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7721 Group User’s Manual

12–10

A-D CONVERTER

12.3 A-D conversion method

The A-D converter compares the comparison voltage (Vref), which is internally generated according to the

contents of the successive approximation register, with the analog input voltage (VIN), which is input from

the analog input pin (ANi). By reflecting the comparison result on the successive approximation register, VIN

is converted into a digital value. When a trigger is generated, the A-D converter performs the following

processing:

Determining bit 7 of the successive approximation register

The A-D converter compares Vref with VIN. At this time, the contents of the successive approximation

Bit 7 of the successive approximation register changes according to the comparison result as follows:

When Vref < VIN, bit 7 = “1”

When Vref > VIN, bit 7 = “0”

Determining bit 6 of the successive approximation register

After setting bit 6 of the successive approximation register to “1,” the A-D converter compares Vref

with VIN. Bit 6 changes according to the comparison result as follows:

When Vref < VIN, bit 6 = “1”

When Vref > VIN, bit 6 = “0”

Determining bits 5 to 0 of the successive approximation register

Operations in are performed for bits 5 to 0.

When bit 0 is determined, the contents (conversion result) of the successive approximation register

is transferred to the A-D register i.

The comparison voltage (Vref) is generated according to the latest contents of the successive approximation

Vref. Table 12.3.2 lists changes of the successive approximation register and Vref during the A-D conversion.

Figure 12.3.1 shows the ideal A-D conversion characteristics.

Table 12.3.1 Relationship between successive approximation register’s contents and Vref

12.3 A-D conversion method

VREFV

256

Successive approximation register’s contents: n

1 to 255

! (n – 0.5)

Vref (V)

VREFV: Reference voltage

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