User`s manual

50
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER with CLOSED CAPTION DECODER
and ON-SCREEN DISPLAY CONTROLLER
M37281MAH–XXXSP,M37281MFH–XXXSP
M37281MKH–XXXSP,M37281EKSP
MITSUBISHI MICROCOMPUTERS
Rev. 1.0
8.8 A-D CONVERTER
8.8.1 A-D Conversion Register (AD)
A-D conversion reigister is a read-only register that stores the result
of an A-D conversion. This register should not be read during A-D
conversion.
8.8.2 A-D Control Register (ADCON)
The A-D control register controls A-D conversion. Bits 2 to 0 of this
register select analog input pins. When these pins are not used as
analog input pins, they are used as ordinary I/O pins. Bit 3 is the A-D
conversion completion bit, A-D conversion is started by writing “0” to
this bit. The value of this bit remains at “0” during an A-D conversion,
then changes to “1” when the A-D conversion is completed.
Bit 4 controls connection between the resistor ladder and VCC. When
not using the A-D converter, the resistor ladder can be cut off from
the internal VCC by setting this bit to “0,” accordingly providing low-
power dissipation.
8.8.3 Comparison Voltage Generator (Resistor
Ladder)
The voltage generator divides the voltage between VSS and VCC by
256, and outputs the divided voltages to the comparator as the refer-
ence voltage V
ref.
8.8.4 Channel Selector
The channel selector connects an analog input pin, selected by bits
2 to 0 of the A-D control register, to the comparator.
8.8.5 Comparator and Control Circuit
The conversion result of the analog input voltage and the reference
voltage “Vref” is stored in the A-D conversion register. The A-D con-
version completion bit and A-D conversion interrupt request bit are
set to “1” at the completion of A-D conversion.
Fig. 8.8.1 A-D Comparator Block Diagram
A-D control register
(address 00EF
16)
A-D control circuit
Data bus
Switch tree
A-D conversion
interrupt request
Resistor ladder
Compa-
rator
Channel selector
A-D conversion register
AD1
AD2
AD3
AD4
(address 00EE16)
b7 b0
3
8
VSS VCC
AD5
AD6
AD7
AD8