Digilent MX7cK User Manual

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Revision: June 24, 2013
Note: This document applies to REV B and C of the board.

1300 Henley Court | Pullman, WA 99163

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Cerebot MX7cK Circuit Diagram

Overview

The Cerebot MX7cK is a microcontroller
development board based on the Microchip
PIC32MX795F512L, a member of the 32-bit
PIC32 microcontroller family. It is compatible

with Digilent’s line of Pmod™ peripheral

modules, and is suitable for use with the
Microchip MPLAB® IDE tools. The Cerebot
MX7cK is also compatible for use with the

chipKIT™ MPIDE development environment.

ChipKIT and MPIDE is a PIC32 based system
compatible with many existing Arduino™ code
examples, reference materials and other
resources.

The Cerebot MX7cK is designed to be easy to
use and suitable for use by anyone from
beginners to advanced users for experimenting
with embedded control and network
communications application. A built in
programming/debugging circuit compatible with
the Microchip MPLAB® IDE is provided, so no
additional hardware is required for use with
MPLAB. The kit contains everything needed to
start developing embedded applications using
either the MPLAB® IDE or the MPIDE.

The Cerebot MX7cK provides 52 I/O pins that
support a number of peripheral functions, such
as UART, SPI and I
pulse width modulated outputs and five
external interrupt inputs. Its network and
communications features also include a 10/100
Ethernet interface, Full Speed USB 2.0 OTG
interface, and dual CAN network interfaces.
Ten of the I/O pins can be used as analog
inputs in addition to their use as digital inputs
and outputs.

The Cerebot MX7cK can be powered in
various ways via USB, or using an external
AC-DC power adapter.

2

C™ ports as well as five

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Cerebot MX7cK Reference Manual

Functional Description

The Cerebot MX7cK is designed for embedded
control and network communications
applications as well as for general
microprocessor experimentation. Firmware
suitable for many applications can be
downloaded to the Cerebot MX7cK’s
programmable PIC32 microcontroller.

Features of the Cerebot MX7cK include:

 a PIC32MX795F512L microcontroller
 support for programming and

debugging within the Microchip MPLAB
development environment

 support for programming within the

chipKIT MPIDE development
environment

 six Pmod connectors for Digilent

peripheral module boards

 10/100 Ethernet
 USB 2.0 compatible Device, Host, and

OTG support

 two CAN network interfaces
 three push buttons
 four LEDs
 multiple power supply options, including

USB powered

 ESD protection and short circuit

protection for all I/O pins.

Features of the PIC32MX795F512L include:

 512KB internal program flash memory
 128KB internal SRAM memory
 USB 2.0 compliant full-speed On-The-

Go (OTG) controller with dedicated
DMA channel

 10/100 Ethernet controller
 two CAN network controllers
 up to four serial peripheral interfaces

(SPI)

 up to six UART serial interfaces
 up to four I2C serial interfaces
 five 16-bit timer/counters
 five timer capture inputs
 five compare/PWM outputs
 sixteen 10-bit analog inputs

 two analog comparators

The Cerebot MX7cK has a number of
input/output connection options, and is
designed to work with the Digilent line of Pmod
peripheral modules that provide various input
and output functions. For more information,
see www.digilentinc.com. In addition to the
Pmod connectors, the board provides three
push buttons and four LEDs for user I/O, as
well as providing connections for two I2C
busses. A serial EEPROM is provided on one
of the I2C busses.

The Cerebot MX7cK features a flexible power
supply system with a number of options for
powering the board as well as powering
peripheral devices connected to the board. It
can be USB powered via the debug USB port,
the USB UART serial port, or the USB device
port. It can also be powered from an external
5V power supply.

Programming Tools

The Cerebot MX7cK can be used with either
the Microchip MPLAB® development
environment or the chipKIT MPIDE
development environment. When used with the
MPLAB® IDE, in-system-programming and
debugging of firmware running on the
PIC32MX795 microcontroller is supported
using an on-board programming/debugging
circuit licensed from Microchip.

The Cerebot MX7cK is immediately useable
with either the MPLAB IDE or the chipKIT
MPIDE. No additional hardware is required to
use the board with the Microchip MPLAB tools.

Using the Cerebot MX7cK with
Microchip Development Tools

The Microchip MPLAB® IDE or the MPLAB® X
IDE can be used to program and debug code
running on the Cerebot MX7cK board using the
built-in programming/debugging circuit.

The MPLAB development environment can be
downloaded from the Microchip web site. This
software suite includes a free evaluation copy

www.digilentinc.com page 2 of 36

Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.

Cerebot MX7cK Reference Manual

of the Microchip C32 compiler for use with the
PIC32 microcontroller family. The licensed
debugger is compatible with the MPLAB IDE
version 8.63 or later or all versions of MPLAB
X.

When creating a new project, use the
“Configure.Select Device…” menu to specify the
PIC32 device being used. Ensure that the
device is set to PIC32MX795F512L.

In order to use the on-board
programming/debugging circuit it must be
selected as the debugger or programmer
within the MPLAB IDE. Use the

“Debugger.Select Tool” menu, or the
“Programmer.Select Tool” menu, and select

“Licensed Debugger” as the programmer or
debugger.

The licensed debugger interface uses USB
connector J15, labeled DEBUG. Connector
J15 is a USB micro-B connector located on the
left side of the board below the Ethernet RJ45
connector. Use a USB-A to micro-B cable
(provided with the board) to connect to an
available USB port on the PC.

When the licensed debugger is selected as the
programming or debugging device, the MPLAB
IDE will check the version number of the
firmware running on the debugger and offer to
update if it is out of date with the version of
MPLAB being used.

The PIC32 in-system programming/debugging
interface uses two pins on the microcontroller.
These devices support two alternate pin pairs
for this interface: PGC1/PGD1 or PGC2/PGD2.
The PGC2/PGD2 pair is used by default. Due
to conflicting uses of the microcontroller pins,
the Cerebot MX7cK is designed to use the
PGC1/PGD1 pair of pins. Because of this, it is
necessary to select the use of PGC1/PGD1 for
the debugging interface. This is done using
configuration variables set using the #pragma

config statement. The following statement

must be used to configure the microcontroller
for use with the on-board licensed debugger
circuit:

www.digilentinc.com page 3 of 36

Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.

#pragma config ICESEL = ICS_PGx1

The MPLAB IDE may report an error indicating
that the device is not configured for debugging
until a program containing this statement has
been programmed into the board.

The MCLR pin on the PIC32 microcontroller is
used by the hardware programming/debugging
interface to reset the processor. This same pin
is used by the USB serial converter to reset the
processor when using the MPIDE. It is possible
that the reset function from the USB serial
interface can interfere with correct operation of
the Microchip programming and debugging
tools. If this happens, jumper JP11 can be
used to disconnect the USB serial converter
reset circuit. Remove the shorting block from
JP11 to disable the reset circuit. If the shorting
block has been removed, it is necessary to
reinstall it on JP11 in order to use the Cerebot
MX7cK board with the MPIDE again.

Using the Microchip development tools to
program the Cerebot MX7cK will erase the
chipKIT boot loader. To use the board with the
chipKIT MPIDE again, it is necessary to
program the boot loader back onto the board.
The programming file for the boot loader
programmed into the board by Digilent at the
factory is available for download from the
product page for the Cerebot MX7cK on the
Digilent web site. Additionally, the boot loader
source code is available in the chipKIT project
repository at www.github.com/chipKIT32/pic32-

Arduino-Bootloader.

To reprogram the boot loader using MPLAB,
perform the following steps:

 Use the “Configure.Select Device …” menu to

select the PIC32MX795F512L

 Use the “Programmer.Select Programmer”

menu to select the “Licensed Debugger”.

 Use the “File Import…” dialog box to

navigate to and select the boot loader
programming downloaded from the Digilent
web site. The file name will be something
like: chipKIT_Bootloader_MX7cK.hex

 Use the “Programmer.Program” command to

program all memories on the device.

Cerebot MX7cK Reference Manual

Using the Cerebot MX7cK with the
chipKIT MPIDE

ChipKIT and the MPIDE is a PIC32 based
hardware and software system compatible with
many existing Arduino™ code examples,
reference materials and other resources. The
MPIDE development platform was produced by

modifying the Arduino™ IDE and is fully

backward compatible with the Arduino IDE.
The Cerebot MX7cK board is designed to be
fully compatible with the chipKIT MPIDE
system, version 20111221 or later.

The MPIDE uses a serial communications port
to communicate with a boot loader running in
the target board. The serial port on the MX7cK
board is implemented using an FTDI FT232R
USB serial converter. Before attempting to use
the MPIDE with the MX7cK, the appropriate
USB device driver must be installed.

The USB serial converter on the Cerebot
MX7cK uses USB connector J2, labeled UART
on the board. This connector is a USB micro­AB, and is located on the lower left side of the
board adjacent to the power switch. Use a
standard USB-A to micro-B cable (provided
with the board) to connect the board to an
available USB port on the PC.

In the MPIDE, use the “Tools.Board” command
to select the Cerebot MX7cK from the list of
available boards. Use the “Tools.Serial Port”
command to choose the appropriate serial port
from the list of available serial ports on the PC.

When the MPIDE needs to communicate with
the MX7cK board, the PIC32 microcontroller is
reset and starts running the boot loader. The
MPIDE then establishes communications with
the boot loader and downloads the user’s
program, programming it into the flash memory
in the microcontroller

When the MPIDE opens the serial
communications connection on the PC, the
DTR pin on the FT232R chip is driven low.
This pin is coupled through a capacitor to the
MCLR pin on the PIC32 microcontroller.

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Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.

Driving the MCLR line low resets the
microcontroller, restarting execution with the
boot loader.

The automatic reset action when the serial
communications connection is opened can be
disabled. To disable this operation, remove the
shorting block from jumper JP11. The shorting
block is reinstalled on JP11 to restore
operation with the MPIDE.

Two red LEDs (LD9 and LD10) will blink when
data is being sent or received between the
Cerebot MX7cK and the PC over the serial
connection.

The header connector J1 provides access to
the other serial handshaking signals provided
by the FT232R. Connector J1 is not loaded at
the factory but can be installed by the user to
access these signals.

Additional Reference Documentation

Additional information about the Cerebot
MX7cK board and the use and operation of the
PIC32MX795F512L microcontroller can be
obtained from the following sources.

The Cerebot MX7cK Schematic, as well as
various support libraries, and example
reference designs are available on the board
product page on the Digilent web site:

www.digilentinc.com

The PIC32MX5XX/6XX/7XX Family Data
Sheet and the PIC32MX Family Reference
Manual available from the Microchip web site:

www.microchip.com

Reference material for the chipKIT MPIDE
system is included in the MPIDE software
download, and on-line in the chipKIT wiki. Help
with questions and problems using the board
with the chipKIT MPIDE software can also be
obtained in the chipKIT forums:

www.github.com/chipKIT32 (software

download)

www.chipKIT.org/wiki
www.chipKIT.org/forum

Cerebot MX7cK Reference Manual

Board Hardware Description

The following describes the various hardware
features of the Cerebot MX7cK board and the
PIC32XM795F512L microcontroller.

Board Power Supply

Switch SW1, in the lower left corner of the
board is the power switch. Place this switch in
the ON position to turn on board power and in
the OFF position to turn off board power.

The Cerebot MX7cK may be USB powered via
either the USB debug port, the USB UART
port, or the USB device port. Alternatively, the
board may be powered via dedicated,
“external”, power supply connectors.

Jumper block J3 selects the power source
used to provide power to the board. This
jumper block provides the following four
positions:

 USB – power is supplied by USB device

connector J19. This is used when the
Cerebot MX7cK is being used to implement
a USB bus powered device.

 EXT – Power is supplied by one of the

external power connectors.

 DBG – Power is supplied by DEBUG USB

connector J15.

 UART – Power is supplied by UART USB

connector J2.

Place the shorting block in the appropriate
position on J3 for the desired power source for
the board.

The Cerebot MX7cK is rated for external power
from 3.6 to 5.5 volts DC. Using a voltage
outside this range will damage the board and
connected devices. For most purposes, when
using external power, a regulated 5V supply
should be used. If the board is operated from
an external supply with a voltage less than 5V,
some features won’t work correctly.

The USB specification requires that USB
devices not draw more than 100mA of current

until they have enumerated on the USB bus
and informed the host that they want to
consume more current. To meet this
specification, the debugger circuit turns on
main board power by driving the PWR_ON
signal high after successfully enumerating on
the USB bus. The bus labeled on the
schematic as VCC5V0 is powered when this
occurs. The VCC5V0 bus powers the input to
the main board voltage regulator, the input
voltage to the USB bus voltage load switch
used when using the board as a USB host, the
power supply voltage for the CAN transceivers,
and the 5V0 side of the power select jumpers
for the Pmod connectors. The voltage on the
VCC5V0 bus will be 5V when the board is
being operated from USB power or an external
regulated 5V supply. If a different external
supply voltage is used, that voltage will appear
on the VCC5V0 bus.

The power supply selected by the shorting
block on J3 will appear on the input power
supply bus, labeled VIN in the schematic. This
voltage is regulated to 3.3V to power the
licensed debugger circuit by IC11, a Microchip
MCP1801 Low Dropout voltage regulator. This
regulator is turned on and the debugger circuit
is powered whenever the power switch is in the
on position. The VIN power bus also supplies
power to IC9, a PFET load switch used to turn
main board power on or off.

When the power switch is turned on, transistor
Q7A attempts to drive the ON/OFF pin of IC9
high to enable the main power supply.

If the licensed debugger is connected to an
active USB port, it initially disables the main
power supply by holding the PWR_ON signal
low. This causes Q8B to hold the ON/OFF pin
of IC9 low, forcing the main supply off. The
licensed debugger will enumerate with the host
computer and once it has successfully
enumerated, will turn on the main board power
supply by driving the PWR_ON signal high.

If the licensed debugger is not connected to an
active USB port, the signal labeled DBG5V0
will not be powered. Transistor Q8B will be
unable to hold the ON/OFF pin low and the

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Cerebot MX7cK Reference Manual

main power supply will be turned on
immediately when the power switch is turned
on. When load switch IC9 is turned on, it
powers the board unregulated power bus
VCC5V0, which provides the input power to
the main regulator.

The main board power supply is a switch mode
voltage regulator implemented using a
Microchip MCP16301 switch mode step-down
regulator, IC10. This regulator provides 3.3V at
up to 600 mA with approximately 96%
efficiency. When the main power supply is
enabled, it powers the regulated power bus
VCC3V3

When powering the board from an external
power supply, there are two connectors that
can be used: J17 and J18.

The barrel connector, J17, is used to power the
board from a “wall wart” style power supply.
This type of power supply is available from
many sources. Digilent has a power supply
available, the 5V Switching Power Supply, that
can be used with connector J17. Connector
J17 is a 2.5mm x 5.5mm coaxial connector
wired with the center terminal as the positive
voltage.

Connector J18 is a screw terminal connector
for an alternative power supply connection for
use with battery packs, bench supplies or other
power sources where use of a hard wired
power supply is desirable.

Connectors J17 and J18 are wired in parallel
and connect to the EXT position on the Power
Select jumper block J3. A shorting block

should be placed on the “EXT” position of J3

when using this option for board power. Only
one of the external power connectors should
be used at a time. If multiple power supplies
are connected simultaneously, damage to the
board or the power supplies may occur.

When the Cerebot MX7cK is operating as a
USB host, an external power supply connected
to either J17 or J18 must be used to power the
board. In addition to powering the logic on the

USB bus voltage supplied to any connected
USB device and must be a regulated 5V with
at least 500mA current capability to meet the
USB specifications.

The CAN bus operates at 5V, and therefore
the transceivers for the two CAN interfaces
require 5V to operate correctly and within the
CAN specification. When using the CAN
network interfaces, the board should be
operated from a 5V supply if using an external
power supply.

The PIC32 microcontroller and on-board I/O
devices operate at a supply voltage of 3.3V
provided by the VCC3V3 bus. The main
voltage regulator is capable of providing a
maximum of 600mA of current. The PIC32
microcontroller will use approximately 85mA
when running at 80MHz. The SMSC LAN8720
Ethernet PHY consumes approximately 45mA
when operating at 100Mbps. The Microchip
MCP2551 CAN transceivers can draw up to
75mA each when operating the CAN busses.
The other circuitry on the board will draw 10-20
mA. The remaining current is available to
provide power to attached Pmods and I2C
devices.

The Cerebot MX7cK can provide power to any
peripheral modules attached to the Pmod
connectors, JA-JF, and to I2C devices powered
from the I2C daisy chain connectors, J7 and J8.
Each Pmod connector provides power pins
that can be powered from either the switched
main power bus, VCC5V0, or regulated
voltage, VCC3V3, by setting the voltage
jumper block to the desired position. The I2C
connectors only provide the regulated voltage,
VCC3V3.

Pmod™ Connectors

The Cerebot MX7cK has six connectors for
connecting Digilent Pmod peripheral modules.
The Pmod connectors, labeled JA–JF, are 2x6
pin, right-angle, female pin header connectors.
Each connector has an associated power
select jumper block labeled JPA–JPF.

Cerebot MX7cK board, this supply provides the

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Cerebot MX7cK Reference Manual

Digilent Pmods are a line of small peripheral
modules that provide various kinds of I/O
interfaces. The Pmod product line includes
such things as button, switch and LED
modules, connector modules, LCD displays,
high current output drivers, various kinds of RF
interfaces, and many others.

There are two styles of Pmod connector: six­pin and twelve-pin. Both connectors use
standard pin headers with 100mil spaced pins.
The six-pin connectors have the pins in a 1x6
configuration, while the twelve-pin connectors
use a 2x6 configuration. All of the Pmod
connectors on the Cerebot MX7cK are twelve
pin connectors.

Six-pin Pmod connectors provide four I/O
signals, ground and a switchable power
connection. The twelve-pin connectors provide
eight I/O signals, two power and two ground
pins. The twelve-pin connectors have the
signals arranged so that one twelve-pin
connector is equivalent to two of the six-pin
connectors. Pins 1–4 and 7–10 are the signal
pins, pins 5 and 11 are the ground pins and
pins 6 & 12 are the power supply pins.

The pin numbering that Digilent uses on the
twelve-pin Pmod connectors is non-standard.
The upper row of pins are numbered 1–6, left
to right (when viewed from the top of the
board), and the lower row of pins are
numbered 7–12, left to right. This is in keeping
with the convention that the upper and lower
rows of pins can be considered to be two six­pin connectors stacked. When viewed from the
end of the connector, pin 1 is the upper right
pin and pin 7 is immediately below it (closer to
the PCB).

Each Pmod connector has an associated
power select jumper. These are used to select
the power supply voltage supplied to the power
supply pins on the Pmod connector. They are
switchable between either the unregulated
power supply, VCC5V0 or the 3.3V main board
supply, VCC3V3. Place the shorting block in
the 3V3 position for regulated 3.3V and in the
5V0 position to use the unregulated supply.

Each signal pin on the Pmod connectors is
connected to an input/output pin on the PIC32
microcontroller. Each pin has a 200 ohm series
resistor and an ESD protection diode. The
series resistor provides short circuit protection
to prevent damaging the I/O block in the
microcontroller if the pin is inadvertently
shorted to VDD or GND, or two outputs are
shorted together. The ESD protection diode
protects the I/O block from damage due to
electro-static discharge.

The 200 ohm resistor in series with each I/O
pin limits the amount of current that can be
sourced from the microcontroller pins. There
will be a 200mV voltage drop per mA of current
sourced by the pin. This will not be a problem
when driving typical, high impedance, logic
inputs, but can be problematic when trying to
drive low impedance inputs. If some cases, it
may be necessary to use external buffers
when trying to drive low impedance inputs.

Although ESD protection is provided between
the connector pins and the microcontroller
pins, ESD safe handling procedures should be
followed when handling the circuit board. The
pins on the microcontroller and other circuits
on the board are exposed and can be
damaged through ESD when handling the
board.

Digilent Pmod peripheral modules can either
be plugged directly into the connectors on the
Cerebot MX7cK or attached via cables.
Digilent has a variety of Pmod interconnect
cables available.

See the Pinout Tables in Appendix C, below,
for more information about connecting
peripheral modules and other devices to the
Cerebot MX7cK. These tables describe the
mapping between pins on the PIC32MX795
microcontroller and the pins on the various
connectors.

The PIC32 microcontroller can source or sink a
maximum of 18mA on all digital I/O pins.
However, to keep the output voltage within the
specified input/output voltage range (VOL 0.4V,
VOH 2.4V) the pin current must be restricted to

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Cerebot MX7cK Reference Manual

+7/-12mA. The maximum current that can be
sourced or sunk across all I/O pins
simultaneously is +/-200mA. The maximum
voltage that can be applied to any digital I/O
pin is 5.5V. The maximum voltage that can be
applied to any analog input capable pin is

3.6V. For more detailed specifications, refer to
the PIC32MX5XX/6XX/7XX Family Data
Sheet.

Digital Inputs and Outputs

The Cerebot MX7cK board provides access to
48 of the I/O pins from the PIC32MX795
microcontroller via the Pmod connectors. Four
additional I/O pins can be accessed via the I2C
connectors, J7 and J8. Any of the pins on the
Pmod or I2C connectors can be individually
accessed for digital input or output. Note that
when the I2C signals on J7 or J8 are being
used for I2C communications, they are not
available for general purpose I/O.

On PIC32 microcontrollers, the input/output
pins are grouped into I/O Ports and are
accessed via peripheral registers in the
microcontroller. There are seven I/O Ports
numbered A–G and each is 16 bits wide.
Depending on the particular PIC32
microcontroller, some of the I/O Ports are not
present, and not all 16 bits are present in all
I/O Ports.

Each I/O Port has four associated registers:
TRIS, LAT, PORT, and ODC. The registers for
I/O Port A are named TRISA, LATA, PORTA
and ODCA. The registers for the other I/O
Ports are named similarly.

The TRIS register is used to set the pin
direction. Setting a TRIS bit to 0 makes the pin
an output. Setting the TRIS bit to 1 makes the
pin an input.

The LAT register is used to write to the I/O
Port. Writing to the LAT register sets any pins
configured as outputs. Reading from the LAT
register returns the last value written.

The PORT register is used to read from the I/O

the current state of all of the pins in the I/O
Port. Writing to the PORT register is equivalent
to writing to the LAT register.

PIC32 microcontrollers allow any pin set as an
output to be configured as either a normal
totem-pole output or as an open-drain output.
The ODC register is used to control the output
type. Setting an ODC bit to 0 makes the pin a
normal output and setting it to 1 makes the pin
an open drain output.

Refer to the PIC32MX5XX/6XX/7XX Family
Data Sheet, and the PIC32 Family Reference
Manual, Section 12, IO Ports, for more detailed
information about the operation of the I/O Ports
in the microcontroller.

Pmod connector JF, pins 8, 9, and 10 are
connected to the signals TCK/RA1, TDI/RA4,
and TDO/RA5 respectively. These
microcontroller pins are shared between
general purpose I/O functions and use by the
JTAG controller. The JTAG controller is
enabled on reset, so these pins are not
available for general purpose I/O until the
JTAG controller is disabled. The following
statement can be used to disable the JTAG
controller:

DDPCONbits.JTAGEN = 0;

The JTAG controller is disabled by the startup
initialization code in the MPIDE runtime. It is
not necessary to disable the JTAG controller
when using the board with MPIDE.

The chipKIT MPIDE system uses logical pin
numbers to identify digital I/O pins on the
connectors. These pin numbers start with pin 0
and are numbered up consecutively.

On the Cerebot MX7cK, pin numbers 0-47 are
used to access the pins on the Pmod
connectors and pin numbers 55-58 are used
for the signal pins on the I2C connectors, J7
and J8. The pin numbers are assigned so that
connector JA pin 1 (JA-01) is digital pin 0, JA
pin 2 (JA-02) is digital pin 1, and so on.

Port. Reading from the PORT register returns

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Cerebot MX7cK Reference Manual

Pins 0-7 are on connector JA, pins 8-15 on JB,
pins 16-23 on JC, pins 24-31 and so on. Refer
to the tables in Appendix B for detailed
information about the pin mapping between
Pmod connector, logical pin number, and
PIC32 microcontroller pin number and pin
function.

When using the Cerebot MX7cK with the
chipKIT MPIDE the functions pinMode(),
digitalRead(), and digitalWrite() are used for
digital pin I/O.

The pinMode() function is used to set the pin
direction. Pin direction can be set to: INPUT,
OUTPUT, or OPEN. OPEN is used for open­drain and implies output.

The digitalRead() and digitalWrite() functions
are used to read or write the pins.
DigitalRead() returns the current state of the
specified pin, and digitalWrite is used to set the
state of an output pin. The pin state can be
either HIGH or LOW.

Push Buttons and LEDs

The Cerebot MX7cK board provides three
push button switches for user input and four
LEDs for output. The buttons, BTN1 and BTN2
are connected to I/O Port G, bits 6 and 7
respectively. BTN3 is connected to I/O Port A,
bit 0. To read the buttons, pins 6 and 7 of I/O
Port G and pin 0 of I/O Port A must be
configured as inputs by setting the
corresponding bits in the TRISG and TRISA
registers. The button state is then obtained by
reading the PORTG or PORTA registers.
When a button is pressed, the corresponding
bit will be high (‘1’). The pins used by the
buttons are dedicated to this use and do not
appear on any connector.

Button BTN3 is connected to the signal
TMS/RA0 on the PIC32 microcontroller. This
microcontroller pin is shared between general
purpose I/O functions and use by the JTAG
controller. The JTAG controller is enabled on
reset, and so BTN3 is not useable as a button
input until the JTAG controller is disabled. The

following statement can be used to disable the
JTAG controller:

DDPCONbits.JTAGEN = 0;

The JTAG controller is disabled by the startup
initialization code in the MPIDE runtime. It is
not necessary to disable the JTAG controller
when using the board with MPIDE.

The four LEDs are connected to bits 12-15 of
I/O Port G. LED 1 is connected to bit 12, LED 2
is connected to bit 13, and so on. These four
pins are dedicated to use with the LEDs and
do not appear on any connector pin. To use
the LEDs, configure the desired bits as outputs
by clearing the corresponding bits in the
TRISG register. The state of an LED is set by
writing values to the LATG register. Setting a
bit to 1 will illuminate the LED and setting the
bit to 0 will turn it off.

When using the MPIDE and the chipKIT
system, the buttons are accessed using
digitalRead() and the LEDs using digitalWrite().
Use the following pins to access them:

 BTN1 – PIN_BTN1, pin 48, RG6
 BTN2 – PIN_BTN2, pin 49, RG7
 BTN3 – PIN_BTN3, pin 50, RA0
 LD1 – PIN_LED1, pin 51, RG12
 LD2 – PIN_LED2, pin 52, RG13
 LD3 – PIN_LED3, pin 53, RG14
 LD4 – PIN_LED4, pin 54, RG15

5V Signal Compatibility

The PIC32 microcontroller operates at 3.3V.
And the I/O pins provide 3.3V logic levels. It is
possible, in some circumstances, to use the
Cerebot MX7cK to operate with 5V logic
devices however.

There are two issues to consider when dealing
with 5V compatibility for 3.3V logic. The first is
protection of 3.3V inputs from damage caused
by 5V signals. The second is whether the 3.3V
output is high enough to be recognized as a
logic high value by a 5V input.

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Cerebot MX7cK Reference Manual

The digital only I/O pins on the PIC32
microcontroller are 5V tolerant. It is safe to
apply 5V logic signals directly to these pins
without risk of damage to the microcontroller.

The analog capable I/O pins on the PIC32 are
not 5V tolerant. The absolute maximum
voltage rating for the analog pins is 3.6V.
Generally, the analog pins are the pins on I/O

with pins that are not 5V tolerant on the PIC32
microcontroller.

RESET

A reset button is at the upper right corner of
the board. Pressing this button will reset the
PIC32 microcontroller.

port B, however, there are other non-5V
tolerant pins on the device.

Refer to the PIC32MX5XX/6XX/7XX Family
Data Sheet for more information about which
pins on the device are 5V tolerant before
applying input signals higher than 3.3V to any
pin on the Cerebot MX7cK board.

If a 5V signal is applied to a non-5V tolerant
pin, some external means must be used to limit
the applied voltage to 3.6V or less. The
Cerebot MX7cK board provides 200 ohm
series resistors between the microcontroller
pins and the Pmod connector pins. These
resistors are primarily intended to provide short
circuit protection on the outputs, but will also
provide limited protection if a 5V signal is
inadvertently applied to a non-5V tolerant pin.

One technique that can be used to limit an
input voltage to a safe level is to use a 200
ohm series resistor and Shottky diode
connected to the 3.3V supply to clamp the
voltage.

The minimum output high voltage of the PIC32
microcontroller is rated at 2.4V when sourcing
12mA of current. When driving a high
impedance input (typical of CMOS logic) the
output high voltage will be close to 3.3V. Some
5V devices will recognize this voltage as a

logic high input, and some won’t. Many 5V

logic inputs will work reliably with 3.3V inputs.
If the 3.3V logic output is not sufficient for 5V

logic input to be reliably seen as a logic high
input signal, some external means must be
used to raise the output level. In some cases, a
pull-up resistor to 5V is sufficient. A pull-up
resistor in the range of 2Kohm–10kOhm can
be used. This technique should not be used

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Cerebot MX7cK Reference Manual

PIC32 Peripheral Devices

The following describes the peripheral devices
available in the PIC32MX795F512L
microcontroller and how they are accessed on
the Cerebot MX7cK board.

CPU Clock Source

The PIC32 microcontroller supports numerous
clock source options for the main processor
operating clock. The Cerebot MX7cK board is
designed to operate with either a silicon
resonator from Discera, IC2, for use with the
EC oscillator option, or an external crystal, X1,
for use with the XT oscillator option. Standard
production boards will have an 8Mhz Discera
silicon resonator loaded and the EC oscillator
option should be used. If IC2 is not loaded, an
8Mhz crystal will be loaded for X1 (on the
bottom of the board) and the XT oscillator
option should be used. Oscillator options are
selected via the configuration settings specified
using the #pragma config statement. Use
#pragma config POSCMOD=EC to select the
EC option and #pragma config
POSCMOD=XT to select the XT option.

Using the internal system clock phase-locked
loop (PLL), it is possible to select numerous
multiples or divisions of the 8Mhz oscillator to
produce CPU operating frequencies up to
80Mhz. The clock circuit PLL provides an input
divider, multiplier, and output divider. The
external clock frequency (8Mhz) is first divided
by the input divider value selected. This is
multiplied by the selected multiplier value and
then finally divided by the selected output
divider. The result is the system clock,
SYSCLK, frequency. The SYSCLK frequency
is used by the CPU, DMA controller, interrupt
controller and pre-fetch cache.

The operating frequency is selected using the
PIC32MX795 configuration variables. These
are set using the #pragma config
statement. Use #pragma config FPLLIDIV
to set the input divider, #pragma config

FPLLMUL to set the multiplication factor and
#pragma config FPLLODIV to set the

output divider. Refer to the
PIC32MX5XX/6XX/7XX Family Data Sheet
and the PIC32MX Family Reference Manual,
Section 6. Oscillators for information on how to
choose the correct values, as not all
combinations of multiplication and division
factors will work.

In addition to configuring the SYSCLK
frequency, the peripheral bus clock, PBCLK,
frequency is also configurable. The peripheral
bus clock is used for most peripheral devices,
and in particular is the clock used by the
timers, and serial controllers (UART, SPI, I2C).
The PBLCK frequency is a division of the
SYSCLK frequency selected using #pragma

config FPBDIV. The PBCLK divider can be

set to divide by 1, 2, 4, or 8.
The following example will set up the Cerebot

MX7cK for operation with a SYSCLK frequency
of 80Mhz and a PBCLK frequency of 10Mhz:

#pragma config FNOSC = PRIPLL
#pragma config POSCMOD = EC
#pragma config FPLLIDIV = DIV_2
#pragma config FPLLMUL = MUL_20
#pragma config FPLLODIV = DIV_1
#pragma config FPBDIV = DIV_8

Documentation for the PIC32 configuration
variables can be found in the PIC32MX
Configuration Settings guide. This is found
using the “Help.Topics…” command in the
MPLAB IDE. Also, refer to Appendix C for an
example of setting the configuration variables.

When using the Cerebot MX7cK with the
chipKIT MPIDE software, the clock source is
set by the boot loader and no action is
required.

Ethernet Interface

The Cerebot MX7cK provides the ability to
interface with 10Mbps or 100Mbps Ethernet
networks. The PIC32MX795 microcontroller
contains a 10/100 Ethernet Medium Access
Controller (MAC). External to the PIC32
microcontroller, the Cerebot MX7cK board
provides an SMSC LAN8720 Ethernet Physical

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