HT46R46/C46/R47/C47/R48/R49
The ADCR control register also contains the
PCR2~PCR0 bits which determine which pins on Port B
are used as analog inputs for the A/D converter and
which pins are to be used as normal I/O pins. If the 3-bit
address on PCR2~PCR0 has a value of ²100² or higher,
then all four pins, namely AN0, AN1, AN2 and AN3 will all
be set as analog inputs. Note that if the PCR2~PCR0 bits
are all set to zero, then all the Port B pins will be setup as
normal I/Os and the internal A/D converter circuitry will be
powered off to reduce the power consumption.
nal interrupt address for processing. If the A/D internal
interrupt is disabled, the microcontroller can be used to
poll the EOCB bit in the ADCR register to check whether
it has been cleared as an alternative method of detect-
ing the end of an A/D conversion cycle.
A/D Converter Clock Source Register - ACSR
The clock source for the A/D converter, which originates
from the system clock fSYS, is first divided by a division
ratio, the value of which is determined by the ADCS1
and ADCS0 bits in the ACSR register.
The START bit in the ADCR register is used to start and
reset the A/D converter. When the microcontroller sets
this bit from low to high and then low again, an analog to
digital conversion cycle will be initiated. When the
START bit is brought from low to high but not low again,
the EOCB bit in the ADCR register will be set to a ²1²
and the analog to digital converter will be reset. It is the
START bit that is used to control the overall on/off opera-
tion of the internal analog to digital converter.
Although the A/D clock source is determined by the sys-
tem clock fSYS, and by bits ADCS1 and ADCS0, there are
some limitations on the maximum A/D clock source speed
that can be selected. As the minimum value of permissible
A/D clock period, tAD, is 0.5ms for the HT46R46, device,
and 1ms for the other devices, care must be taken for sys-
tem clock speeds in excess of 2MHz. With the exception of
the HT46R46 device, for system clock speeds in excess of
2MHz, the ADCS1 and ADCS0 bits should not be set to
²00². For the HT46R46 device, for system clock speeds in
excess of 4MHz, the ADCS1 and ADCS0 bits should not
be set to ²00². Doing so will give A/D clock periods that are
less than the minimum A/D clock period which may result
in inaccurate A/D conversion values. Refer to the following
table for examples, where values marked with an asterisk
* show where, depending upon the device, special care
must be taken, as the values may be less than the speci-
fied minimum A/D Clock Period.
The EOCB bit in the ADCR register is used to indicate
when the analog to digital conversion process is com-
plete. This bit will be automatically set to ²0² by the
microcontroller after a conversion cycle has ended. In
addition, the corresponding A/D interrupt request flag
will be set in the interrupt control register, and if the inter-
rupts are enabled, an appropriate internal interrupt sig-
nal will be generated. This A/D internal interrupt signal
will direct the program flow to the associated A/D inter-
b
7
b
0
E
O
C
B
P
C
R
2
P
C
R
1
P
C
R
0
A
C
S
2
A
C
S
1
A
C
S
0
S
T
A
R
T
A
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C
R
R
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g
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2
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1
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0
0
0
0
0
1
0
0
1
1
0
1
0
1
:
:
:
:
:
A
A
A
A
u
N
N
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2
3
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:
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1
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:
S
t
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t
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"
1
"
A/D Converter Control Register
b
7
b
0
T
E
S
T
A
D
C
S
1
A
D
C
S
0
A
C
S
R
R
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g
i
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C
0
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:
s
y
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t
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c
l
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k
/
2
:
s
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8
3
:
:
s
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2
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f
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F
o
t
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m
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p
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t
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d
a
s
"
0
"
o
r
t
s
t
o
d
e
e
o
n
l
y
A/D Converter Clock Source Register
Rev. 1.00
28
April 18, 2007
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
