Is it possible to have the USB and RTC working at the same time?
USB needs a 48 MHz derived from a 4 MHz or more crystal,RTC needs a 32768 KHz.
Is it possible to have them working together without using USB_CLKIN=48MHz?
I believe that the KL series is designed to work primarily with a low power external 32.768kHz clock module, supplying a 32.768kHz clock to the RTC_CLKIN input; see the TWR-KL46Z48M board for an example.
The main crystal (typically 8MHz) is then suitable for USB use.
For "simple" RTC use (not high accuracy) the internal LPO 1kHz clock can be used to drive the RTC.
Hi Mark,thanks for your reply
I checked RTC_CLKIN but it looks that is suitable for an oscillator,and not for a crysta.In battery RTC application this could be a problem.
What do you think about it?
I stated that a clock module (oscillator) is required to drive the input.
Whether this is suitable for a particular application depends on battery life requirements and also the processing requirements during the life time (sleeping/wakup/processing states). As I understand it, the KL series are aimed at low power applications where the processor is "always" powered but can be set to very low power modes (less that 1uA max. at room temperature is possible, but which mode is acceptable will depend on what low power state the application can accept and what peripherals are required in the low power state). The external oscillator on the TWR board is not particularly good since it requires about 2uA (and is huge!!). There are ultra-low power ones available that will require (much) less that 1uA.
Since the KL has no Vbat domain it can not be powered down without losing the RTC information. You need also to be aware that the processor can also not be held in reset when the RTC is clocked from the RTC_CLKIN. In reset it will not lose the original time count value but it will not be clocked by the 32kHz oscillator since its RTC_CLKIN connection is removed from the RTC (this needs to be reconfigured after each reset cycle and the seconds spent in the reset state are 'lost').
In cases where the RTC only is to remain powered (to monitor date/time) it may, in some applications, make more sense to use a K series device with a Vbat domain and a 32kHz crystal since the RTC in the K devices actually require less that 1uA for the RTC, including its internal 32kHz oscillator, which is a very respectable value (previous RTC generations required 20x that value). To match this, a KL device will need a very good external ultra-low-power oscillator and also be able to use a very low leakage stop mode.
Therefore there is no YES/NO answer. It depends on your requirements (and size of battery that can be connected ;-)
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