Cerebot 32MX7 Reference Manual
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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 I
2
C
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 I
2
C devices
powered from the I
2
C 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 I
2
C 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
