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A-D CONVERTER

7906 Group User’s Manual Rev.2.0

12-12

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:

➀ Determining bit 9 of the successive approximation register

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

Bit 9 of the successive approximation register depends on the comparison result as follows:

When Vref < VIN, bit 9 = “1”

When Vref > VIN, bit 9 = “0”

➁ Determining bit 8 of the successive approximation register

After setting bit 8 of the successive approximation register to “1,” the A-D converter compares Vref with

VIN. Bit 8 depends on the comparison result as follows:

When Vref < VIN, bit 8 = “1”

When Vref > VIN, bit 8 = “0”

➂ Determining bits 7 to LSB of the successive approximation register

Operation ➁ is performed for each of bits 7 to 0 in the 10-bit resolution mode.

Operation ➁ is performed for each of bits 7 to 2 in the 8-bit resolution mode.

When the LSB is determined, the contents of the successive approximation register (in order words,

conversion result) are transferred to the A-D register i.

Vref is generated according to the latest contents of the successive approximation register. Table 12.3.1 lists

the relationship between the successive approximation register’s contents and Vref. Tables 12.3.2 and 12.3.3

list the changes of the successive approximation register and Vref during the A-D conversion, respectively.

Figure 12.3.1 shows the ideal A-D conversion characteristics in the 10-bit resolution mode.

Successive approximation register’s contents: n

VREF

1024

1 to 1023

× (n – 0.5)

Vref (V)

VREF: Reference voltage

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

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