35
38C2 Group
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
PRELIMINAR
Y
Notice:
This
is not
a final
specification.
Some
parametric
limits
are
subject
to change.
A-D CONVERTER
The 38C2 group has a 10-bit A-D converter. The A-D converter per-
forms successive approximation conversion.
[A-D Conversion Register (ADL, ADH)]
One of these registers is a high-order register, and the other is a low-
order register. The high-order 8 bits of a conversion result is stored in
the A-D conversion register (high-order) (address 001B16), and the
low-order 2 bits of the same result are stored in bit 7 and bit 6 of the
A-D conversion register (low-order) (address 001A16).
During A-D conversion, do not read these registers.
Also, the connection between the resistor ladder and reference volt-
age input pin (VREF) can be controlled by the VREF input switch bit (bit
0 of address 001A16). When “1” is written to this bit, the resistor ladder
is always connected to VREF. When “0” is written to this bit, the resistor
ladder is disconnected from VREF except during the A-D conversion.
[A-D Control Register (ADCON)]
This register controls A-D converter. Bits 2 to 0 are analog input pin
selection bits. Bit 3 is an AD conversion completion bit and “0” during A-
D conversion. This bit is set to “1” upon completion of A-D conversion.
A-D conversion is started by setting “0” in this bit.
[Comparison Voltage Generator]
The comparison voltage generator divides the voltage between AVSS
and VREF, and outputs the divided voltages.
[Channel Selector]
The channel selector selects one of the input ports P47/AN7–P40/
AN0 and inputs it to the comparator.
[Comparator and Control Circuit]
The comparator and control circuit compares an analog input volt-
age with the comparison voltage and stores the result in the A-D
conversion register. When an A-D conversion is completed, the con-
trol circuit sets the AD conversion completion bit and the AD conver-
sion interrupt request bit to “1.”
The comparator uses internal capacitors whose charge will be lost if
the clock frequency is too low.
Therefore, set the A-D clock frequency to 250 kHz or more.
Also, when the STP instruction is executed during the A-D conver-
sion, the A-D conversion is stopped immediately, the A-D conversion
completion bit is set to “1”, and the interrupt request is generated.
Fig. 31 Block diagram of A-D converter
Fig. 30 Structure of A-D control register
Data bus
AVSS
A-D interrupt request
b7
b0
3
P40/OOUT0/AN0
P41/OOUT1/AN1
P42/AN2
P43/AN3
P44/AN4
P45/AN5
P46/AN6
P47/AN7
A-D control register
Channel
selector
Comparator
A-D control circuit
A-D conversion register (H)
A-D conversion register (L)
(Address 001B 16)
(Address 001A 16)
Resistor ladder
VREF
Analog input pin selection bits
b2 b1 b0
00 0: P40/AN0
00 1: P41/AN1
01 0: P42/AN2
01 1: P43/AN3
10 0: P44/AN4
10 1: P45/AN5
11 0: P46/AN6
11 1: P47/AN7
AD conversion completion bit
0: Conversion in progress
1: Conversion completed
AD conversion clock selection bits
b5 b4
00: XIN/2
01: XIN/4
10: XIN/8
11: XIN/16
10-bit or 8-bit conversion switch bit
0: 10-bit AD
1: 8-bit AD
Booster selection bit
(When A-D conveter is used at VCC =
2.5 V or less, write “1” to this bit.)
0: Booster not used
1: Booster used
A-D control register
(ADCON: address 001916)
b7
b0
10-bit reading
(Read address 001B16 before 001A16)
A-D conversion register 1
(Address 001B 16)
A-D conversion register 2
(Address 001A 16)
8-bit reading
(Read only address 001B16)
(Address 001B 16)
b0
b7
b0
b1
* VREF input switch bit
b9 b8 b7 b6 b5 b4 b3 b2
b7
b0
b9 b8 b7 b6 b5 b4 b3 b2
b7
b0
*
(high-order)
(low-order)
Note : The bit 5 to bit 1 of address 001A 16 becomes “0” at reading.
Also, bit 0 is undefined at reading.
1: ON
0: ON only during A-D conversion