LPC51u68 internal fractional n PLL for ext. clock generation

[michael6]

LPC51u68 project PLL question. We are using USB interface and need to generate PLL generated signal to drive external IC on its osc in. We need a frequency range from 9MHz - ~ 14 MHz with a resolution of approx 50KHz. (42-60KHz). Is this possible without influencing USB communication. Data sheet not entirely clear in this respect. We think it shall work but need coinfirmation or hints if this is possible or if we need to employ an external PLL for that purpose. Many thanks Michael

[ZhangJennie]

Hi Michael

Yes, you can output pll clock to CLKOUT

for example, if your pll clock is 98Mhz, CLKOUT=pll_clk/8=12M. 12M is for output. It won't affect USB clock.

Best Regards
Jun Zhang

[michael6]

Hi Jenny

Thanks for the answer. What we are looking for is USB operation independent fro PLL.

We would like to have USB comm not impacted by the PLL which we need to sweep in a range from 9MHz to 14 Mhz with a resolution of in the range of 40-60KHz.

So while we communicate with the MCU via USb we want to be able to use the PLL independently to drive an external IC oszillator input with a clock we found suitable for the application.

So question is. Is there an independent clock source for USB, or are we always influencing USB clock when using different generated PLLL frequency on MCLK OUT?

Many thanks

Michael

with best regards / mit freundlichen Grüßen

DI Michael Ganzera

General Manager / Geschäftsführer

GMMC the home of SANGOMA RFID solutions

GMMC GmbH | Kehlbergstrasse 109 | 8054 Graz | Austria | Europe

Tel: +43 664 415 62 60

e-mail: <mailto:michael@gmmc-biz.com> michael@gmmc-biz.com

web: <http://www.gmmc-biz.com/> www.gmmc-biz.com

[ZhangJennie]

Hi Michael

You can also use FRO_HF as USB clock source. it can be configured to adjust automatically to the USB bus rate.

it can work with full speed usb without issue.

Call the FRO high frequency output setup API. for selecting 48MHz or 96 MHz with trim. see USB clock config path in blue

Best Regards

Jun Zhang