Digilent 410-295P-KIT - Manual (Page 21)

ChipKIT Pro MX4 Reference Manual
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Page 21 of 37
13 A/D Converter Voltage Reference
The PIC32 microcontroller provides two voltage reference inputs to the analog to digital converter. Vref- is used to
set the lower reference level and Vref+ is used to set the upper reference level. These reference inputs can be
connected to internal references or to external references using two of the analog input pins.
When the internal references are being used, Vref- is connected to VSS and Vref+ is connected to VDD. This means
that the voltage input range at the analog input pins is 0V–3.3V. In this case, an input voltage of 0V will convert to
~0, an input voltage of 1.65V will convert to ~511, and an input voltage of 3.3V will convert to ~1023.
Either one, or both, of the references can be connected to external reference pins. When this is done, the
references can be set to voltages other than 0V and 3.3V.
If, for example, both references were selected to use external references, with 1V applied to Vref- and 2V applied
to Vref+, the input voltage range at the analog input pins would be from 1V to 2V. An applied voltage of 1V would
have a converted value of ~0, 1.5V would have a converted value of ~511, and 2V would have a converted value of
~1023.
When both external references are being used, Vref+ must have a higher voltage applied to in than Vref-.
The analog reference input pins appear on Pmod connector JK, pins 7 & 8. Vref- is on pin JK-07, and Vref+ is on pin
JK-08. These pins are not available to be used for digital I/O when being used as an external reference.
When using the MPIDE software, the use of external analog references is selected using the analogReference()
function. The following values can be used with analogReference():
• DEFAULT – Vref- = 0V, Vref+ = 3.3V
• INTERNAL – same as default
• EXTERNAL – Vref- = 0V, Vref+ = voltage at A2
• EXTMINUS – Vref- = voltage at A3, Vref+ = 3.3V
• EXTPLUSMINUS – Vref- = voltage at A3, Vref+ = voltage at A2
14 Timers
The PIC32 microcontroller provides five timers that can be used for various timing functions. These timers are each
16 bits wide, although two pairs, TIMER2/TIMER3 and TIMER4/TIMER5 can be combined to produce 32 bit wide
timers.
A timer consists of a control register, a counter register, and a period register. The control register is used to
configure the timer for various modes of operation. The count register counts cycles of the clock source selected
via the control register. This clock source can be the peripheral bus clock or a division of the peripheral bus clock
via a pre-scaler divider. The period register can be used to generate an interrupt and/or reset the count register
when a pre-determined value is reached.
Each timer has an associated input pin. In some operating modes, this pin can be used as an external clock input to
the timer, or as a gate input to turn on/off incrementing of the counter register under control of an external signal.
The following gives the Pmod connector position, chipKIT pin number, and microcontroller I/O port and bit number
for the timer input pins.
• TCK1 – not available
• TCK2 – JD-04, digital pin 27, RC01
• TCK3 – JD-10, digital pin 31, RC02