PIC18F23K22-ESLASHML datasheet(318/496 Pages) MICROCHIP

318 / 496 page

PIC18(L)F2X/4XK22

DS41412C-page 318

Preliminary

 2010 Microchip Technology Inc.

19.1

CTMU Operation

The CTMU works by using a fixed current source to

charge a circuit. The type of circuit depends on the type

of measurement being made. In the case of charge

measurement, the current is fixed, and the amount of

time the current is applied to the circuit is fixed. The

amount of voltage read by the A/D is then a measure-

ment of the capacitance of the circuit. In the case of

time measurement, the current, as well as the capaci-

tance of the circuit, is fixed. In this case, the voltage

read by the A/D is then representative of the amount of

time elapsed from the time the current source starts

and stops charging the circuit.

If the CTMU is being used as a time delay, both

capacitance and current source are fixed, as well as the

voltage supplied to the comparator circuit. The delay of

a signal is determined by the amount of time it takes the

voltage to charge to the comparator threshold voltage.

19.1.1

THEORY OF OPERATION

The operation of the CTMU is based on the equation

for charge:

More simply, the amount of charge measured in

coulombs in a circuit is defined as current in amperes

(I) multiplied by the amount of time in seconds that the

current flows (t). Charge is also defined as the

capacitance in farads (C) multiplied by the voltage of

the circuit (V). It follows that:

The CTMU module provides a constant, known current

source. The A/D Converter is used to measure (V) in

the equation, leaving two unknowns: capacitance (C)

and time (t). The above equation can be used to calcu-

late capacitance or time, by either the relationship

using the known fixed capacitance of the circuit:

or by:

using a fixed time that the current source is applied to

the circuit.

19.1.2

CURRENT SOURCE

At the heart of the CTMU is a precision current source,

designed to provide a constant reference for measure-

ments. The level of current is user-selectable across

three ranges or a total of two orders of magnitude, with

the ability to trim the output in ±2% increments

(nominal). The current range is selected by the

IRNG<1:0> bits (CTMUICON<1:0>), with a value of

‘00’ representing the lowest range.

Current trim is provided by the ITRIM<5:0> bits

(CTMUICON<7:2>). These six bits allow trimming of

the current source in steps of approximately 2% per

step. Note that half of the range adjusts the current

source positively and the other half reduces the current

source. A value of ‘000000’ is the neutral position (no

change). A value of ‘100000’ is the maximum negative

adjustment (approximately -62%) and ‘011111’ is the

maximum positive adjustment (approximately +62%).

19.1.3

EDGE SELECTION AND CONTROL

CTMU measurements are controlled by edge events

occurring on the module’s two input channels. Each

channel, referred to as Edge 1 and Edge 2, can be con-

figured to receive input pulses from one of the edge

input pins (CTED1 and CTED2) or ECCPx Special

Event Triggers. The input channels are level-sensitive,

responding to the instantaneous level on the channel

rather than a transition between levels. The inputs are

selected using the EDG1SEL and EDG2SEL bit pairs

(CTMUCONL<3:2 and 6:5>).

In addition to source, each channel can be configured for

event polarity using the EDGE2POL and EDGE1POL

bits (CTMUCONL<7,4>). The input channels can also

be filtered for an edge event sequence (Edge 1 occur-

ring before Edge 2) by setting the EDGSEQEN bit

(CTMUCONH<2>).

19.1.4

EDGE STATUS

The CTMUCONL register also contains two Status bits:

EDG2STAT and EDG1STAT (CTMUCONL<1:0>).

Their primary function is to show if an edge response

has occurred on the corresponding channel. The

CTMU automatically sets a particular bit when an edge

response is detected on its channel. The level-sensitive

nature of the input channels also means that the Status

bits become set immediately if the channel’s configura-

tion is changed and is the same as the channel’s

current state.

-------



CV.



tC V



 I



CI t



 V



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