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Cerebot MC7™ Board Reference Manual
RC Servos use a pulse width modulated signal, PWM, to control the servo position. The dsPIC33F microcontroller
provides eight output compare units that can be used to generate the necessary pulse width modulated signal.
PORTD bits 0-7 correspond to output compare unit outputs OC1-OC8. The output compare units can be operated
from either Timer 2 or Timer 3. It is also possible to use timer interrupts to accomplish the generation of the PWM
outputs to drive the servo connectors. Using timer interrupts allows a single timer to be used to control the signal
timing for all eight servo connectors. This can be useful if some timers need to be used for other purposes.
The pulse width modulation timing varies between servo manufacturers. Generally, a pulse width of 1000us (1
milli-second) corresponds to the center of rotation, with a 500us (0.5ms) pulse width being full rotation in one
direction and 1500us (1.5ms) being full rotation in the other direction. Servo manufacturers also vary on which
direction of rotation is produced by narrower and wider pulses. Finally, some manufacturers produce continuous
rotation servos. These generally require wider pulses. For these servos, a 1500us pulse width will result in the
servo stopped and no rotation, 500us will produce rotation at full speed in one direction and 2500us full speed
rotation in the other direction. This varies widely between manufacturers, however. Refer to manufacturer data
sheets for the servos, or experiment to determine how specific servos operate.
RC hobby servos generally operate at 5V. Many are specified for operation at up to 6V, and some specialty servos
require higher voltages. There are two power supply options for providing power to the servo connectors: The on-
board 5V switching regulator, or an external power supply.
For the first case: Install shorting blocks on JP5 and JP8 to connect the VS servo power bus to the REG5V0 power
bus. This connection allows for operation from a single high voltage power supply and allowing use of the servos.
There are two jumpers, as the current rating on these jumper blocks is 2A, and the eight servos can potentially
draw up to 4A of current from the REG5V0 power bus.
For operation from an external servo power supply, remove the shorting blocks from jumpers JP5 and JP8 and
connect the external power supply to screw terminal connector J4. An external power supply should not be
connected to J4 without removing the shorting blocks from JP5 and JP8. Be sure to observe the proper polarity
when connecting the external power supply.
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9
I
C Interfaces
2
The Inter-Integrated Circuit (I
C
) Interface provides a medium speed (100K or 400K bps) synchronous serial
TM
2
communications bus. The I
C interface provides master and slave operation using either 7 bit or 10 bit device
addressing. Each device is given a unique address, and the protocol provides the ability to address packets to a
specific device or to broadcast packets to all devices on the bus. Refer to the data sheet for the dsPIC
microcontroller and the dsPIC33F Family Reference Manual for detailed information on configuring and using the
2
I
C interface.
2
The dsPIC33F microcontroller provides two independent I
C interfaces. There are two sets of connectors in the
2
2
lower left corner of the board for access to the two I
C ports. Connector J6 provides access to I
C port #1 while
2
connector J8 provides access to I
C port #2.
2
Each I
C connector provides two positions for connecting to the I2C signals, ground and the 3.3V power supply. By
using two-wire or four-wire MTE cables (available separately from Digilent) a daisy chain of multiple Cerebot MC7
2
boards or other I
C-capable boards can be created.
2
2
The I
C bus is an open-collector bus. Devices on the bus actively drive the signals low. The high state on the I
C
2
signals is achieved by pull-up resistors when no device is driving the lines low. One device on the I
C bus must
Copyright Digilent, Inc. All rights reserved.
Page 8 of 22
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