LPC18xx RTC Clocking From External Crystal

[workalot]

Workalot is taking in UM10430 looking for what register bits need to be set to have an the LPC1837 RTC clocked by an external crystal. And I suppose I would need to know where the bypass register bits are if the external clock is an oscillator.

Oh well, maybe I'll have another look tomorrow, maybe a guru is about.

thank you, Workalot

[ZhangJennie]

Hi 

If you use oscillator, Both crystal (XTAL) and the capacitances CRTCX1 and CRTCX2 need to be connected externally.

If you use external clock, only RTCX1 needs to be connected, and leave RTCX2 open.

The 1 kHz output of the 32 kHz oscillator must be enabled in the CREG0 register in the CREG block. No other bypass settings.

Best Regards

Jun Zhang

[workalot]

Thank you Jun,

I'm wondering then (and I will check), if the bit in CREG0 is not set does the RTC run from the internal clock.

regards,

[ZhangJennie]

If bit 0 doesn't set, the default value is 0 which disable 1kHz output.

So we need to enable it in code.

[workalot]

Bit rude of Workalot not to respond sooner, apologies. Other work takes my focus at the moment.

I was trying to understand how the RTC could be clocked by an external source or by an internal source. For platforms not having an external 32768Hz, the RTC needs a clock source.

regards, Workalot. 

[ZhangJennie]

If RTC uses external clock, we need bit 0 in CREG0. Otherwise LPC18xx will use internal clock as RTC clock source.

[workalot · Accepted solution]

There you go - such a simple answer - thank you.

If only your...

'If RTC uses external clock, we need bit 0 in CREG0. Otherwise LPC18xx will use internal clock as RTC clock source.'

... appeared in UM10430.

regards

Workalot