PRELIMINARY
CYW43438
3.3 External 32.768 kHz Low-Power Oscillator
The CYW43438 uses a secondary low-frequency sleep clock for low-power mode timing. Either the internal low-precision LPO or an
external 32.768 kHz precision oscillator is required. The internal LPO frequency range is approximately 33 kHz ± 30% over process,
voltage, and temperature, which is adequate for some applications. However, one trade-off caused by this wide LPO tolerance is a
small current consumption increase during power save mode that is incurred by the need to wake up earlier to avoid missing beacons.
Whenever possible, the preferred approach is to use a precision external 32.768 kHz clock that meets the requirements listed in
Table 4.
Note: The CYW43438 will auto-detect the LPO clock. If it senses a clock on the EXT_SLEEP_CLK pin, it will use that clock. If it
doesn't sense a clock, it will use its own internal LPO.
■ To use the internal LPO: Tie EXT_SLEEP_CLK to ground. Do not leave this pin floating.
■ To use an external LPO: Connect the external 32.768 kHz clock to EXT_SLEEP_CLK.
Table 4. External 32.768 kHz Sleep-Clock Specifications
Parameter
Nominal input frequency
LPO Clock
Units
kHz
ppm
%
32.768
Frequency accuracy
Duty cycle
±200
30–70
Input signal amplitude
Signal type
200–3300
mV, p-p
–
Square wave or sine wave
>100
<5
kΩ
Input impedance1
pF
Clock jitter
<10,000
ppm
1. When power is applied or switched off.
Document Number: 002-14796 Rev. *K
Page 15 of 101
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