Digilent 410-297P-KIT - Manual (Page 10)

ChipKIT MX3 Board Reference Manual
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Page 10 of 25
4.1 User LEDs
Two LEDs are provided, LD4 and LD5, connected to I/O port F bits 0 and 1 (RF00 and RF01). LATF Bit 0 is connected
to LD4 and bit 1 is connected to LD5. The LEDs are turned on or off by configuring these two pins as outputs and
driving them high or low. Driving the pin high turns the LED on, driving it low turns it off. These I/O pins are
dedicated to use with the LEDs and are not available at any connector.
When using the MPIDE and the chipKIT system, the LEDs are accessed as digital pins 42 (LD4) and 43 (LD5), or
preferably, using the symbols PIN_LED1 and PIN_LED2.
4.2 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 MX3 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 MX3 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 MX3 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 and input voltage to a safe level is to use a 200 ohm series resistor and
Shotkey 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 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.
5 CPU Clock Source
The PIC32 microcontroller supports numerous clock source options for the main processor operating clock. The
ChipKIT MX3 uses an 8Mhz external crystal for use with the XT oscillator option. Oscillator options are selected via