7477 datasheet(157/354 Pages) RENESAS

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7470/7471/7477/7478 GROUP USER’S MANUAL

1-140

HARDWARE

1.14 A-D converter

1.14.1 A-D conversion method

The A-D conversion method is of successive approximation comparison.

The reference voltage Vref generated internally is compared with the analog input voltage VIN which is

input from the analog input pin (P20/IN0–P27/IN7), and its result is stored into each bit of the A-D conversion

[Internal operation]

After A-D conversion is started, the following operations are automatically performed.

1 The contents of the A-D conversion register is set to “0016.”

2 The most significant bit (bit 7) of the A-D conversion register is set to “1.”

3 The reference voltage Vref is input to the comparator. The reference voltage Vref is specified by

the contents n of the A-D conversion register and the reference voltage VREF input from the VREF

pin.

An expression for the reference voltage Vref is shown below.

Relational expression between Vref and VREF

When n = 0

Vref = 0

When n = 1 to 255

Vref = VREF/256

× (n – 0.5)

n: The values of A-D conversion register (decimal notation)

4 The reference voltage Vref and the analog input voltage VIN are compared with 8 times. Upon

completion of each comparison, the comparison result is stored into the A-D conversion register.

As the A-D conversion register changes, the reference voltage V ref changes.

[1] Determination of the most significant bit (bit 7) of the A-D conversion register (in the 1st

comparison)

The reference voltage Vref and the analog input voltage VIN are compared. Bit 7 is determined

according to its result as follows.

If Vref < VIN , then bit 7 = “1.”

If Vref > VIN , then bit 7 = “0.”

[2] Determination of the bit 0 - 6 of the A-D conversion register (after the 2nd comparison).

First, bit 6 of the A-D conversion register is set to “1.” Next, the reference voltage Vref is

compared with the analog input voltage VIN. Bit 6 is determined according to its result as

follows.

If Vref < VIN , then bit 6 = “1.”

If Vref > VIN , then bit 6 = “0.”

7477 Datasheet(PDF) 157 Page - Renesas Technology Corp