Drive K64 EXTAL32 with signal from PCF2123

[vincentfillatre]

I am currently designing  an application based on K64F. It also includes an external RTC (NXP PCF2123). To save one 32.768kHz crystal I would like to use the clock output of the PCF2123 (open drain)# to drive the K64F. Could you please advise me if such a configuration is possible an how to configure the K64F to do so.

Thanks in advance for your answer.

[xiangjun_rong]

Hi, Vincent,

I suppose you want to connect external SPI real time clock/calendar PCF2123, in the case, the internal RTC module of K64F is unnecessary. The PCF2123 outputs a 32.768KHz clock signal from CLKOUT pin, I think it is okay to connect the 32.768KHz clock signal to EXTAL pin of K64 so that you can use FLL of K64 to get the clock frequency you desired for core/system of K64F. You can not use PLL, because the PLL input frequency shohld range from 2MHz to 4Mhz.

Hope it can help you

BR

Xiangjun Rong

[vincentfillatre]

Hi Xiangjun,

Thanks for your answer. You are right, I want to connect the 32.768kHz CLKOUT pin  of the PCF2123 to the K64F. The idea is to be able to go in the lowest power consumption mode of the K64F while keeping the ability to wake-up at a precise time thanks to the PCF2123. I could use the K64K RTC module my goal is to save one crystal.

During normal operation mode,  the K64F will be clocked at 120MHz using a 12MHz crystal connected between EXTAL and XTAL pins. That's why I'm trying to use the EXTAL32 pin and not the EXTAL.

[xiangjun_rong]

Hi, Vincent,

I see that you want to connect the 12MHz crystal to the Extal/Xtal pins, connect the 32.768kHz CLKOUT pin  of the PCF2123 to the Extal32 pin of K64F so that you can save a 32.768KHz crystal, but the solution is infeasible. It is okay to connect 12MHz crystal to Extal/Xtal pins. But Extal32/Xtal32 pins can only interface with crystal, it can not interface with clock source, so it is not feasible to interface the 32.768KHz clock source from PCF2123 to Extal32 pin.

Hope it can help you

BR

Xiangjun Rong

[vincentfillatre]

Hi Xiangjun,

Thanks for your answer. that's fully clear. I will use a dedicated crystal for the K64F.

Best regards,

Vincent Fillâtre

[egoodii]

The K64 datasheet supplies this option for driving EXTAL32:

I presume this means 'AC coupled'?  I'm not quite sure how you would get this drive-level from your open-drain output in a 'very low power' way...