PIC18F452-I/ML【PDF 119/134 页】IC MCU FLASH 16KX16 A/D 44QFN

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1997 Microchip Technology Inc.

DS30444E - page 85

PIC16C9XX

12.4.1

FASTER CONVERSION - LOWER

RESOLUTION TRADE-OFF

Not all applications require a result with 8-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 res-

olution required, the acquisition time is the same. To

speed up the conversion, the clock source of the A/D

module may be switched so that the TAD time violates

the minimum specied time (see the applicable electri-

cal specication). Once the TAD time violates the mini-

mum specied time, all the following A/D result bits are

not valid (see A/D Conversion Timing in the Electrical

Specications section.) The clock sources may only be

switched between the three oscillator versions (cannot

be switched from/to RC). The equation to determine

the time before the oscillator can be switched is as fol-

lows:

Conversion time = 2TAD + N TAD + (8 - N)(2TOSC)

Where: N = number of bits of resolution required.

Since the TAD is based from the device oscillator, the

user must use some method (a timer, software loop,

etc.) to determine when the A/D oscillator may be

changed. Example 12-3 shows a comparison of time

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

sus the 8-bit resolution conversion. The example is for

devices operating at 8 MHz (The A/D clock is pro-

grammed for 32TOSC), and assumes that immediately

after 6TAD, the A/D clock is programmed for 2TOSC.

The 2TOSC violates the minimum TAD time, therefore

the last 4-bits will not be converted to correct values.

EXAMPLE 12-3: 4-BIT vs. 8-BIT CONVERSION TIMES

Freq. (MHz)

Resolution

4-bit

8-bit

TAD

1.6

TOSC

12.5 ns

125 ns

2TAD + N TAD + (8 - N)(2TOSC)

10.6

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