3885 Group
SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER
MITSUBISHI MICROCOMPUTERS
66
COMPARATOR CIRCUIT
Comparator Configuration
The comparator circuit consists of the ladder resistors, the analog
comparators, a comparator control circuit, the comparator refer-
ence input selection bit (bit 7 of PCTL2), a comparator data
register (CMPD), the comparator reference power source input pin
(P20/CMPREF) and analog input pins (P30–P37). The analog input
pin (P30–P37) also functions as an ordinary digital port.
Comparator Operation
To activate the comparator circuit, first set port P3 to input mode
by setting the corresponding direction register (P3D) to “0” to use
port P3 as an analog voltage input pin. The internal fixed analog
voltage (VCC 29/32) can be generated by setting “1” to the com-
parator reference input selection bit (bit 7 of PCTL2). The internal
fixed analog voltage becomes about 2.99 V at VCC = 3.3 V. When
setting “0” to the comparator reference input selection bit, the P20/
CMPREF pin becomes the comparator reference power source in-
put pin and it is possible to input the comparator reference power
source optionally from the external. The voltage comparison is im-
mediately performed by the writing operation to the comparator
data register (CMPD). After 14 cycles of the internal system clock
φ (the time required for the comparison), the comparison result is
stored in the comparator data register (CMPD).
If the analog input voltage is greater than the internal reference
voltage, each bit of this register is “1”; if it is less than the internal
reference voltage, each bit of this register is “0”. To perform an-
other comparison, the voltage comparison must be performed
again by writing to the comparator data register (CMPD).
Read the result when 14 cycles of
φ or more have passed after the
comparator operation starts. The ladder resistor is turned on dur-
ing 14 cycles of
φ , which is required for the comparison, and the
reference voltage is generated. An unnecessary current is not
consumed because the ladder resistor is turned off while the com-
parator operation is not performed. Since the comparator consists
of capacitor coupling, the electric charge may lost if the clock fre-
quency is low.
Keep the clock frequency more than 1 MHz during the comparator
operation. Do not execute the STP, WIT, or port P3 I/O instruction.
Fig. 64 Comparator circuit
VSS
8
VCC
P3 (8)
P37
P36
P30
b0
Comparator reference input selection bit
(bit 7 of PCTL2)
Comparator data register
Compar-
ator
Ladder resistor
connecting signal
Comparator
control circuit
Comparator connecting
signal
Compar-
ator
Compar-
ator
P20/CMPREF
VCC29/32
“1”
“0”
Data bus