topic Re: RTC oscillator taking time to start oscillations in LPC Microcontrollers

RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:02 GMT

Content originally posted in LPCWare by Ramana on Thu May 29 02:41:39 MST 2014
Dear all,
I am using RTC in LPC1778 controller. I have connected 32.768KHz crystal via 22pf capacitors as given in the controller datasheet. In some of my cards, RTC oscillation taking 10 to 15mintues to start oscillations. In some of my cards it is working fine. Is it controller issue or any?
Can u help to solve this issue.

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:03 GMT

Content originally posted in LPCWare by R2D2 on Thu May 29 03:20:46 MST 2014

Quote: Ramana
Can u help to solve this issue.

No :)

Any hint about schematic?

Load capacitance (CL)?

See: http://www.farnell.com/datasheets/62327.pdf

and Chapter 8 Long Start-Up Time of

http://www.freescale.com/files/microcontrollers/doc/app_note/AN3208.pdf

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:03 GMT

Content originally posted in LPCWare by Ramana on Thu May 29 04:15:32 MST 2014
Thank you for the replay please see attached circuit

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:04 GMT

Content originally posted in LPCWare by R2D2 on Thu May 29 04:37:11 MST 2014
A better solution is to add a serial resistor as shown in:

http://www.embeddedartists.com/sites/default/files/docs/schematics/LPCXpressoLPC1769revB.pdf

Any hint about CL of your crystal?

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:04 GMT

Content originally posted in LPCWare by Ramana on Fri May 30 05:07:08 MST 2014
Crystal load capacitance = 12pF

I have connected two 22pF capacitor at Xtal pins. so load capacitance will be half, i.e. 11pF

I have placed crystal very near to the controller pins around 0.5inch

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:05 GMT

Content originally posted in LPCWare by R2D2 on Fri May 30 05:34:13 MST 2014

Quote: Ramana
Crystal load capacitance = 12pF

I have connected two 22pF capacitor at Xtal pins. so load capacitance will be half, i.e. 11pF

I have placed crystal very near to the controller pins around 0.5inch

:quest:

Is this a language problem?

The question is which CL is required for your crystal?

Could be useful to look in your crystal datasheet for that value...

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:06 GMT

Content originally posted in LPCWare by Ramana on Fri May 30 21:05:52 MST 2014
Please see the attached datasheet

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:06 GMT

Content originally posted in LPCWare by mubes on Sun Jun 01 08:01:59 MST 2014
Ramana,

The datasheet says the CL is 12.5pF max. As a rule of thumb you're already carrying 6pF from the tracking so the 11pF is going to be over the top...and probably lead to the symptoms you're seeing.

Try removing the load capacitors and see what happens...

Dave

Re: RTC oscillator taking time to start oscillations

lpcware — Wed, 15 Jun 2016 18:06:07 GMT

Content originally posted in LPCWare by mubes on Mon Jun 09 03:50:21 MST 2014
As an extension to this subject, there's a decent writeup over at Adafruit that can explain things better than my two liner previously.

DAVE