PIC16C717T-E/SS【PDF 21/220 页】IC MCU OTP 2KX14 A/D PWM 20SSOP

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2002 Microchip Technology Inc.

DS41120B-page 115

PIC16C717/770/771

11.7

Use of the ECCP Trigger

An A/D conversion can be started by the “special

event trigger” of the CCP module. This requires that

the CCP1M<3:0> bits be programmed as 1011b and

that the A/D module is enabled (ADON is set). When

the trigger occurs, the GO/DONE bit will be set on Q2

to start the A/D conversion and the Timer1 counter will

be reset to zero. Timer1 is RESET to automatically

repeat the A/D conversion cycle, with minimal soft-

ware overhead (moving the ADRESH and ADRESL to

the desired location). The appropriate analog input

channel must be selected before the “special event

trigger” sets the GO/DONE bit (starts a conversion

cycle).

If the A/D module is not enabled (ADON is cleared),

then the “special event trigger” will be ignored by the

A/D module, but will still RESET the Timer1 counter.

11.8

Effects of a RESET

A device RESET forces all registers to their RESET

state. This forces the A/D module to be turned off, and

any conversion is aborted. The value that is in the

ADRESH and ADRESL registers are not modified.

The ADRESH and ADRESL registers will contain

unknown data after a Power-on Reset.

11.9

Faster Conversion - Lower

Resolution Trade-off

Not all applications require a result with 12 bits of reso-

lution, but may instead require a faster conversion

time. The A/D module allows users to make the trade-

off of conversion speed to resolution. Regardless of

the resolution required, the acquisition time is the

same. To speed up the conversion, the A/D module

may be halted by clearing the GO/DONE bit after the

desired number of bits in the result have been con-

verted. Once the GO/DONE bit has been cleared, all

of the remaining A/D result bits are ‘0’.

The equation

to determine the time before the GO/DONE bit can be

switched is as follows:

Conversion time = (N+1)TAD

Where: N = number of bits of resolution required,

and 1TAD is the amplifier settling time.

Since TAD is based from the device oscillator, the user

must use some method (a timer, software loop, etc.) to

determine when the A/D GO/DONE bit may be

cleared. Table 11-4 shows a comparison of time

required for a conversion with 4 bits of resolution, ver-

sus the normal 12-bit resolution conversion. The

example is for devices operating at 20 MHz. The A/D

clock is programmed for 32 TOSC.

EXAMPLE 11-4:

4-BIT vs. 12-BIT

CONVERSION TIME

Example

4-Bit Example:

Conversion Time = (N + 1) TAD

= (4 + 1) TAD

= (5)(1.6

= 8

12-Bit Example:

Conversion Time = (N + 1) TAD

= (12 + 1) TAD

= (13)(1.6

= 20.8

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