ML7406 datasheet(84/104 Pages) LAPIS

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FEDL7406-02

ML7406

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○Low speed clock shift detection function

ML7406 has low speed shift detection function to compensate inaccurate clock generated by RC oscillator (external clock or

internal RC oscillation circuits). By detecting frequency shift of the wake up timer, host can set wake-up timer parameters

which taking frequency shift into consideration. More accurate timer operation is possible by adjusting wake-up timer interval

setting ([WUT_INTERVAL_H/L: B0 0x2F/0x30]) or continuous operation timer interval ([WU_DURATION: B0 0x31]).

Setting

Frequency shift detection clock frequency setting

[CLK_CAL_SET: B0 0x70]

Clock calibration time

[CLK_CAL_TIME: B0 0x71]

Clock calibration result value

[CLK_CAL_H: B0 0x72], [CLK_CAL_L: B0 0x73]

This function is to measure low speed wake-up timer cycle by using accurate high speed internal clock and count result will

be stored in [CLK_CAL_H/L: B0 0x72/0x73] registers. Above setting and count numbers are as follows:

High speed clock counter = {Wakeup timer clock cycle[SLEEP/WU_SET:B0 0x2D(2)] *

Clcok calibration time setting ([CLK_CAL_TIME:B0 0x71(5-0)]) /

{master clock cycle (26MHz) / clock division setting value ([CLK_CAL_SET: B0

0x70(7-4)])}

Clock calibration time is as follows:

Clock calibration time[s] = Wakeup timer clock cycle * Clock calibration time setting

[Example]

Assuming no division in the internal high speed clock, calibration time is set as 10 cycle. Set 1,000 to Wake-up interval

timer:

condition: wake-up timer clock frequency = 32.768kHz

detection clock division setting CLK_CAL_DIV[3:0][CLK_CAL_SET: B0 0x70(7-4)] = 0b0000

clock calibration time setting [CLK_CAL_TIME] = 0x0A

wake-up timer interval [WUT_INTERVAL_H/L:B0 0x2F,30] = 0x03E8

Theorical high speed clock count = (1/32.768kHz) * 10 / (1/26MHz)

= 7934(0x1EFE)

If getting [CLK_CAL_H/L:B0 0x72,73] = 0x1E17 (7703)

Counter difference = 7703 - 7934 = -231

Frequency shift = 1/[{1/32.768kHz + (-231) / 10 * 1/26MHz } - 1/32.768kHz] = 0.983 kHz

Then finding wake-up timer clock frequency accuracy is +3% higher. And the compensation vale (C) is calcurared as

below:

C= Wake-up timer interval([WUT_INTERVAL_H/L:B0 0x2F,30]) * frequecy shift / 32.768

= 1000 * 0.983kHz / 32.768kHz

=30

Therefore, setting [WUT_INTERVAL_H/L:B0 0x2F,30] = 1000 +30 = 1030 = 0x0406 to achive more accurate inteval

timinig.

(Note)

If calibration time is too short or if high speed counter is divided into low speed clock, calibration may not be accurate.

ML7406 Datasheet(PDF) 84 Page - LAPIS Semiconductor Co., Ltd.

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