ADE5166_08 AD [Analog Devices], ADE5166_08 Datasheet - Page 119

• Manufacturer Part Number: ADE5166_08
• Description: Single-Phase Energy Measurement IC with 8052 MCU, RTC, and LCD Driver
• Manufacturer: AD [Analog Devices]

RTC CRYSTAL COMPENSATION
The RTC provides registers to compensate for the tolerance of
the crystal frequency and its variation over temperature. Up to
±248 ppm frequency error can be calibrated out by the RTC
circuitry. The compensation is fully digital and implemented
by adding or subtracting pulses from the crystal clock signal.
The resolution of the RTC nominal compensation SFR
(RTCCOMP, Address 0xF6) is ±2 ppm/LSB, or 0.17 sec/day/LSB.
The RTC compensation circuitry adds the RTC temperature com-
pensation SFR (TEMPCAL, Address 0xF7) and the RTC nominal
compensation SFR (RTCCOMP, Address 0xF6) to determine how
much compensation is required. The sum of these two registers is
limited to ±248 ppm, or 42.85 sec/day.
RTC Calibration
The nominal crystal frequency can be calibrated by adjusting
the RTCCOMP SFR so that the clock going into the RTC is
precisely 32.768 kHz at 25°C.
Calibration Flow
An RTC calibration pulse output is on up to four pins configured
by the four LSBs in the RTC calibration configuration register
(RTC_CAL, Address 0x0F). Enable the RTC output by setting
the CAL_EN bit (Bit 6) in the RTC calibration configuration
register (RTC_CAL, Address 0x0F).
The RTC calibration is accurate to within ±2 ppm over a 30.5 sec
window in all operational modes: PSM0, PSM1, and PSM2. Two
output frequencies are offered for the normal RTC mode: 1 Hz
with FSEL[1:0] = 00 and 512 Hz with FSEL[1:0] = 01 in the RTC
calibration configuration register (RTC_CAL, Address 0x0F).
A shorter window of 0.244 sec is offered for fast calibration during
PSM0 or PSM1mode . Two output frequencies are offered for this
RTC calibration output mode: 500 Hz with FSEL[1:0] = 10 and
16 kHz with FSEL[1:0] = 11 in the RTC calibration configuration
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register (RTC_CAL, Address 0x0F). Note that for the 0.244 sec
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 138. RTC Calibration Options
Option
Normal Mode 0
Normal Mode 1
Calibration Mode 0
Calibration Mode 1
When no RTC compensation is applied, with RTCCOMP and
TEMPCAL equal to zero, the nominal compensation required
to account for the error in the external crystal can be deter-
mined. 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 RTC calibration pin is sufficient.
The value to write to the RTCCOMP SFR is calculated from the
% error or seconds per day error on the frequency output. Each
LSB of the RTCCOMP SFR represents 2 ppm of correction
where 1 sec/day error is equal to 11.57 ppm.
During calibration, user software writes the current time to the
RTC. Refer to the Access to Internal RTC Registers section for
more information on how to read and write to the RTC time-
keeping registers.
RTCCOMP = 5000 × (% Error)
RTCCOMP =
2
×
11
FSEL[1:0]
00
01
10
11
1
.
57
× (sec/day Error)
ADE5166/ADE5169
Calibration
Window (sec)
30.5
30.5
0.244
0.244
f
(Hz)
1
512
500
16000
RTCCAL