Microchip Technology MCU PIC PIC18F87K22-I/PTRSL TQFP-80 MCP PIC18F87K22-I/PTRSL Data Sheet

DS39960D-page 398

 2009-2011 Microchip Technology Inc.

Time can be precisely measured after the ratio (C/I) is
measured from the current and capacitance calibration
step. To do that:
1.

Initialize the A/D Converter and the CTMU.

2.

Set EDG1STAT.

3.

Set EDG2STAT.

4.

Perform an A/D conversion.

5.

Calculate the time between edges as T = (C/I) * V,
where:
• I is calculated in the current calibration 

step (

)

• C is calculated in the capacitance calibra-

tion step (

)

• V is measured by performing the A/D conversion

It is assumed that the time measured is small enough
that the capacitance, C

OFFSET

, provides a valid voltage

to the A/D Converter. For the smallest time measure-
ment, always set the A/D Channel Select register
(AD1CHS) to an unused A/D channel, the correspond-
ing pin for which is not connected to any circuit board
trace. This minimizes added stray capacitance, keep-
ing the total circuit capacitance close to that of the A/D
Converter itself (25 pF).
To measure longer time intervals, an external capacitor
may be connected to an A/D channel and that channel
selected whenever making a time measurement.

A/D Converter

CTMU

CTED1

CTED2

AN

X

Output  Pulse

EDG1

EDG2

C

AD

R

PR

Current Source