UPD17708 datasheet(284/356 Pages) NEC

284 / 356 page

284

µPD17704, 17705, 17707, 17708, 17709

18.4.2 Program example of IF counter

A program example of the IF counter is shown below.

As shown in this example, make sure that a wait time elapses after an instruction that selects the P1C0/FMIFC

or P1C1/AMIFC pin for the IF counter function has been executed until counting is started.

This is because, as described in 18.4.1, the internal AC amplifier does not operate normally immediately after a

pin has been selected for the IF counter.

Example To count the frequency input to the P1C0/FMIFC pin (FMIF count mode) (gate time: 8 ms)

INITFLG IFCMD1, NOT IFCMD0, IFCCK1, NOT IFCCK0

; Selects FMIFC pin (FMIF count mode), and sets gate time to 8 ms

Wait

; Internal AC amplifier stabilization time

SET1

IFCRES

; Resets counter

SET1

IFCSTRT

; Starts counting

LOOP:

SKT1

IFCG0STT

; Detects opening or closing of gate

READ

; Branches to READ: if gate is closed

Processing A

LOOP

; Do not read data of IF counter with this processing A

READ:

GET

DBF, IFC

; Reads value of IF counter data register to data buffer

18.4.3 Error of IF counter

The errors of the IF counter include a gate time error and a count error. The following paragraphs (1) and (2)

describe each of these errors.

(1) Gate time error

The gate time of the IF counter is created by dividing the 4.5-MHz clock. Therefore, if the system clock is

shifted from 4.5 MHz by “+x” ppm, the gate time is shifted by “–x” ppm.

(2) Count error

The IF counter counts frequency by the rising edge of the input signal.

If a high level is input to the pin when the gate is open, therefore, one excess pulse is counted.

If the gate is closed, however, a count error due to the status of the pin does not occur.

Therefore, the count error is “+1, –0”.

UPD17708 Datasheet(PDF) 284 Page - NEC

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