Our customer is using KW21Z for an 802.15.4 application design.
To save BOM cost, while RTC function is not needed, can we remove the external 32kHz crystal oscillator part?
We are trying to use VLLS1 with LPTMR + LPO mode for low power, and its related interrupts for wakeup. We do not need an RTC function. And 32 MHz reference oscillator is still existing. So we think this "external 32kHz crystal oscillator" can be removed.
From MKWxxZRM Rev. 3, p.157, we found "ERCLK32K - Clock source for some modules. Can be selected as either
OSC32KCLK (default), RTC_CLKIN or LPO." and " This clock is used to provide the 32kHz clock to the Radio; when using Radio, LPO should not be selected" worried us if this 32kHz clock is necessary for normal 802.15.4 Tx/Rx?
Also in p.2239, "when the link layer has to enter low-power state, and the reference oscillator must be switched off, and alternate clock source must be temporarily employed to keep the timebase running. This low power state is known as Deep Sleep Mode, or DSM", seems it only related to DSM.
In p.2241, "When the 802.15.4 Link Layer anticipates long periods of inactivity, the Link Layer controller can be put into a Deep Sleep Mode (DSM)", and it is not clearly stated when and if it is necessary to enter this DSM?
Please check and comment, if we can remove this external 32kHz crystal oscillator part, without any impact to normal 802.15.4 operation?