Renesas rl78 User Manual

RL78/G1A

CHAPTER 7 REAL-TIME CLOCK

Correction example 2

Example of correcting from 32767.4 Hz to 32768 Hz (32767.4 Hz + 18.3 ppm)

[Measuring the oscillation frequency]

Note

The oscillation frequency

of each product is measured by outputting about 1 Hz from the RTC1HZ pin when the

watch error correction register (SUBCUD) is set to its initial value (00H).

Note See 7.4.5 1 Hz output of real-time clock for the setting procedure of outputting about 1 Hz from the RTC1HZ

pin.

[Calculating the correction value]

(When the output frequency from the RTCCL pin is 0.9999817 Hz)

Oscillation frequency = 32768

¯ 0.9999817

≈ 32767.4 Hz

Assume the target frequency to be 32768 Hz (32767.4 Hz + 18.3 ppm) and DEV to be 1.

The expression for calculating the correction value when DEV is 1 is applied.

Correction value = Number of correction counts in 1 minute

= (Oscillation frequency

÷ Target frequency − 1) ¯ 32768 ¯ 60

= (32767.4

÷ 32768 − 1) ¯ 32768 ¯ 60

−36

[Calculating the values to be set to (F6 to F0)]

(When the correction value is

−36)

If the correction value is 0 or less (when quickening), assume F6 to be 1.

Calculate (F5, F4, F3, F2, F1, F0) from the correction value.

− {(/F5, /F4, /F3, /F2, /F1, /F0) − 1} ¯ 2

−36

(/F5, /F4, /F3, /F2, /F1, /F0)

= 17

(/F5, /F4, /F3, /F2, /F1, /F0)

= (0, 1, 0, 0, 0, 1)

(F5, F4, F3, F2, F1, F0)

= (1, 0, 1, 1, 1, 0)

Consequently, when correcting from 32767.4 Hz to 32768 Hz (32767.4 Hz + 18.3 ppm), setting the correction

−36 (bits 6 to 0 of the SUBCUD register: 1101110) results in

32768 Hz (0 ppm).

Figure 7-26 shows the operation when (DEV, F6, F5, F4, F3, F2, F1, F0) is (1, 1, 1, 0, 1, 1, 1, 0).

R01UH0305EJ0200 Rev.2.00

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Jul 04, 2013