Microchip Technology MA160014 Data Sheet

 2010-2012 Microchip Technology Inc.

DS41412F-page 331

Time can be precisely measured after the ratio (C/I) is
measured from the current and capacitance calibration
step by following these steps:

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 calibration step
(

) and

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 (4-5 pF). To measure longer time
intervals, an external capacitor may be connected to an
A/D channel and this channel selected when making a
time measurement.

A/D Converter

CTMU

CTED1

CTED2

AN

X

Output  Pulse

EDG1

EDG2

C

AD

R

PR

Current Source