MB89190/190A series
Chapter 12 A/D Converter
12-19
12.
7 Notes on Using A/D Converter
The notes on using the A/D converter are explained.
T Notes on using A/D converter
Input impedance of analog input pin
As shown in
Figure 12.7, the A/D converter incorporates a sample hold circuit, and stores the voltage
of the analog input pin in the sample hold capacitor during about 8 instruction cycles after starting A/D
conversion (sense function).
Therefore, when the output impedance of the external circuit of the
analog input is high, the analog input voltage may not stabilize during the analog input sampling
period.
So, reduce the output impedance of the external circuit (10 k
or less). If the output
impedance of the external circuit cannot be lowered, provide an external capacitor of about 0.1
F at
the analog input pin.
Fig. 12.7 Analog Input Equivalent Circuit
Notes when setting with program
– When the A/D conversion function is used, the ADCD register holds the previous value until A/D
conversion is started, but the contents of this register become undefined immediately after A/D
conversion starts.
– When the A/D conversion function or the sense function is operating, do not reselect the analog
input channel (ADC1: ANS3 to ANS0) and do not switch between the A/D conversion and sense
functions (ADC2: ADMD).
In particular, when continuous starting is being performed, do not
perform reselection or switching before disabling the continuous starting (ADC2: EXT = 0) and
ensuring that the conversion-on flag bit (ADC1: ADMV) is 0.
Also, when the sense function is
operating, do not rewrite the comparison condition setting bit (ADC1: SIFM) before stopping the
sense function.
– When the sense function is operating, stop it before writing to the ADCD register.
– Clear the interrupt request flag bit (ADC1: ADI = 0) before switching between the A/D conversion
function and the sense function.
– The A/D converter is stopped when a reset occurs or the stop mode or timer mode is started.
– Control cannot return from interrupt processing when the interrupt request flag bit (ADC1: ADI) is 1
and the interrupt request is enabled (ADC2: ADIE = 1). Always clear the ADI bit.
Comperator
controller
Closed for 8 instruction cycles after starting A/D conversion
Sample hold circuit
Analog channel
selector
MB89190/190A series
AN0 to AN7
C = about 33 pF
R = about 6 k