Digilent Cerebot II Reference Manual - Page 4

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Cerebot II Reference Manual
the amount of current being used is close to its
limit.
Power Supply Monitor Circuit
The Cerebot II microcontroller can measure
the power supply voltage on the VU and VS
power busses using the provided power supply
monitor circuits. This feature is especially
useful when using batteries because it allows
the microcontroller firmware to determine the
charge state of the battery and potentially
notify the user when a battery supply is low.
Each power supply monitor circuit made up of
a voltage divider that divides the power bus
voltage by four, and a zener diode to clamp the
resulting voltage to no greater than 3.3V.
Jumper JP3 enables the supply monitor circuit
for VU power, and jumper JP4 enables the
supply monitor circuit for VS power. The
analog to digital converter built into the
ATmega64 is used to measure the power
supply voltages. ADC channel 0 is used to
measure VU and ADC channel 1 is used to
measure VS.
When the power supply monitor circuit is
enabled the maximum safe voltage on VU is
9V and the maximum safe voltage on VS is
12V.
RC Servo Connectors
The Cerebot II provides eight 3-pin RC hobby
servo connectors for direct control of servos in
robotics and embedded hardware actuator
applications. The connectors share I/O pins
with Pmod connector JH on the left lower side
of the Cerebot II. Individual I/O pins may be
accessed through the JH header if they're not
in use by a servo. Refer to the ATmega64
data sheet for information on how to access
the I/O pins.
The I/O pins shared between the servo
connectors and connector JH are the analog to
digital converter inputs on the ATmega64
microcontroller. If servos are being driven on
some channels and analog voltages are being
read on other pins simultaneously, it is
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possible for digital switching noise to reduce
the accuracy of the analog to digital
conversions. If this is an issue, perform the
analog-to-digital conversions at times when the
servo pins are not switching. In normal
applications, there will be a great deal of dead
time when the servo pins are not switching.
There are three power options for servo
connections: a common power bus (VU) for the
Cerebot II and servos, separate on-board
power busses (VU and VS) for the Cerebot II
and servos, and an external power bus for
servos.
Install the shorting block on JP2 to connect the
VS servo power bus to the VU power bus. The
VU bus can be powered from the coax power
connector, J7, the screw terminal connector,
J9, or the 2-pin battery connector, J8.
The VU bus can also be powered from any of
the Pmod header interface connectors by
setting the corresponding power jumper block
to the VU position. This option is not suitable
for providing power for large numbers of
servos or servos that have a high current
demand.
Remove the shorting block from jumper JP2 to
make the VS servo power bus independent
from the VU bus. In this case, the VS bus is
powered from screw terminal connector J10.
Finally, for very high current applications, a
separate power bus external to the Cerebot II
can be used to provide servo power. In this
case, remove the shorting block on JP2, tie the
external servo power bus ground to the
Cerebot II ground through the ground terminal
on J10, and use pin 1 on the servo connectors
to bring the servo control signals out to the
servos. The servo power and ground
connections are made off-board.
The on-board servo power bus can be used to
provide a maximum of 2A to each servo
connector and 5A total to all servo connectors.
Digilent, Inc.
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