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3.11.2
Description of Operation
(1) Analog reference voltage
A high-level analog reference voltage is applied to the VREFH pin; a low-level
analog reference voltage is applied to the VREFL pin. To perform AD conversion, the
reference voltage as the difference between VREFH and VREFL, is divided by 1024
using string resistance. The result of the division is then compared with the analog
input voltage.
To turn off the switch between VREFH and VREFL, write 0 to ADMOD1
<VREFON> in AD mode control register 1. To start AD conversion in the off state,
first write 1 to ADMOD1<VREFON>, wait 3 µs until the internal reference voltage
stabilizes (This is not related to fc), then set ADMOD0<ADS> to 1.
(2) Analog input channel selection
The analog input channel selection varies depends on the operation mode of the AD
converter.
•
In analog input channel fixed mode (ADMOD0<SCAN> = 0)
Setting ADMOD1<ADCH2:0> selects one of the input pins AN0 to AN7 as the input
channel.
•
In analog input channel scan mode (ADMOD0<SCAN> = 1)
Setting ADMOD1<ADCH2:0> selects one of the 8 scan modes.
Table 3.11.1 illustrates analog input channel selection in each operation mode.
After reset, ADMOD0<SCAN> = 0 and ADMOD1<ADCH2:0> = 000. Thus pin AN0
is selected as the fixed input channel. Pins not used as analog input channels can be
used as standard input port pins.
<ADCH2:0>
000
001
010
011
100
101
110
111
Table 3.11.1 Analog Input Channel Selection
Channel Fixed
<SCAN> = 0
AN0
AN1
AN2
AN3
AN4
AN5
AN6
AN7
91C824-187
Channel Scan
<SCAN> = 1
AN0
AN0 → AN1
AN0 → AN1 → AN2
AN0 → AN1 → AN2 → AN3
AN4
AN4 → AN5
AN4 → AN5 → AN6
AN4 → AN5 → AN6 → AN7
TMP91C824
2008-02-20
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