301
11.6 Notes on Using A/D Converter
This section lists points to note when using the A/D converter.
s Notes on Using A/D Converter
r Input impedance of analog input pins
The A/D converter contains a sample hold circuit as shown in Figure 11.6-1 "Analog Input
Equivalent Circuit" to fetch analog input voltage into the sample hold capacitor for eight
instruction cycles after activating A/D conversion (or the sense function). For this reason, if the
output impedance of the external circuit for the analog input is high, analog input voltage might
not stabilize within the analog input sampling period. Therefore, it is recommended to keep the
output impedance of the external circuit low (below 10 k
). Note that if the impedance cannot
be kept low, it is recommended to connect an external capacitor of about 0.1F for the analog
input pin.
Figure 11.6-1 Analog Input Equivalent Circuit
r Notes on setting by program
For the A/D conversion function, the ADCD register maintains previous value until the next
A/D conversion is activated.
However, the content of the ADCD register becomes
indeterminate immediately after activating A/D conversion.
Do not re-select the analog input channel (ADC1: ANS3 to ANS0) or do not switch between
the A/D conversion and sense functions (ADC2: ADMD) while the A/D conversion or sense
function is operating. Particularly, when continuous activation is enabled, only perform such
operations after disabling continuous activation (ADC2: EXT = "0") and waiting for the
conversion-in-progress flag bit (ADC1: ADMV) to go to "0". Stop operation before modifying
the compare condition setting bit (ADC1: SIFM) in the same way when the sense function is
operating.
When using the sense function, stop operation before writing to the ADCD register.
Clear the interrupt request flag bit (ADC1: ADI = "0") before switching between the A/D
conversion and sense functions.
A reset or activation of stop mode stops the A/D converter and initializes all registers.
Interrupt processing cannot return if the interrupt request flag bit (ADC1: ADI) is "1" and the
interrupt request enable bit is enabled (ADC2: ADIE = "1"). Always clear the ADI bit.
MB89620160/160A series
Sample hold circuit
R = 6 k
C = 33 pF
Comparator
controller
Close for 8 instruction cycles after activating A/D conversion.
Analog channel selector
AN0 to AN3