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CHAPTER 12 A/D CONVERTER
12.7 Notes on Using A/D Converter
This section describes notes on using the A/D converter.
s Notes on Using the A/D Converter
r Input impedance of the analog input
The A/D converter contains the sample hold circuit as shown in
Figure 12.7-1 "Equivalent Circuit
of Analog Input" and captures the voltage of the analog input and holds it in the capacitor for
sample hold in about 16 instruction cycles, after activation of A/D conversion.
Accordingly,
when the output impedance of the external circuit of the analog input is high, the analog input
voltage may not be stabilized during analog input sampling period. Therefore, set the output
impedance of the external circuit to a sufficiently low level (lower than about 4 k
). If the output
impedance of the external circuit cannot be set low, it is recommended that a capacitor with
about 0.1
F be added externally to the analog input.
Figure 12.7-1 Equivalent Circuit of Analog Input
r Notes on setting using a program
When A/D conversion functions are enabled, the values in the ADDH and ADDL registers
are held without being changed until the activation of A/D conversion. However, once A/D
conversion is activated, the values in the ADDH and ADDL registers become undefined
immediately.
When A/D conversion functions are enabled, do not reselect an analog input channel (ADC1:
ANS3 to ANS0).
Especially, during continuous activation, disable continuous activation
(ADC2: EXT = 0), and wait for the conversion in-progress flag bit (ADC1: ADMV) to be 0 for
reselection.
The A/D converter is stopped via a reset and activation of the stop mode, and all registers
are initialized.
When the interrupt request flag bit (ADC1: ADI) is 1 and an interrupt request is enabled
(ADC2: ADIE = 1), recovery from interrupt handling is no longer possible. Be sure to clear
the ADI bit.
Tip:
When A/D conversion is completed, if the next conversion is reactivated, the interrupt
request flag bit (ADC1: ADI) is not set.
RC
MB89930A series
Sample hold circuit
AN0 to AN7
Converter
Selecting
an analog
channel
After activation of A/D conversion,
close 16 instruction cycles