TMP86FS49AIUG datasheet(235/296 Pages) TOSHIBA

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Page 219

TMP86FS49AIUG

17.6 Precautions about AD Converter

17.6.1 Restrictions for AD Conversion interrupt (INTADC) usage

When an AD interrupt is used, it may not be processed depending on program composition. For example, if

an INTADC interrupt request is generated while an interrupt with priority lower than the interrupt latch IL15

(INTADC) is being accepted, the INTADC interrupt latch may be cleared without the INTADC interrupt being

processed.

The completion of AD conversion can be detected by the following methods:

(1) Method not using the AD conversion end interrupt

Whether or not AD conversion is completed can be detected by monitoring the AD conversion end flag

(EOCF) by software. This can be done by polling EOCF or monitoring EOCF at regular intervals after start of

AD conversion.

(2) Method for detecting AD conversion end while a lower-priority interrupt is being processed

While an interrupt with priority lower than INTADC is being processed, check the AD conversion end flag

(EOCF) and interrupt latch IL15. If IL15 = 0 and EOCF = 1, call the AD conversion end interrupt processing

routine with consideration given to PUSH/POP operations. At this time, if an interrupt request with priority

higher than INTADC has been set, the AD conversion end interrupt processing routine will be executed first

against the specified priority. If necessary, we recommend that the AD conversion end interrupt processing

routine be called after checking whether or not an interrupt request with priority higher than INTADC has been

set.

17.6.2 Analog input pin voltage range

Make sure the analog input pins (AIN0 to AIN15) are used at voltages within VAREF to VSS. If any volt-

age outside this range is applied to one of the analog input pins, the converted value on that pin becomes uncer-

tain. The other analog input pins also are affected by that.

17.6.3 Analog input shared pins

The analog input pins (AIN0 to AIN15) are shared with input/output ports. When using any of the analog

inputs to execute AD conversion, do not execute input/output instructions for all other ports. This is necessary

to prevent the accuracy of AD conversion from degrading. Not only these analog input shared pins, some other

pins may also be affected by noise arising from input/output to and from adjacent pins.

17.6.4 Noise Countermeasure

The internal equivalent circuit of the analog input pins is shown in Figure 17-5. The higher the output

impedance of the analog input source, more easily they are susceptible to noise. Therefore, make sure the out-

put impedance of the signal source in your design is 5 k

Ω or less. Toshiba also recommends attaching a capac-

itor external to the chip.

Figure 17-5

Analog Input Equivalent Circuit and Example of Input Pin Processing

DA converter

AINi

Analog comparator

Internal resistance

Permissible signal

source impedance

Internal capacitance

5 k

Ω (typ)

C = 12 pF (typ.)

5 k

Ω (max)

Note) i = 15 to 0

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