Renesas RL78 Series User Manual page 385

16-bit single-chip microcontrollers

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C = 100 to 1000 pF

(5) Analog input (ANIn) pins

<1> The analog input pins (ANI0 to ANI3) are also used as input port pins (P20 to P23).

When A/D conversion is performed with any of the ANI0 to ANI3 pins selected, do not change to output value

P20 to P23 while conversion is in progress; otherwise the conversion resolution may be degraded.

<2> If a pin adjacent to a pin that is being A/D converted is used as a digital I/O port pin, the A/D conversion result

might differ from the expected value due to a coupling noise. Be sure to prevent such a pulse from being input

(6) Input impedance of analog input (ANIn) pins

This A/D converter charges a sampling capacitor for sampling during sampling time.

Therefore, only a leakage current flows when sampling is not in progress, and a current that charges the capacitor

flows during sampling. Consequently, the input impedance fluctuates depending on whether sampling is in progress,

and on the other states.

To make sure that sampling is effective, however, we recommend using the converter with analog input sources that

have output impedances no greater than 1 kΩ. If a source has a higher output impedance, lengthen the sampling

time or connect a larger capacitor (with a value of about 0.1 µF) to the pin from among ANI0 to ANI3 and ANI16 to

ANI19 to which the source is connected (see Figure 12-44). The sampling capacitor may be being charged while the

setting of the ADCS bit is 0 and immediately after sampling is restarted and so is not defined at these times.

Accordingly, the state of conversion is undefined after charging starts in the next round of conversion after the value

of the ADCS bit has been 1 or when conversion is repeated. Thus, to secure full charging regardless of the size of

fluctuations in the analog signal, ensure that the output impedances of the sources of analog inputs are low or secure

sufficient time for the completion of conversion.

R01UH0515EJ0120 Rev.1.20

Dec 16, 2016

Figure 12-44. Analog Input Pin Connection

If there is a possibility that noise equal to or higher than AV

or equal to or lower than AV

a diode with a small V

ANI0 to ANI3, ANI16 to ANI19

CHAPTER 12 A/D CONVERTER

value (0.3 V or lower).

may enter, clamp with

Rl78/g1d R5f11agg R5f11agh R5f11agj