参数资料
型号: KSZ8864RMNI
厂商: Micrel Inc
文件页数: 96/111页
文件大小: 0K
描述: IC ETHERNET SWITCH 4PORT 64QFN
特色产品: KSZ8864RMN Ethernet Switches
标准包装: 348
系列: *
其它名称: 576-3995
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
8
1.6
s
1.6
s
TOSC
8
12.5 ns
125 ns
2TAD + N TAD + (8 - N)(2TOSC)
8
10.6
s
16
s
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