Datasheet

 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 T
AD time violates
the minimum specified time (see the applicable electri-
cal specification). Once the T
AD 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 = 2T
AD + N • TAD + (8 - N)(2TOSC)
Where: N = number of bits of resolution required.
Since the T
AD 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 32T
OSC), and assumes that immediately
after 6T
AD, the A/D clock is programmed for 2TOSC.
The 2T
OSC 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 8 1.6 µs 1.6 µs
T
OSC 8 12.5 ns 125 ns
2T
AD + N • TAD + (8 - N)(2TOSC) 8 10.6 µs 16 µs