LPCXpresso IDEのトピックRe: CLKOUT question - lpc11c14

CLKOUT question - lpc11c14

lpcware — Thu, 16 Jun 2016 02:40:40 GMT

Content originally posted in LPCWare by charithjperera on Sun Jun 12 08:01:39 MST 2011
Hi all,

Just a quick question; is it possible to get a 32.768kHz clock out signal from the lpc11c14?

It's got a 12MHz crystal, the maximum dividing ratio seems to be 255 with the CLKOUT pin for 47kHz. Would anybody know of another way to lower this?

Thanks!
Charith

Re: CLKOUT question - lpc11c14

lpcware — Thu, 16 Jun 2016 02:40:41 GMT

Content originally posted in LPCWare by Ex-Zero on Sun Jun 12 08:32:52 MST 2011
You have 2 options:

1. Use the lowest possible crystal = 10 MHZ -> 10MHz / 255 = 39.22 kHz :mad:

2. Use 'Programmable watchdog oscillator with a frequency range of 7.8 kHz to 1.8 MHz.' So with watchdog oscillator you should be able to get lower frequencies :)
But: 'The watchdog oscillator nominal frequency is programmable between 7.8 kHz and 1.7 MHz. The frequency spread over processing and temperature is[B][COLOR=Red] ±40 %[/COLOR][/B] (see Table 11).' :eek:

Re: CLKOUT question - lpc11c14

lpcware — Thu, 16 Jun 2016 02:40:42 GMT

Content originally posted in LPCWare by Rob65 on Sun Jun 12 12:25:27 MST 2011
Charith,

just a quick answer: No.

In order to get 32.768 kHz from a 12 MHz crystal you need to divide by 366.2109375 which is kind of impossible with a simple divider.

Rob

Re: CLKOUT question - lpc11c14

lpcware — Thu, 16 Jun 2016 02:40:42 GMT

Content originally posted in LPCWare by charithjperera on Sat Jun 18 04:03:58 MST 2011
Dang,

Thanks very much for the replies! :)

I suppose the watchdog oscillator wouldn't be good for me due to the massive temperature variation possible.
I was trying to use this clock as a stable clock for an ITG3200 gyro to reduce the variation with temperature. The gyro has a 2% accurate IRC oscillator so using the lpc11c14's IRC wouldn't be of much help; i'll just try calibrating over a temperature range :)

Thank again,
Charith

Re: CLKOUT question - lpc11c14

lpcware — Thu, 16 Jun 2016 02:40:43 GMT

Content originally posted in LPCWare by larryvc on Sat Jun 18 08:47:17 MST 2011
You could try one of these:

http://para.maxim-ic.com/en/results.mvp?fam=osc_mod&285=32.768kHz&980=TCXO

Re: CLKOUT question - lpc11c14

lpcware — Thu, 16 Jun 2016 02:40:43 GMT

Content originally posted in LPCWare by Ex-Zero on Sat Jun 18 10:54:05 MST 2011

Quote: charithjperera
The gyro has a 2% accurate...:eek:

A simple PWM at 48MHz clock and Match register = 1465 should generate a nice 32764.5 Hz signal. That's 0.01% :)

Re: CLKOUT question - lpc11c14

lpcware — Thu, 16 Jun 2016 02:40:44 GMT

Content originally posted in LPCWare by charithjperera on Thu Aug 11 05:55:12 MST 2011

Quote: Zero
A simple PWM at 48MHz clock and Match register = 1465 should generate a nice 32764.5 Hz signal. That's 0.01% :)

Thanks Zero, that worked out perfectly!