Digilent 410-295P-KIT - Manual (Page 13)

ChipKIT Pro MX4 Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page 13 of 37
are rated for a maximum of 2A of current. USB power (J12 in the USB, DBG, or URT positions) should only be used
to power a couple of servos to avoid exceeding the 500mA that a USB device is allowed to use.
For the second case above: Remove the shorting block from jumper JP2 to make the VS servo power bus
independent from the BRD_VU bus. Attach the servo power supply to screw terminal connector J5.
Finally, for very high servo current applications, a separate power bus external to the ChipKIT Pro MX4 can be used
to provide servo power. In this case, remove the shorting block on JP1, tie the external servo power bus ground to
the ChipKIT Pro MX4 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.
6 5V Signal Compatibility
The PIC32 microcontroller operates at 3.3V. And the I/O pins provide 3.3V logic levels. It is possible, in some
circumstances, to use the ChipKIT Pro MX4 to operate with 5V logic devices.
There are two issues to consider when dealing with 5V compatibility for 3.3V logic. The first is protection of 3.3V
inputs from damage caused by 5V signals. The second is whether the 3.3V output is high enough to be recognized
as a logic high value by a 5V input.
The digital I/O pins on the PIC32 microcontroller are 5V tolerant. It is safe to apply 5V logic signals directly to these
pins without risk of damage to the microcontroller.
The analog capable I/O pins on the PIC32 are not 5V tolerant. The absolute maximum voltage rating for the analog
pins is 3.6V. Generally, the analog pins are the pins on I/O port B, however, there are other non-5V tolerant pins on
the device.
Refer to the PIC32MX3XX/4XX Family Data Sheet for more information about which pins on the device are 5V
tolerant before applying input signals higher than 3.3V to any pin on the ChipKIT Pro MX4 board.
If a 5V signal is applied to a non-5V tolerant pin, some external means must be used to limit the applied voltage to
3.6V or less. The ChipKIT Pro MX4 board provides 200 ohm series resistors between the microcontroller pins and
the Pmod connector pins. These resistors are primarily intended to provide short circuit protection on the outputs,
but will also provide limited protection if a 5V signal is inadvertently applied to a non-5V tolerant pin.
One technique that can be used to limit an input voltage to a safe level is to use a 200 ohm series resistor and
Shottky diode connected to the 3.3V supply to clamp the voltage.
The minimum output high voltage of the PIC32 microcontroller is rated at 2.4V when sourcing 12mA of current.
When driving a high impedance input (typical of CMOS logic) the output high voltage will be close to 3.3V. Some 5V
devices will recognize this voltage as a logic high input, and some won’t. Many 5V logic inputs will work reliably
with 3.3V inputs.
If the 3.3V logic output is not sufficient for the 5V logic input to be reliably seen as a logic high input signal, some
external means must be used to raise the output level. In some cases, a pull-up resistor to 5V is sufficient. A pull-up
resistor in the range of 2Kohm–10kOhm can be used. This technique should not be used with pins that are not 5V
tolerant on the PIC32 microcontroller.