Data Sheet

LC840PA
Datasheet
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5.16 External 32.768 kHz Crystal
This is not required for normal BL654PA module operation. The ProFLEX firmware requires the external 32.768kHz crystal
to enable high baud rate communications and ensure lowest power operation in sleep mode.
The LC840PA uses the on-chip 32.76 kHz RC oscillator (LFCLK) by default (which has an accuracy of ±500 ppm) which
requires regulator calibration (every eight seconds) to within ±500 ppm. This is insufficient for ProFLEX operation.
An external high accuracy (±20 ppm) 32.768 kHz crystal (and associated load capacitors) must be connected to the
LC840PASIO_01/XL2 (pin 41) and SIO_00/XL1 (pin 42) to provide improved protocol timing and to help with radio power
consumption in the system standby doze/deep sleep modes by reducing the time that the RX window needs to be open. Table
23 compares the current consumption difference between RC and crystal oscillator.
Table 23: Comparing current consumption difference between LC840PA on-chip RC 32.76 kHz oscillator and optional external crystal
(32.768kHz) based oscillator
LC840PA On-chip 32.768 kHz RC Oscillator
(±500 ppm) LFRC
Optional External Higher Accuracy (±20
ppm) 32.768 kHz Crystal-based Oscillator
LFXO
Current Consumption
of 32.768 kHz Block
0.7 uA
0.23 uA
Standby Doze Current
(SYSTEM ON IDLE +full
RAM retention +RTC
run current + LFRC or
LFXO)
3.1 uA
2.6 uA
Calibration
Calibration required regularly (default eight
seconds interval).
Calibration takes 33 ms; with DCDC used, the
total charge of a calibration event is 16 uC.
The average current consumed by the
calibration depends on the calibration interval
and can be calculated using the following
formula:
CAL_charge/CAL_interval – The lowest
calibration interval (0.25 seconds) provides an
average current of (DCDC enabled):
16uC/0.25s = 64uA
To get the 500-ppm accuracy, the Bluetooth
LE stack specification states that a calibration
interval of eight seconds is enough. This gives
an average current of:
16uC/8s = 2 uA
Added to the LFRC run current and Standby
Doze (IDLE) base current shown above results
in a total average current of:
LFRC + CAL = 3.1 + 2 = 5.1 uA
Not applicable
Total
5.1 uA
2.6 uA
Summary
▪ Low current consumption
▪ Accuracy 500 ppm
▪ Lowest current consumption
▪ Needs external crystal
▪ High accuracy (depends on the crystal,
usually 20 ppm)