Digilent 32MX7 User Manual

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Cerebot 32MX7 Circuit Diagram

Revision: October 20, 2011
Note: This document applies to REV C of the board.

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Overview

The Cerebot 32MX7 board is a useful tool for
embedded control and network
communications projects for both students and
hobbyists.

Its versatile design and programmable
microcontroller lets you access numerous
peripheral devices and program the board for
multiple uses. The board has many I/O
connectors and power supply options. It’s
network and communications features include
10/100 Ethernet interface, Full Speed USB 2.0
OTG interface, dual CAN network interfaces,
dual I2C buses, up to three UART ports and up
to three SPI ports.

The Cerebot 32MX7 works with the Microchip
MPLAB development environment and
provides built in programming and debugging
support within MPLAB.

The Cerebot 32MX7 provides a number of
connections for peripheral devices. It has six
connectors for attaching Digilent Pmod™
peripheral modules. Digilent Pmod peripheral
modules include H-bridges, analog-to-digital
and digital-to-analog converters, a speaker
amplifier, switches, buttons, LEDs, as well as
converters for easy connection to RS232,
screw terminals, BNC jacks, servo motors, and
more.

Features include:

• a PIC32MX795F512L microcontroller

• support for programming and

debugging within the Microchip MPLAB
development environment

• six Pmod connectors for Digilent
peripheral module boards

• 10/100 Ethernet

• USB 2.0 Device, Host, and OTG

support

• two CAN network interfaces

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• three push buttons

• four LEDs

• multiple power supply options, including

USB powered

• ESD protection and short circuit
protection for all I/O pins.

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Cerebot 32MX7 Reference Manual

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

For more information on the
PIC32MX795F512L microcontroller, refer to
the PIC32MX5XX/6XX/7XX Family Data Sheet
and the PIC32 Family Reference Manual
available at www.microchip.com.

Functional Description

The Cerebot 32MX7 is designed for embedded
control and network communications
applications as well as general microprocessor
experimentation. Firmware suitable for many
applications can be downloaded to the Cerebot
32MX7’s programmable PIC32 microcontroller.

The board has a number of input/output
connection options, and is specially designed
to work with the Digilent line of Pmod
peripheral modules with 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 32MX7 can be used with the
Microchip MPLAB development environment.
In-system-programming and debug of firmware

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running on the PIC32MX795 microcontroller is
supported using an on-board program/debug
circuit licensed from Microchip.

The Cerebot 32MX7 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 either the debug USB
port or the USB device port, or it can be
powered from an external power supply or
batteries.

Programming and In-System
Debugging Using the MPLAB® IDE

The Cerebot 32MX7 board is intended to be
used with the Microchip MPLAB® IDE for
firmware development, programming and in­system debugging using a circuit licensed from
Microchip. MPLAB version 8.63 or later is
required for use of the on-board
program/debug circuit. The licensed debugger
is accessed via USB, using connector J15.
This connector is a micro-USB connector on
the lower left side of the board, near the power
switch. The provided USB cable should be
connected from J15 to a USB port on the
development PC for access to the board.

When creating a new project, use the
Configure.Select Device menu to specify the
PIC32 device in use. Ensure that the device is
set to PIC32MX795F512L.

To use the on-board program/debug 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 in-system programming/debugging
interface uses two pins on the PIC32
microcontroller. The PIC32 devices support
two alternate pin pairs for this interface:
PGC1/PGD1 or PGC2/PGD2. PIC32 devices
use PGC2/PGD2 by default. Due to conflicting
uses of the microcontroller pins, the Cerebot

Cerebot 32MX7 Reference Manual

32MX7 is designed to use PGC1/PGD1.
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:

#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.

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.

There are three power options for main power
to the board: USB powered from the debug
USB connector, USB powered from the USB
device connector, or external, non-USB
powered. Jumper block J16, (above the
Ethernet connector, J11) is used to select the
main power source. To select USB powered
from the debug connector, place the shorting
block in the DBG position. To select USB
power from the USB device connector, place
the shorting block in the USB position. This
option is used when the board is being used to
implement a bus powered USB device. To
power the board from an external power
supply, place the shorting block in the EXT
position. The board comes from the factor
jumpered for USB power from the debug USB
connector.

When powering the board from an external
power supply, there are two power supply
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 an optional 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.

The Cerebot 32MX7 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. When operating the board
from an external supply with a voltage less
than 5V, some features of the board won’t
work correctly.

When the Cerebot 32MX7 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
Cerebot 32MX7 board, this supply provides 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.

Connectors J17, and J18 are wired in parallel
and connect to the “External Power” position
(center position) on the Power Select jumper
block J16. A shorting block should be placed
on the “EXT” position of J16 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.

supply available, the 5V Switching Power

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Cerebot 32MX7 Reference Manual

The power supply selected by the shorting
block on J16 will appear on the input power
supply bus, labeled VIN in the schematic. This
voltage is regulated to 3.3V to power the
debug circuit by IC11, a Microchip MCP1801
Low Dropout voltage regulator. This regulator
is turned on and the debug circuit is powered
whenever the power switch is in the on
position.

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 debug 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 switched on 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.

Note: The signal labeled DBG5V0 on the
schematic comes from the debug USB
connector. If the debug USB connector is not
connected to a live USB port, this voltage will
not be present and the debug circuit is not
involved in turning on board power. In this
case, the board power is turned on when the
power switch is placed in the ON position.

The PIC32 microcontroller and on-board I/O
devices operate at a supply voltage of 3.3V
provided by the VCC3V3 bus. The regulated
voltage on this bus is provided by a Microchip
MCP1726 Low Dropout voltage regulator,
IC10. This regulator is capable of providing a
maximum of 1A 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 voltage regulator is on the
bottom of the board, approximately under the
“3” in the Cerebot 32MX7 logo, and will get
warm when the amount of current being used
is close to its limit.

The Cerebot 32MX7 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 power connectors only provide the
regulated voltage, VCC3V3.

USB Interface

The PIC32MX795 microcontroller contains a
USB 2.0 Compliant, Full Speed Device and
On-The-Go (OTG) controller. This controller
provides the following features:

• USB full speed host and device support

• Low speed host support

• USB OTG support

• Endpoint buffering anywhere in system

RAM

• Integrated DMA to access system RAM
and Flash memory.

The USB controller uses a phased lock loop,
PLL, to generate the necessary USB clock
frequency from the external primary oscillator
input frequency. By default, this PLL is
disabled. In order to use the USB controller, it
is necessary to enable the USB PLL, and set
the input divider to the correct value to
generate a valid USB clock. The input to the
USB PLL must be 4Mhz. The Cerebot 32MX7
provides an 8Mhz clock to the PIC32

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Cerebot 32MX7 Reference Manual

microcontroller, so a USB PLL input divider
value of 2 must be used. These parameters
are set in the PIC32 microcontroller
configuration registers using the

config

statement. The following statements

#pragma

must be used to configure the PIC32
microcontroller for use of the USB controller:

#pragma config UPLLEN = ON
#pragma config UPLLIDIV = DIV_2

When operating as a USB device, the Cerebot
32MX7 can be used as a self powered device
or as a bus powered device. To operate as a
self powered device, an external power supply
should be connected to one of the external
power connectors (J17 or J18) and a shorting
block placed on the center, “EXT” position of
J16. The external power supply must be a
regulated 5V supply. To operate as a bus
powered device, the shorting block should be
placed in the USB Device position, “USB”, on
J16.

Connector J19, on the bottom of the board in
the lower right corner is the Device/OTG
connector. This is a standard USB micro-AB
connector. Connect a cable with a micro-A
plug (optionally available from Digilent) from
this connector to an available USB port on a
PC or USB hub for device operation.

When operating as a USB host, the Cerebot
32MX7 must be externally powered. Connect
a regulated 5V power supply to one of the
external power connectors (J17, or J18) and
ensure that the shorting block is in the center,
“EXT” position of J16. The power supply used
must be a regulated 5V supply. The Cerebot
32MX7 board provides power to the attached
USB device when operating as a host, and the
USB specification requires the use of a 5V
power supply. NOTE: Providing a voltage
greater than 5V can damage the Cerebot
32MX7 board and/or the USB device being
used.

Jumper JP10 is used to route power to the
host connector being used. Place the shorting
block in the “A” position when using the

Place the shorting block in the “MICRO”
position for use with the USB micro-AB (OTG)
connector, J19.

When operating as a USB host, the
PIC32MX795 microcontroller controls
application of power to the connected device
via the VBUSON control pin (labeled
P32_VBUSON in the schematic). Bus power
is applied to the device by driving the VBUSON
pin high. Power is removed from the device by
driving the VBUSON pin low. The VBUSON
pin is accessed via bit 3 of the U1OTGCON
register.

The VBUSON pin drives the enable input of a
TPS2051B Current-Limited Power Distribution
Switch to control the application of USB power
to the host connector. This switch has over­current detection capability and provides an
over-current fault indication by pulling the
signal P32_USBOC low. The over-current
output pin can be monitored via the INT1/RE8
pin on the PIC32MX795 microcontroller.
Details about the operation of the TPS2051B
can be obtained from the data sheet available
at the Texas Instruments web site.

There are reference designs available on the
Microchip web site demonstrating both device
and host operation of PIC32 microcontrollers.
These reference designs are suitable to use for
developing USB firmware for the Cerebot
32MX7 board.

Ethernet Interface

The Cerebot 32MX7 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
microcontroller, the Cerebot 32MX7 board
provides an SMSC LAN8720 Ethernet Physical
Layer Transceiver (PHY). Together, the MAC
and PHY in combination with an appropriate
coupling transformer and RJ45 jack provide a
standard 10/100 Ethernet interface.

standard USB type A (host) Connector, J20.

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Cerebot 32MX7 Reference Manual

The RJ45 connector J11, provides the physical
connection to an Ethernet network using a
standard Ethernet cable.

All devices on an Ethernet network must have
a unique address. This address is used to
direct packets on the network to a specific
device and to identify the device that originated
a packet. An Ethernet MAC uses a 48-bit
address value, commonly called the “MAC
Address”. These address values are globally
unique to ensure that no two devices on a
network can have conflicting addresses. MAC
addresses are assigned by the IEEE. The
address to use with the Cerebot 32MX7 is
printed on a sticker attached to the bottom of
the board. The address is a twelve digit
hexadecimal number of the form:
00183Exxxxxx, where xxxxxx represents six
hexadecimal digits. This value is used to
initialize the Ethernet Controller MAC Station
Address registers in the Ethernet controller of
the PIC32MX795 microcontroller.

In order to connect to and operate with an
Ethernet network, the PIC32 microcontroller
must be running network protocol stack
firmware. Normally, the TCP/IP (Transmission
Control Protocol/Internet Protocol) network
protocol is used and “TCP/IP Stack” software
must be used. The Microchip Applications
Library, available for download from the
Microchip web site provides full protocol stack
support compatible with the PIC32MX795 MAC
and the LAN8720 PHY. Microchip also
provides numerous example programs
illustrating the use of their network protocol
stack for various applications.

When not using the Microchip network protocol
stack, refer to the manufacturer documentation
for the PIC32MX795 and LAN8720, plus
network protocol documentation, for operation
of the Ethernet interface.

The PIC32MX795 microcontroller provides two
alternate sets of pins that can be used to
connect the MAC to the external PHY. It also
provides two alternate standard MAC/PHY
interface signaling conventions. The Cerebot

the alternate) pins, and to use the RMII (not
the MII) interface signaling convention. These
options are selected using the configuration
variables in the PIC32 microcontroller and are
specified using the

#pragma config

statement. To enable the Ethernet controller in
the correct configuration, the following
statements must appear in the main program
module:

#pragma config FETHIO=ON
#pragma config FMIIEN=OFF

The LAN8720 PHY has a reset signal, labeled
NRST in the schematic, that can be used to
reset the PHY. This signal is connected to the
INT2/RE9 pin on the PIC32 microcontroller.
The NRST signal is active low. Configure the
microcontroller pin as an output and drive it low
to reset the PHY, or drive it high to allow the
PHY to come out of reset and begin operation.
The NRST signal is pulled low on the board, so
that the PHY is held in reset by default. To
allow the PHY to operate, this pin must be
driven high. This reset operation is not part of
the Microchip network protocol stack, and so
driving NRST high must be done before
initializing the Microchip network stack.

CAN Interfaces

The Controller Area Network (CAN) standard is
a control networking standard originally
developed for use in automobile systems, but
has since become a standard used in various
industrial control and building automation
networking applications as well.

The PIC32MX795 microcontroller contains two
independent CAN network controllers. These
CAN controllers in combination with two
Microchip MCP2551 CAN transceivers allow
the Cerebot 32MX7 board to operate on up to
two independent CAN networks. Refer to the
PIC32MX7XX data sheet and the PIC32
Family Reference Manual, plus CAN network
documentation for information on operation of
the CAN controllers and CAN networking in
general.

32MX7 is designed to use the standard (not

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