Cerebot MX7cK Reference Manual
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page 13 of 34
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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.
The Digilent chipKIT network libraries perform
this automatically, and so no action is
necessary when using the board with the
MPIDE software system and the Digilent
libraries.
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 MX7cK
provides an 8Mhz clock to the PIC32
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
#pragma
config
statement. The following statements
must be used to configure the PIC32
microcontroller for use of the USB controller:
#pragma config UPLLEN = ON
#pragma config UPLLIDIV = DIV_2
When using the chipKIT MPIDE development
environment, these will have been set by the
boot loader, so no action is needed.
When operating as a USB device, the Cerebot
MX7cK 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 “EXT” position of J3. To
operate as a bus powered device, the shorting
block should be placed in the USB Device
position, “USB”, on J3.
Note that when operating as a bus powered
device, the Cerebot MX7cK and all devices
connected to it are limited to using no more
than 500mA of current to remain in compliance
with the USB specification.
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
MX7cK 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, “EXT”
position of J3. The power supply used must be
a regulated 5V supply. The Cerebot MX7cK
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 MX7cK
board and/or the USB device being used.
Connector J20, on the top of the board in the
lower right corner is the USB host connector.
This is a standard USB type A receptacle. USB
devices may be connected either directly or
through a standard USB cable.
Jumper JP10 is used to route power to the
host connector being used. Place the shorting
