Clock distribution/synchronisation

[lpcware]

Content originally posted in LPCWare by wlamers on Fri May 24 02:14:01 MST 2013

<p class="MsoNormal">My current design requires to have the main clock of two LPC4357's to be synchronized. Additionally I require the clock to be routed to another external PCB over some longer distance (12 MHz clock). Distributing the clock can be done using a crystal and clock buffer (like the TI LMK00105). But the voltage input requirement of XTAL1 is &lt;= 1.2V according to the datasheet of the LPC43xx. Maybe I can use the GP_CLKIN pin for the input to solve the voltage requirement. Who knows the best way to clock both uC's with the same clock source? Maybe there are other alternatives than using a clock buffer?

[lpcware]

Content originally posted in LPCWare by suckfish on Thu May 30 23:57:37 MST 2013

The LMK00104 sounds like overkill for this; I'd just use a cheap CMOS 12MHz oscillator driving the multiple outputs. If you really need a buffer a 74HC04 or similar would probably be good enough.

If you use the XTAL input pin, the voltage requirement can be handled via a resistor divider? Either that or use a clock-in pin. The choice probably depends on what package you're using and what else you need to use the pins for.

Alternatively, you could connect a crystal to one CPU, and use one or more clk-outs on that CPU to drive a clock input pin on both CPUS (looping back on the first cpu if you need the phases to be aligned) and the external clock.

The clocking network inside the processor is pretty flexible, and you have several pins capable of inputing/outputing clocks so you have a good deal of choice about how you do this.