial2 D1 D0 D10 D2 D3
Serial3 D29 D30 D31 D32 D33

Low-Noise ADC Pins

The six pins at the bottom right of the board (D15—D20) generally offer lower-noise ADC performance than other pins on the board. If you’re concerned about getting good ADC readings, we recommend using one of these pins to take your measurements.

Maple has an electrically isolated analog power plane with its own regulator, and a geometrically isolated ground plane. Pins D15—D20 are laid out to correspond with these analog planes, and our measurements indicate that they generally have the lowest noise of all the analog lines. However, analog performance may vary depending upon the activity of the other GPIOs. Consult the Maple hardware design files for more details.

Board-Specific Values

This section lists the Maple’s board-specific values.

  • BOARD_NR_GPIO_PINS: 44 (however, pin D43 is not usable)
  • boardPWMPins: 0, 1, 2, 3, 5, 6, 7, 8, 9, 11, 12, 14, 24, 27, 28
  • boardADCPins: 0, 1, 2, 3, 10, 11, 12, 15, 16, 17, 18, 19, 20, 27, 28

Hardware Design Files

The hardware schematics and board layout files are available in the Maple GitHub repository. The design files for Rev 1, Rev 3, and Rev 5 are respectively in the maple-r1, maple-r3, and maple-r5 subdirectories. A schematic for a JTAG adapter suitable for use with Maple is available in the jtagadapter directory.

From the GitHub repository main page, you can download the entire repository by clicking the “Download” button. If you are familiar with Git, you can also clone the repository at the command line with

$ git clone git://

Failure Modes

The following are known failure modes. The failure modes aren’t design errors, but are easy ways to break or damage your board permanently.

  • High voltage on non-tolerant pins: not all header pins are 5V compatible; so e.g. connecting certain serial devices in the wrong way could over-voltage the pins. The pin-mapping master table details which pins are 5 V tolerant.


This section documents design flaws and other errors.


  • Barrel jack power supply voltage mistake: The acceptable voltage range given next to the barrel jack on the Maple through Rev 5s manufactured in Spring 2011 is incorrect. The given range is 7V — 18V. In fact, 18V is too high and should not be supplied to your board. The original voltage regulators used on the Maple were rated up to 18V. However, the voltage regulators on current Maple Revs are rated up to only 16V, and due to the current draw requirements of the board, operate properly only up to 12V. The recommended maximum voltage you should apply is 12V, and potentially even lower depending upon the current draw requirements of the application. Please see Power Regulation on the Maple for more information.
  • Reset and PB4 tied together: The Maple’s reset line is also connected to PB4, which is labeled on the silkscreen as pin 43. Thus, attempting to use pin 43 as a GPIO can reset your board. This has other implications. Since PB4 is also the JTAG NJTRST line, this prevents the JTAG “reset halt” command from working properly.
  • Power supply marketing mistake: We originally sold the Maple advertising that it was capable of supplying up to 800 mA; the correct value is 500 mA.
  • PWM marketing mistake: We originally advertised the Maple as having 22 PWM-capable pins; the correct number is 15.
  • ADC marketing mistake: We originally advertised the Maple as having 16 analog input pins. Due to the following issue, the correct number is 15.
  • ADC on BOARD_LED_PIN: We originally sold the Maple RET6 Edition advertising 16 analog input lines. However, one of them (the one on pin 13) is also connected to the built-in LED. The voltage drop across the LED means that the analog to digital converter on that pin is not really useful. While it is still usable, its readings will be incorrect.

By Rev

The following subsections lists known issues and warnings for each revision of the Maple board.

Rev 5

  • Pin 3 AIN missing: Pin 3 is capable of analog input, but on Rev 5s manufactured during Fall 2010, the corresponding “AIN” is missing from its silkscreen. This mistake was fixed in later manufacturing runs.

Rev 3

  • Pin 3 AIN missing: Pin 3 is capable of analog input, but the corresponding “AIN” is missing from the Rev 3 silkscreen.
  • Bad/Sticky Buttons: a number of Rev 3 boards sold in May-June 2010 have questionable RESET and BUT buttons.

    What seems to have happened is that the flux remover we used to clean the boards before shipping eroded the plastic internals, which resulted in intermittent functionality. All buttons on all shipped boards did function in testing, but some may have been unreliable in regular use.

    If you have this problem, we will be happy to ship you new buttons if you think you can re-solder them yourself, or you can ship us your board and we will swap out that part.

    For reference, the button part number is KMR211GLFS and the flux remover we used is “Precision Electronics Cleaner” from RadioShack, which is “Safe on most plastics” and contains: dipropylene glycol monomethyl ether, hydrotreated heavy naphtha, dipropylene glycol methyl ether acetate (say that three times fast!), and carbon dioxide.

  • Resistors on pins 0 and 1: these header pins, which are RX/TX on USART2 (Serial2), have resistors in-line between the STM32 and the headers. These resistors increase the impedance of the lines for ADC reads and affect the open drain GPIO functionality of the pins.

    These resistors were accidentally copied over from older Arduino USB designs, where they appear to protect the USB-Serial converter from TTL voltage on the headers.

  • Silkscreen Errors: the silkscreen on the bottom indicated PWM functionality on pin 25 and listen the external header GND pin as number 38 (actually 38 is connected to the BUT button). We manually sharpied over both of these mistakes.

Rev 1

  • ADC noise: generally very high, in particular when the USB port is being used for communications (including keep-alive pings when connected to a computer).

    This issue was resolved in Rev 3 with a 4-layer design and a geometrically isolated ADC Vref plane.

  • Resistors on pins 0 and 1: these header pins, which are RX/TX on USART2 (Serial2), have resistors in-line between the STM32 and the headers. These resistors increase the impedance of the lines for ADC reads and affect the open drain GPIO functionality of the pins.

    These resistors were accidentally copied over from older Arduino USB designs, where they appear to protect the USB-Serial converter from TTL voltage on the headers.

  • Silkscreen Differences: the pin numbering scheme on Rev 1 is different from Rev 3, and thus Rev 3 software is difficult to use with Rev 1 boards. Notably, the analog input bank is labeled A0-A4 on Rev 1 but 15-20 on Rev 3, and the extra header bank does not have a pinout table on the bottom.

  • No BUT Button: the BUT button, useful for serial bootloading, was only added in Rev 3. As a workaround, you can directly short the appropriate MCU pin to Vcc; see this forum posting.