Cpu Clock Source - Digilent chipKIT Pro MX7 Reference Manual

chipKIT™ Pro MX7 Board Reference Manual
 LD1 – PIN_LED1, pin 51, RG12
 LD2 – PIN_LED2, pin 52, RG13
 LD3 – PIN_LED3, pin 53, RG14
 LD4 – PIN_LED4, pin 54, RG15
4.2 5V Signal Compatibility
The PIC32 microcontroller operates at 3.3V and the I/O pins provide 3.3V logic levels; however, it is possible in
some circumstances to use the chipKIT Pro MX7 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 only 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 PIC32MX5XX/6XX/7XX 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 MX7 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 MX7 board provides 200 ohm series resistors between the microcontroller pins and
the Pmod port 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
Schottky 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 Pro MX7 board is designed to operate with either a silicon resonator from Discera, IC2, for use with the EC
oscillator option, or an external crystal, X1, for use with the XT oscillator option. Standard production boards will
have an 8 MHz Discera silicon resonator loaded and the EC oscillator option should be used. If IC2 is not loaded, an
8 MHz crystal will be loaded for X1 (on the bottom of the board) and the XT oscillator option should be used.
Oscillator options are selected via the configuration settings specified using the #pragma config statement.
Use #pragma config POSCMOD=EC to select the EC option and #pragma config POSCMOD=XT to select
the XT option.
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