7480 Group and 7481 Group User's Manual
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HARDWARE
1.15.3 Operations of A-D Converter
The A-D conversion system of the 7480 Group and 7481 Group is successive comparison conversion. The
comparison result of internally generated comparison voltage Vref with input voltage VIN from an analog
input pin is stored in the A-D conversion register.
The operations of the A-D converter are described below.
Start of A-D Conversion
When the A-D conversion completion bit of the A-D control register is cleared to ‘0’, A-D conversion is
started.
A-D Conversion
The A-D conversion register goes to ‘0016’.
Analog input voltage VIN is compared with comparison voltage Vref 8 times. The contents of the A-D
conversion register are determined by the bit from the MSB, each time a comparison is performed.
Comparison voltage Vref is determined by the following formula depending on the contents of the A-
D conversion register and reference voltage VREF, which is input from the VREF pin.
Expression of comparison voltage Vref
Vref =
0 ........................................when n = 0
VREF
256
VREF: Reference voltage input from the VREF pin
n
: The contents of the A-D conversion register
The first comparison (determination of bit 7 of A-D conversion register)
Bit 7 of the A-D conversion register is set to ‘1’, and comparison voltage Vref obtained by the above
formula is input to the comparator. Vref is compared with VIN, and bit 7 of the A-D conversion register
is determined, depending on the result of the comparison as follows:
Bit 7 remains ‘1’ (retention) if Vref < VIN.
Bit 7 is converted to ‘0’ if Vref > VIN.
Comparison from the second time (determination of bits 6 to 0 of A-D conversion register)
Every bit of bits 6 to 0 of the A-D conversion register is successively determined as bit 7 is done
in the first time. (The next bit to be determined is set to ‘1’, and the value of the bit is determined
by the comparison result of VIN with Vref.)
Figure 1.15.5 shows the change of the A-D conversion register and the comparison voltage during
A-D conversion.
A-D conversion is completed when comparison voltage Vref is compared with analog input voltage VIN
8 times and all bits of the A-D conversion register are determined. At this time, the A-D conversion
completion bit of the A-D control register becomes ‘1’.
× (n – 0.5) ........... when n = 1 to 255
1.15 A-D Converter