3874 Group User's Manual
APPLICATION
2-149
2.6 A-D converter and D-A converter
2.6.5 Notes on use
(1)
Analog input pin
Make the signal source impedance for analog input low, or equip an analog input pin with an external
capacitor of 0.01
F to 1F. Further, be sure to verify the operation of application products on the
user side.
q Reason
An analog input pin includes the capacitor for analog voltage comparison. Accordingly, when
signals from signal source with high impedance are input to an analog input pin, charge and
discharge noise generates. This may cause the A-D conversion precision to be worse.
(2)
A-D converter power source pin
Pins AVSS are A-D converter power source pins. Regardless of using the A-D conversion function
or not, connect them as following :
AVSS : Connect to the VSS line
q Reason
If the AVCC and the AVSS pin are opened, the microcomputer may have a failure because of noise
or others. Also, if the AVCC pin is connected to the VSS pin, current flows from AVCC to VSS.
(3)
Reference voltage input pin VREF
When using A-D converter or D-A converter, apply a voltage of 2 V to Vcc to the reference voltage
input pin VREF. Note that if the VREF value is lowered, the accuracy degrades.
(4)
Clock frequency during A-D conversion
The comparator consists of a capacity coupling, and a charge of the capacity will be lost if the clock
frequency is too low. Thus, make sure the following during an A-D conversion.
f(XIN) is 500 kHz or more
Do not execute the STP instruction and WIT instruction
(5)
A-D/D-A conversion register
The A-D/D-A conversion register functions as an A-D conversion register during a read and a D-A
conversion during a write. Accordingly, the D-A conversion register set value cannnot be read out.
(6)
Inputting falling signal to ADT pin during A-D conversion when using external trigger
When inputting the falling signal to the ADT pin during A-D conversion, A-D conversion which is in
progress is stopped, and A-D conversion is started again.
(7)
Set analog input pin to input mode
When using the A-D converter, set port P6 pins used as the analog input pins to an input port.
When using the D-A converter, set P80/DA pin as input port.
(8)
Connecting D-A output pin to low-impedance load when using D-A converter
The DA output pin does not include a buffer, so that use an external buffer when connecting a
peripheral circuit having a low-impedance load.
(9)
Vcc when using D-A converter
The D-A converter accuracy when Vcc is 4.0 V or less differs from that of when Vcc is 4.0 V or more.
When using the D-A converter, we recomend that Vcc is used at 4.0 V or more.