BCM4390DKWBGT Datasheet, PDF (20/78 Pages) Cypress Semiconductor – WICED™ Wi-Fi IEEE 802.11 b/g/n SoC with Embedded Application Processor

BCM4390 Advance Data Sheet
External 32.768 KHz Low-Power Oscillator
Table 2: Crystal Oscillator and External Clock – Requirements and Performance (Cont.)
Crystal^a
External Frequency
Reference^{b c}
Parameter
Conditions/Notes
Min Typ Max Min Typ Max Units
Duty cycle
37.4 MHz clock
– – – 40 50 60 %
Phase Noise^f
37.4 MHz clock at 10 kHz offset – – – – – –129 dBc/Hz
(IEEE 802.11b/g)
37.4 MHz clock at 100 kHz offset –
–
–
–
–
–136 dBc/Hz
Phase Noise^f
(IEEE 802.11n,
2.4 GHz)
37.4 MHz clock at 10 kHz offset – – – – – –134 dBc/Hz
37.4 MHz clock at 100 kHz offset – – – – – –141 dBc/Hz
a. (Crystal) Use WRF_XTAL_IN and WRF_XTAL_OUT.
b. See “External Frequency Reference” on page 18 for alternative connection methods.
c. For a clock reference other than 37.4 MHz, 20 × log10(f/ 37.4) dB should be added to the limits, where f = the
reference clock frequency in MHz.
d. The frequency step size is approximately 80 Hz resolution.
e. It is the responsibility of the equipment designer to select oscillator components that comply with these
specifications.
f. Assumes that external clock has a flat phase noise response above 100 kHz.
External 32.768 KHz Low-Power Oscillator
The BCM4390 uses a secondary low frequency 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 Figure 3 on page 20.
The external 32.768 kHz crystal provides:
• A real-time clock for the apps core
• Accurate timing for the WLAN power-save modes
BROADCOM ®
February 5, 2014 • 4390-DS103-R
WICED Wi-Fi IEEE 802.11 SoC w/Embedded App Processor
Page 19
BROADCOM CONFIDENTIAL