7480 datasheet(161/337 Pages) RENESAS

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7480 Group and 7481 Group User's Manual

1-144

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

: 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

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