Digilent MX7cK - Manual (Page 5)

Cerebot MX7cK Reference Manual
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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