PIC16C76-04E/SP View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

PIC16C76-04E/SP

8-Bit CMOS Microcontrollers with A/D Converter

Microchip Technology 

PIC16C7X

13.4.1 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 specified time (see the applicable electri-

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

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

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

Specifications 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 13-3 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 20 MHz and 16 MHz (The A/D

clock is programmed for 32TOSC), and assumes that

immediately after 6TAD, the A/D clock is programmed

for 2TOSC.

The 2TOSC violates the minimum TAD time since the last

4-bits will not be converted to correct values.

EXAMPLE 13-3: 4-BIT vs. 8-BIT CONVERSION TIMES

Freq. (MHz)(1)

TAD

20

16

TOSC

20

16

2TAD + N • TAD + (8 - N)(2TOSC)

20

16

Note 1: PIC16C7X devices have a minimum TAD time of 1.6 µs.

Resolution

4-bit

1.6 µs

2.0 µs

50 ns

62.5 ns

10 µs

12.5 µs

8-bit

1.6 µs

2.0 µs

50 ns

62.5 ns

16 µs

20 µs

© 1997 Microchip Technology Inc.

DS30390E-page 123

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