PIC16C71X
7.4.1
FASTER CONVERSION - LOWER
RESOLUTION TRADE-OFF
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 7-3 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.
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
follows:
The 2TOSC violates the minimum TAD time since the
last 4-bits will not be converted to correct values.
Conversion time = 2TAD + N • TAD + (8 - N)(2TOSC)
Where: N = number of bits of resolution required.
EXAMPLE 7-3: 4-BIT vs. 8-BIT CONVERSION TIMES
Resolution
(1)
Freq. (MHz)
4-bit
8-bit
TAD
20
16
20
16
20
16
1.6 µs
2.0 µs
50 ns
1.6 µs
2.0 µs
50 ns
TOSC
62.5 ns
10 µs
62.5 ns
16 µs
2TAD + N • TAD + (8 - N)(2TOSC)
12.5 µs
20 µs
Note 1: The PIC16C71 has a minimum TAD time of 2.0 µs.
All other PIC16C71X devices have a minimum TAD time of 1.6 µs.
1997 Microchip Technology Inc.
DS30272A-page 43
MICROCHIP [ MICROCHIP ]
