Cerebot MC7™ Board Reference Manual
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These outputs from the bridges are available on connectors J14 & J15 or J16 & J18. Connectors J14 & J15 are
screw terminal connectors for wire attachment. Connectors J16 & J18 are 6-pin JST connectors used by the gear
head motors available from Digilent.
Connectors J14 & J16 provide the outputs VMOT1 and VMOT2, while J15 & J18 provide the outputs VMOT3 and
VMOT4.
5
Current and Voltage Feedback
Each of the four half bridges is configured for monitoring the current flow through the low side transistor of the
bridge. A voltage proportional to the bridge current is applied to one of the A/D inputs on the dsPIC
microcontroller for each half bridge.
A 10 milli-ohm (0.01 ohm), 1% current sense resistor is connected between the source of each low side FET and
ground. A differential amplifier with a fixed gain of 21 and an offset voltage of 1.0V is used to buffer and amplify
the voltage across the current sense resistors before being applied to the inputs to the A/D converter. The
following formula relates the voltage applied to the A/D converter input to the sensed bridge current:
V = (0.21 * I) + 1.0
Refer to Appendix C for tables showing data measured from an example board.
In addition to the bridge current, the output voltage of each half bridge can be monitored as well. The output
voltage of each half bridge can be connected via a voltage divider to one of the inputs to the A/D converter. The
voltage divider ratio is as follows:
Vo = 2.2/17.2 * Vi = 0.128 * Vi
The voltage monitoring can be enabled/disabled via jumpers JP9-JP12.
The current/voltage monitoring signals are connected to the A/D converter inputs in the following way:
IMON1
AN0
IMON2
AN1
IMON3
AN2
IMON4
AN3
VMON1
AN10
VMON2
AN11
VMON3
AN12
VMON4
AN13
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Over-current Detection
The Cerebot MC7 provides an over-current monitoring mechanism for detecting excessive current flow through
any of the half bridge circuits.
Four comparators are used to monitor the current measurement from each half bridge compared to a fixed
reference voltage. The outputs of these comparators are OR’d together and applied to external interrupt INT1 on
the dsPIC microcontroller.
