topic Necessity for external crystal? in LPC Microcontrollers

Necessity for external crystal?

lpcware — Wed, 15 Jun 2016 20:09:05 GMT

Content originally posted in LPCWare by cdoerr on Fri Mar 07 01:04:55 MST 2014
Hello,

For other microcontroller lines it is often recommended or even required to use an external oscillator for anything high-speed with precise times, such as SPI and USB, as the internal clock is not accurate enough. The LPC800-MAX board does have a crystal mounted, but it is not connected, yet I have so far not run into any issues with the board.

Does this rule of thumb not apply to the LPC812 anymore with its internal oscillator? The user manual says "PLL allows CPU operation up to the maximum CPU rate without the need for a
high-frequency crystal." When does it then still make sense to add a crystal to one's design?

Thanks
Christian

Re: Necessity for external crystal?

lpcware — Wed, 15 Jun 2016 20:09:06 GMT

Content originally posted in LPCWare by MarcVonWindscooting on Fri Mar 07 16:58:38 MST 2014
From datasheet:
IRC frequency (-40C .. +105C, 2.7V..3.6V): 11.82MHz..12.18MHz.

This accuracy (+- 1.5%) is fine for SPI, UART, ...
But a simple 24hour clock would be off by up to 21.6 minutes per day. Not acceptable for most people.

USB has different clock speed tolerances. The higher the speed, the tighter the tolerance.

The reason, why the crystal is not unconditionally connected on your board is the temptation of using two more GPIOs of a chip that doesn't have plenty of these ;-)

Re: Necessity for external crystal?

lpcware — Wed, 15 Jun 2016 20:09:07 GMT

Content originally posted in LPCWare by starblue on Sat Mar 08 04:04:59 MST 2014
The issue is how many bits you can transmit before you need to resynchronize.

At ±1.5% clock error you can be off by one bit time after about 60 bits, and the actual bit sample time needs to be within some smaller window.

SPI and I2C have a separate clock line, so this shouldn't be an issue.

With UART it is about eight or nine bits, should also be fine.

CAN has bit stuffing, so the bit value changes at least every 6 bits. But depending on speed and length of the bus the sample window can be quite small (so it might need a crystal).

Basically you need to do the math for your situation.

The problem with higher speeds is that higher speed protocols typically have longer packets.

Re: Necessity for external crystal?

lpcware — Wed, 15 Jun 2016 20:09:08 GMT

Content originally posted in LPCWare by cdoerr on Wed Mar 12 06:31:39 MST 2014
Great, thank you for the extensive information. This makes it very clear.

Re: Necessity for external crystal?

lpcware — Wed, 15 Jun 2016 20:09:08 GMT

Content originally posted in LPCWare by salamlora1 on Tue Mar 18 00:53:27 MST 2014
Hi
Most of my designs with lpc812 are without of external ocl and works fine
And also in our company QC it’s under the test of -12C and + 50C and works fine for 7 days test
(Don’t forget WD) ;-)