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authorMarti Bolivar <mbolivar@leaflabs.com>2011-05-25 01:14:33 -0400
committerMarti Bolivar <mbolivar@leaflabs.com>2011-05-26 02:42:07 -0400
commit6bf24f14a33b7e6d4565cd147ad4d2252dba77ff (patch)
tree5b1daa2343b00254f9be514b5b6adef6a23b5018
parent69068c8ac050f6b6202da64e22de384af21b7339 (diff)
downloadlibrambutan-6bf24f14a33b7e6d4565cd147ad4d2252dba77ff.tar.gz
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Docs: Add Maple Mini information
Flesh out and uncomment maple-mini.rst. Add links to pieces of it in various appropriate places throughout the rest of the documentation.
-rw-r--r--docs/source/adc.rst53
-rw-r--r--docs/source/external-interrupts.rst9
-rw-r--r--docs/source/gpio.rst6
-rw-r--r--docs/source/hardware/maple-mini.rst342
-rw-r--r--docs/source/index.rst6
-rw-r--r--docs/source/lang/api/hardwarespi.rst2
-rw-r--r--docs/source/timers.rst16
-rw-r--r--docs/source/usart.rst4
8 files changed, 396 insertions, 42 deletions
diff --git a/docs/source/adc.rst b/docs/source/adc.rst
index f3d5f55..a749201 100644
--- a/docs/source/adc.rst
+++ b/docs/source/adc.rst
@@ -20,31 +20,42 @@ number of techniques must be used to get good precision and accuracy.
Noise and Bias
--------------
-.. FIXME [0.0.11, Maple form factor-specific]
-
-The biggest issues with analog-digital conversion are noise and bias.
-With the Maple, we have tried to isolate the ADC pins and traces from
-strong noise sources, but there are always trade--offs between noise,
-additional functionality, cost, and package size.
-
-The 6 ADC pins in a bank (D15--D20) generally have the least
-noise, and should be used for fine measurements. If the input voltage
-changes relatively slowly, a number of samples can be taken in
-succession and averaged together, or the same voltage can even be
-sampled by multiple ADC pins at the same time.
-
-An important factor when taking a voltage reading is the reference
-voltages that the sample is being compared against. In the case of the
-Maple, the high reference is |vcc| and the low reference is ground.
-This means that noise or fluctuations on either |vcc| or ground will
-affect the measurement. It also means that the voltage you are trying
-to sample must be between ground and 3.3V.
+.. FIXME [0.0.12, Maple Native]
+
+The biggest issues with analog to digital conversion are noise and
+bias. With the Maple line, we have tried to isolate the ADC pins and
+traces from strong noise sources, but there are always trade-offs
+between noise, additional functionality, cost, and package size.
+We've tried to enable good analog performance by isolating as many ADC
+pins as possible from digital noise on each board.
+
+More information on these isolated pins is available in each board's
+hardware documentation:
+
+* :ref:`Maple <maple-adc-bank>`
+* :ref:`Maple RET6 Edition <maple-ret6-adc-bank>`
+* :ref:`Maple Mini <maple-mini-adc-bank>`
+
+.. :ref:`Maple Native <maple-native-adc-bank>`
+
+That said, there are a number of more general things you can do to try
+to get good readings. If your input voltage changes relatively
+slowly, a number of samples can be taken in succession and averaged
+together, or the same voltage can even be sampled by multiple ADC pins
+at the same time.
+
+Another important factor when taking a voltage reading is the
+reference voltages that the sample is being compared against. In the
+case of the Maple, the high reference is |vcc| and the low reference
+is ground. This means that noise or fluctuations on either |vcc| or
+ground will affect the measurement. It also means that the voltage you
+are trying to sample must be between ground and 3.3 V.
.. _adc-range:
In the case of a variable reading, it is best if the voltage varies
-over the entire range of 0--3.3V; otherwise, only a fraction of the
-sensitivity is being leveraged. Some basic tools to accomplish this
+over the entire range of 0 through 3.3 V; otherwise, only a fraction
+of the sensitivity is being used. Some basic tools to accomplish this
are `resistor dividers
<http://en.wikipedia.org/wiki/Voltage_divider>`_ and `Zener diodes
<http://en.wikipedia.org/wiki/Voltage_source#Zener_voltage_source>`_\
diff --git a/docs/source/external-interrupts.rst b/docs/source/external-interrupts.rst
index ac065a4..ccb9afd 100644
--- a/docs/source/external-interrupts.rst
+++ b/docs/source/external-interrupts.rst
@@ -37,17 +37,20 @@ means that every pin on the Maple connects to what is called an *EXTI
line*, and within an EXTI line, only one of the pins that connects to
it can be used to detect external interrupts at a time.
+.. TODO [0.0.12] Maple Native links
+
The EXTI Line Pin Map for your board lists which pins connect to which
EXTI lines:
* :ref:`Maple <maple-exti-map>`
* :ref:`Maple RET6 Edition <maple-ret6-exti-map>`
+* :ref:`Maple Mini <maple-mini-exti-map>`
.. note::
- You should set the :ref:`pin mode <lang-pinmode>` of your
- desired pin to an input mode (e.g ``INPUT`` or ``INPUT_FLOATING``,
- ``INPUT_PULLUP``, ``INPUT_PULLDOWN``).
+ You should set the :ref:`pin mode <lang-pinmode>` of your desired
+ pin to an input mode (e.g. ``INPUT``, ``INPUT_PULLUP``,
+ ``INPUT_PULLDOWN``).
Function Reference
------------------
diff --git a/docs/source/gpio.rst b/docs/source/gpio.rst
index 74be3d4..64ad494 100644
--- a/docs/source/gpio.rst
+++ b/docs/source/gpio.rst
@@ -20,19 +20,19 @@ Pin Maps
The hardware documentation for your board lists each pin's
capabilities, by pin number:
-.. TODO [0.1.0] Uncomment Mini and Native GPIO links
+.. TODO [0.0.12] Native link
* :ref:`Maple <maple-gpios>`
* :ref:`Maple RET6 Edition <maple-ret6-gpios>`
+* :ref:`Maple Mini <maple-mini-gpios>`
-.. * :ref:`Maple Mini <maple-mini-gpios>`
.. * :ref:`Maple Native <maple-native-gpios>`
The current and voltage limitations were determined using the STM32
datasheets. In particular, only some GPIO pins are **5V tolerant**,
which means that applying 5 volts to a pin and reading it as input or
allowing it to drain to ground will not damage that pin. Connecting a
-voltage higher than 3.3V to a non-5V tolerant pin may damage your
+voltage higher than 3.3 V to a non-5 V tolerant pin may damage your
board.
.. _gpio-modes:
diff --git a/docs/source/hardware/maple-mini.rst b/docs/source/hardware/maple-mini.rst
index a8a4186..3e79ed0 100644
--- a/docs/source/hardware/maple-mini.rst
+++ b/docs/source/hardware/maple-mini.rst
@@ -1,6 +1,344 @@
-.. _hardware-maple-mini:
+.. _maple-mini:
Maple Mini
==========
-Stub.
+This page is a general resource for information specific to the Maple
+Mini. The Maple Mini is a smaller version of the Maple that fits on a
+breadboard.
+
+.. contents:: Contents
+ :local:
+
+.. TODO [dma.rst] Ref to dma.rst in sequel instead of libmaple-dma
+
+Technical Specifications
+------------------------
+
+* MCU: **STM32F103RCBT6**, a 32-bit ARM Cortex M3 microprocessor
+* Clock Speed: **72 MHz**
+* **128 KB Flash** and **20 KB SRAM**
+* 34 Digital I/O Pins (:ref:`gpio`)
+* 12 **PWM** pins at 16 bit resolution (:ref:`pwm`)
+* 9 analog input pins, 12 bit **ADC** resolution (:ref:`adc`)
+* 2 **SPI** peripherals (:ref:`spi`)
+* 2 **I2C** peripherals (:ref:`i2c`)
+* 7 Channels of Direct Memory Access (**DMA**) (:ref:`libmaple-dma`)
+* 3 **USART** peripherals (:ref:`usart`)
+* 1 advanced and 3 general-purpose **timers** (:ref:`timers`)
+* Dedicated **USB** port for programming and communications (:ref:`usb`)
+* **JTAG** (:ref:`jtag`)
+* Nested Vectored Interrupt Controller (NVIC) (including
+ :ref:`external interrupt <external-interrupts>` on GPIOs)
+* Supplies up to 500 mA at 3.3 V, with :ref:`separate 250 mA digital
+ and analog regulators <maple-mini-adc-bank>` for low-noise analog
+ performance
+* :ref:`Four layer design <maple-mini-hardware>`
+* Support for low power, sleep, and standby modes (<500 μA)
+* Operating Voltage: 3.3 V
+* Input Voltage (recommended): 3 V — 12 V
+* Dimensions: 2.02″ × 0.72″
+
+.. _maple-mini-powering:
+
+Powering the Maple Mini
+-----------------------
+
+You can power the Maple Mini via the USB plug or by powering Vin
+directly.
+
+.. warning:: The silkscreen on the Maple Mini suggests it will accept
+ an input voltage up to 16 V. We recommend applying **no greater
+ than 12 V**.
+
+ See :ref:`this erratum <maple-mini-vin>` for more information.
+
+.. _maple-mini-gpios:
+
+GPIO Information
+----------------
+
+The Maple Mini features 34 total input/output pins, numbered ``D0``
+through ``D33``. These numbers correspond to the numeric values next
+to each header on the Maple Mini's silkscreen. However, some of them
+have special uses by default [#fusedpins]_.
+
+.. _maple-mini-usb-pins:
+
+Pin ``D23`` is the :ref:`USB <usb>` D+ line, and ``D24`` is the USB D-
+line. To use them as GPIOs, your program will need to :ref:`disable
+SerialUSB <lang-serialusb-end>` first. Be aware, however, that
+disabling SerialUSB means that the :ref:`bootloader <bootloader>`
+won't work properly, and you'll need to use
+:ref:`troubleshooting-perpetual-bootloader` to make your next upload.
+
+.. _maple-mini-but:
+
+Pin ``D32`` is the Mini's :ref:`button pin <lang-board-values-but>`.
+It is thus mainly useful as an :ref:`input <lang-pin-levels>`. The
+pin will :ref:`read <lang-digitalread>` ``HIGH`` when the :ref:`button
+is pressed <lang-isbuttonpressed>`.
+
+.. _maple-mini-led:
+
+Pin ``D33`` is the Mini's :ref:`LED pin <lang-board-values-led>`. It
+is thus mainly useful as an :ref:`output <lang-pin-levels>`. The LED
+will glow when ``HIGH`` is :ref:`written <lang-digitalwrite>` to it.
+(It also supports :ref:`pwm`, for finer-grained brightness control).
+
+.. TODO [0.1.0] silkscreen pictures which expand abbreviations
+
+.. _maple-mini-pin-map-master:
+
+Master Pin Map
+^^^^^^^^^^^^^^
+
+This table shows a summary the available functionality on every GPIO
+pin, by peripheral type. The "5 V?" column documents whether or not
+the pin is 5 volt tolerant.
+
+.. csv-table::
+ :header: Pin, :ref:`GPIO <gpio>`, :ref:`ADC <adc>`, :ref:`Timer <timers>`, :ref:`I2C <i2c>`, :ref:`UART <usart>`, :ref:`SPI <spi>`, 5 V?
+
+ D0, PB11, -, -, 2_SDA, 3_RX, -, Yes
+ D1, PB10, -, -, 2_SCL, 3_TX, -, Yes
+ D2, PB2, -, -, -, -, -, Yes
+ D3, PB0, CH8, 3_CH3, -, -, -, -
+ D4, PA7, CH7, 3_CH2, -, -, 1_MOSI, -
+ D5, PA6, CH6, 3_CH1, -, -, 1_MISO, -
+ D6, PA5, CH5, -, -, -, 1_SCK, -
+ D7, PA4, CH4, -, -, 2_CK, 1_NSS, -
+ D8, PA3, CH3, 2_CH4, -, 2_RX, -, -
+ D9, PA2, CH2, 2_CH3, -, 2_TX, -, -
+ D10, PA1, CH1, 2_CH2, -, 2_RTS, -, -
+ D11, PA0, CH0, 2_CH1_ETR, -, 2_CTS, -, -
+ D12, PC15, -, -, -, -, -, -
+ D13, PC14, -, -, -, -, -, -
+ D14, PC13, -, -, -, -, -, -
+ D15, PB7, -, 4_CH2, 1_SDA, -, -, Yes
+ D16, PB6, -, 4_CH1, 2_SCL, -, -, Yes
+ D17, PB5, -, -, 1_SMBA, -, -, -
+ D18, PB4, -, -, -, -, -, Yes
+ D19, PB3, -, -, -, -, -, Yes
+ D20, PA15, -, -, -, -, -, Yes
+ D21, PA14, -, -, -, -, -, Yes
+ D22, PA13, -, -, -, -, -, Yes
+ D23, PA12, -, 1_ETR, -, 1_RTS, -, Yes
+ D24, PA11, -, 1_CH4, -, 1_CTS, -, Yes
+ D25, PA10, -, 1_CH3, -, 1_RX, -, Yes
+ D26, PA9, -, 1_CH2, -, 1_TX, -, Yes
+ D27, PA8, -, 1_CH1, -, 1_CK, -, Yes
+ D28, PB15, -, -, -, -, 2_MOSI, Yes
+ D29, PB14, -, -, -, 3_RTS, 2_MISO, Yes
+ D30, PB13, -, -, -, 3_CTS, 2_SCK, Yes
+ D31, PB12, -, 1_BKIN, 2_SMBA, 3_CK, 2_NSS, Yes
+ D32, PB8, -, 4_CH3, -, -, -, Yes
+ D33, PB1, CH9, 3_CH4, -, -, -, -
+
+.. _maple-mini-timer-map:
+
+Timer Pin Map
+^^^^^^^^^^^^^
+
+The following table shows what pins are associated with a particular
+timer's capture/compare channels.
+
+.. csv-table::
+ :header: Timer, Ch. 1, Ch. 2, Ch. 3, Ch. 4
+ :delim: |
+
+ 1 | D27 | D26 | D25 | D24
+ 2 | D11 | D10 | D9 | D8
+ 3 | D5 | D4 | D3 | :ref:`D33 <maple-mini-led>`
+ 4 | D16 | D15 | :ref:`D32 <maple-mini-but>` |
+
+.. _maple-mini-exti-map:
+
+EXTI Line Pin Map
+^^^^^^^^^^^^^^^^^
+
+The following table shows which pins connect to which :ref:`EXTI lines
+<external-interrupts-exti-line>` on the Maple.
+
+.. list-table::
+ :widths: 1 1
+ :header-rows: 1
+
+ * - EXTI Line
+ - Pins
+ * - EXTI0
+ - 3, 11
+ * - EXTI1
+ - 10, 33
+ * - EXTI2
+ - 2, 9
+ * - EXTI3
+ - 8, 19
+ * - EXTI4
+ - 7, 18
+ * - EXTI5
+ - 6, 17
+ * - EXTI6
+ - 5, 16
+ * - EXTI7
+ - 4, 15
+ * - EXTI8
+ - 27, 32
+ * - EXTI9
+ - 26
+ * - EXTI10
+ - 1, 25
+ * - EXTI11
+ - 0, 24
+ * - EXTI12
+ - 23, 31
+ * - EXTI13
+ - 14, 22, 30
+ * - EXTI14
+ - 13, 21, 29
+ * - EXTI15
+ - 12, 20, 28
+
+.. _maple-mini-usart-map:
+
+USART Pin Map
+^^^^^^^^^^^^^
+
+The Maple RET6 Edition has three serial ports whose pins are broken
+out to headers (also known as :ref:`USARTs <usart>`). They communicate
+using the pins summarized in the following table:
+
+.. csv-table::
+ :header: Serial Port, TX, RX, CK, CTS, RTS
+ :delim: |
+
+ ``Serial1`` | 26 | 25 | 27 | 24 | 23
+ ``Serial2`` | 9 | 8 | 7 | 11 | 10
+ ``Serial3`` | 1 | 0 | 31 | 30 | 29
+
+.. _maple-mini-adc-bank:
+
+Low-Noise ADC Pins
+^^^^^^^^^^^^^^^^^^
+
+Analog input pins D4 — D11 on the Maple Mini generally offer
+lower-noise ADC performance than D3. If you’re concerned about
+getting good ADC readings, we recommend using one of these pins to
+take your measurements.
+
+Maple Mini has an electrically isolated analog power plane with its
+own regulator, and a geometrically isolated ground plane, connected to
+the digital plane by an inductor. Pins D4 — D11 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 :ref:`Maple Mini hardware design files
+<maple-mini-hardware>` for more details.
+
+.. _maple-mini-board-values:
+
+Board-Specific Values
+---------------------
+
+This section lists the Maple Mini's :ref:`board-specific values
+<lang-board-values>`.
+
+- ``CYCLES_PER_MICROSECOND``: 72
+- ``BOARD_BUTTON_PIN``: 32
+- ``BOARD_LED_PIN``: 33
+- ``BOARD_NR_GPIO_PINS``: 34
+- ``BOARD_NR_PWM_PINS``: 12
+- ``boardPWMPins``: 3, 4, 5, 8, 9, 10, 11, 15, 16, 25, 26, 27
+- ``BOARD_NR_ADC_PINS``: 9
+- ``boardADCPins``: 3, 4, 5, 6, 7, 8, 9, 10, 11
+- ``BOARD_NR_USED_PINS``: 4
+- ``boardUsedPins``: ``BOARD_LED_PIN``, ``BOARD_BUTTON_PIN``, 23, 24
+ (23 and 24 are used by :ref:`USB <maple-mini-usb-pins>`)
+- ``BOARD_NR_USARTS``: 3
+- ``BOARD_USART1_TX_PIN``: 26
+- ``BOARD_USART1_RX_PIN``: 25
+- ``BOARD_USART2_TX_PIN``: 9
+- ``BOARD_USART2_RX_PIN``: 8
+- ``BOARD_USART3_TX_PIN``: 1
+- ``BOARD_USART3_RX_PIN``: 0
+- ``BOARD_NR_SPI``: 2
+- ``BOARD_SPI1_NSS_PIN``: 7
+- ``BOARD_SPI1_MOSI_PIN``: 4
+- ``BOARD_SPI1_MISO_PIN``: 5
+- ``BOARD_SPI1_SCK_PIN``: 6
+- ``BOARD_SPI2_NSS_PIN``: 31
+- ``BOARD_SPI2_MOSI_PIN``: 28
+- ``BOARD_SPI2_MISO_PIN``: 29
+- ``BOARD_SPI2_SCK_PIN``: 30
+- ``BOARD_JTMS_SWDIO_PIN``: 22
+- ``BOARD_JTCK_SWCLK_PIN``: 21
+- ``BOARD_JTDI_PIN``: 20
+- ``BOARD_JTDO_PIN``: 19
+- ``BOARD_NJTRST_PIN``: 18
+
+.. _maple-mini-hardware:
+
+Hardware Design Files
+---------------------
+
+The hardware schematics and board layout files are available in the
+`Maple Mini GitHub repository <https://github.com/leaflabs/maplemini>`_
+
+From the GitHub repository main page, you can download the entire
+repository by clicking the "Download" button. If you are familiar
+with `Git <http://git-scm.com/>`_, you can also clone the repository
+at the command line with ::
+
+ $ git clone git://github.com/leaflabs/maplemini.git
+
+Failure Modes
+-------------
+
+The following known failure modes apply to all Maple boards. 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 5 V
+ compatible; so e.g. connecting certain serial devices in the wrong
+ way could over-voltage the pins. The :ref:`pin-mapping master table
+ <maple-mini-pin-map-master>` details which pins are 5 V-tolerant.
+
+Errata
+------
+
+This section lists known issues and warnings for the Maple Mini Rev 2
+(the first Rev sold to the public).
+
+.. _maple-mini-vin:
+
+* **Silkscreen Vin voltage mistake**: The silkscreen on the Maple Mini
+ falsely indicates that Vin may be supplied with up to 16 V. We
+ recommend an input voltage **no greater than 12 V**.
+
+ The voltage regulator on the Mini is rated up to 16 V. However, our
+ tests indicate that as its input voltage approaches 16 V, its output
+ begins to rise to levels higher than those recommended by ST for
+ supplying the STM32F103CB. The limit of 12 V keeps the voltage
+ supplied to the processor at safe levels.
+
+Recommended Reading
+-------------------
+
+STMicro documentation for STM32F103CB microcontroller:
+
+* `Datasheet
+ <http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00161566.pdf>`_
+ (PDF); covers STM32F103x8, STM32F103xB.
+* `Reference Manual RM0008
+ <http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/REFERENCE_MANUAL/CD00171190.pdf>`_
+ (PDF); general, definitive resource for STM32F1 line.
+* `Programming Manual PM0056
+ <http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/PROGRAMMING_MANUAL/CD00228163.pdf>`_
+ (PDF); assembly language and register reference.
+* `STM32F103CB <http://www.st.com/internet/mcu/product/189782.jsp>`_
+ overview page with links to further references.
+
+.. rubric:: Footnotes
+
+.. [#fusedpins] See :ref:`boardUsedPins <lang-board-values-used-pins>`
+ for more information.
diff --git a/docs/source/index.rst b/docs/source/index.rst
index ce3985f..d17c4db 100644
--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@@ -67,11 +67,7 @@ Have fun!
hardware/maple.rst
hardware/maple-ret6.rst
+ hardware/maple-mini.rst
.. TODO write/include these upon Mini and Native release
-
-.. .. toctree::
-.. :hidden:
-
-.. hardware/maple-mini.rst
.. hardware/maple-native.rst
diff --git a/docs/source/lang/api/hardwarespi.rst b/docs/source/lang/api/hardwarespi.rst
index a80e429..054d1a8 100644
--- a/docs/source/lang/api/hardwarespi.rst
+++ b/docs/source/lang/api/hardwarespi.rst
@@ -43,7 +43,7 @@ Turning the SPI Port On
Now it's time to turn your SPI port on. Do this with the ``begin()``
function (an example is given below).
-.. FIXME [0.0.11] Breathe doesn't include the class; fix & submit pull req
+.. FIXME [Breathe] Output doesn't include the class; fix & submit pull req
.. doxygenfunction:: HardwareSPI::begin
diff --git a/docs/source/timers.rst b/docs/source/timers.rst
index 403193f..e23579d 100644
--- a/docs/source/timers.rst
+++ b/docs/source/timers.rst
@@ -77,11 +77,17 @@ your program.
PWM Conflicts
^^^^^^^^^^^^^
-Because PWM functionality on a given pin depends on
-the configuration of the timer and channel, you must chose your
-channels carefully if you want to use both timer interrupts and PWM in
-the same program. Refer to your board's :ref:`Timer Pin Map
-<gpio-pin-maps>` to match up timer channels and pin numbers.
+Because PWM functionality on a given pin depends on the configuration
+of the timer and channel, you must chose your channels carefully if
+you want to use both timer interrupts and PWM in the same program.
+Refer to your board's timer pin map to match up timer channels and pin
+numbers:
+
+.. TODO [0.0.12] Native links
+
+* :ref:`Maple <maple-timer-map>`
+* :ref:`Maple RET6 Edition <maple-ret6-timer-map>`
+* :ref:`Maple Mini <maple-mini-timer-map>`
Overhead
^^^^^^^^
diff --git a/docs/source/usart.rst b/docs/source/usart.rst
index b3e1ea1..8766e3f 100644
--- a/docs/source/usart.rst
+++ b/docs/source/usart.rst
@@ -9,8 +9,7 @@ USART
Hardware/Circuit Design
-----------------------
-.. FIXME [0.0.12/Maple Native] UART4 and UART5, Native links
-.. FIXME [0.0.11] Mini info and links
+.. TODO [0.0.12/Maple Native] UART4 and UART5, Native links
Each LeafLabs board has at least 3 separate USART devices. In the most
simple use case, the RX and TX pins are used to send data at a
@@ -24,6 +23,7 @@ available here:
* :ref:`Maple <maple-usart-map>`
* :ref:`Maple RET6 Edition <maple-ret6-usart-map>`
+* :ref:`Maple Mini <maple-mini-usart-map>`
If you use a particular serial port, you cannot also use its
communication pins for other purposes at the same time.