ADE75 datasheet(122/148 Pages) AD

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ADE75xx/ADE71xx

Preliminary Technical Data

Rev. PrE | Page 122 of 148

Calibration Flow:

A RTC calibration pulse output is provided on the

P0.2/CF1/RTCCAL pin. Enable the RTC output by setting the

RTCCAL bit in the INTPR SFR.

The RTC calibration is accurate to within ±2ppm over a 30.5

second window in all operational modes: PSM0, PSM1 and

PSM2. Two output frequencies are offered for the normal RTC

mode: 1Hz with FSEL[1:0]=00 and 512Hz with FSEL[1:0]=01 in

the INTPR register.

A shorter window of 0.244 seconds is offered for fast calibration

during PSM0 or PSM1. Two output frequencies are offered for

this RTC calibration output mode: 500Hz with FSEL[1:0]=01

and 16.384kHz with FSEL[1:0]=11 in the INTPR register. Note

that for the 0.244s calibration window, the RTC is clocked 125

times faster than in the normal mode, resulting in timekeeping

registers that represent seconds/125, minutes/125 and

hours/125 instead of seconds, minutes and hours. Therefore this

mode should be used for calibration only.

Table 120: RTC calibration options

When no RTC compensation is applied, when RTCCOMP and

TEMPCAL equal to zero, the nominal compensation required

to account for the error in the external crystal can be

determined. In this case, it is not necessary to wait for an entire

calibration window to determine the error in the pulse output.

Calculating at the error in frequency between two consecutive

pulses on the P0.2/CF1/RTCCAL pin is enough.

The value to write to the RTCCOMP register is calculated from

the % error or seconds per day error on the frequency output.

Each LSB of the RTCCOMP SFR represents 2ppm of correction

where1s/day error is equal to 11.57ppm.

)

5000

Error

RTCCOMP

)

(

Error

day

RTCCOMP

to determine how much compensation is required and the sum

of these two registers is limited to ±248ppm.

During calibration, user software writes the RTC with the

current time. Refer to the RTC Read and Write operations

section for more information on how to read and write the RTC

timekeeping registers.

Option

FSEL[1:0]

Calibration

Window (s)

FRTCCAL (Hz)

Normal Mode 0

30.5

Normal Mode 1

30.5

512

Calibration

Mode 0

0.244

500

Calibration

Mode 1

0.244

16384

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