Figure 13.6 Analog Input Equivalent Circuit; Notes On Using A/D Converter - Fujitsu F2MC-8L MB89620 Series Hardware Manual

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13.

Notes on Using A/D Converter

This section lists points to note when using the A/D converter.
n Notes on Using A/D Converter
l Input impedance of analog input pins
The A/D converter contains a sample hold circuit as shown in Figure 13.6 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 Ω ). Note that if the impedance cannot be kept low, it is recommended to connect an
external capacitor of about 0.1µF for the analog input pin.
AN0 to AN7
l 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.
l Note on interrupt requests
The interrupt request flag bit (ADC1: ADI) is not set if A/D conversion is reactivated (ADC1: AD
= "1") at the same time as the previous A/D conversion completes, or if the sense function is
reactivated (ADC1: AD = "1") at the same time as the comparison condition is satisfied.
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CHAPTER 13 A/D CONVERTER
MB89620 series
Close for 8 instruction cycles after activating A/D conversion.
Analog channel selector

Figure 13.6 Analog Input Equivalent Circuit

Sample hold circuit
R = 6 k Ω
C = 33 pF
Comparator
controller
MB89620 series