Rev.3.02
Apr 10, 2008
Page 125 of 131
REJ03B0177-0302
38D2 Group
Notes on A/D Conversion
1. Analog Input Pin
Set the signal source impedance for analog input low, or equip an
analog input pin with an external capacitor of 0.01
μF to 1 μF.
In addition, operations of application products should be verified
thoroughly on the user side.
<Reason>
An analog input pin has a built-in capacitor for analog voltage
comparison. Thus if a signal from the high impedance signal
source is input to the analog input pin, charge and discharge
noise w i ll b e g e n e r a ted. This may cause th e A/D
conversion/comparison accuracy to drop.
2. Clock Frequency during A/D Conversion
The comparator input consists of a capacity coupling. If the
conversion rate is too low, the A/D conversion accuracy may
deteriorate due to a charge lost, so set f(XIN) 500 kHz or more for
A/D conversion in XIN mode. Also, do not execute the STP or
WIT instruction during A/D conversion.
In low-speed mode (when on-chip oscillator is selected), as A/D
conversion is performed using the internal on-chip oscillator,
there is no limit on the minimum frequency for f(XIN).
3. ADKEY Function
When the ADKEY enable bit is set to “1”, the analog input pin
selection bits are disabled. Do not execute the A/D conversion by
a program while ADKEY is enabled. Enabling ADKEY does not
change bits 0 to 2 of ADCON.
4. A/D Conversion Immediately After ADKEY Function
Started
In the ADKEY function, A/D conversion is not performed to the
analog input voltage immediately after starting the function. This
causes the A/D conversion result immediately after starting the
function to be undefined. If the A/D conversion result of the
analog input voltage applied to the ADKEY pin is required,
select the analog input pin corresponding to ADKEY before
performing A/D conversion.
5. Input Voltage Applied to ADKEY Pin
Set the input to the ADKEY pin into a steep falling waveform
and stabilize the input voltage within eight cycles (1
μs when
f(XIN) = 8 MHz) from the moment the input voltage reaches VIL
or lower.
The actual threshold voltage for the ADKEY pin is between VIH
and VIL.
To prevent unnecessary ADKEY operation due to noise or other
factors, set the ADKEY pin voltage to VIH (0.9 VCC) or more
while the input is waited.
6. Register Operation during A/D Conversion
The A/D conversion operation is not guaranteed if the following
are preformed:
The CPU mode register is operated during A/D conversion
operation
The AD control register is operated during A/D conversion
operation
The STP or WIT instruction is executed during A/D
conversion operation
7. A/D Converter Power Source Pin
Connect to the A/D converter power source pin to AVSS or VSS
whether the A/D conversion function is used or not.
<Reason>
If the AVSS pin is left open, the MCU may operate incorrectly
because the pin will be affected by noise or other factors.