LM8335: Alternate to LM8335 General Purpose Output Host Interface Expander

Is there a way to check the speed that my device is operating at normally? Is there a way to change/increase the speed to 20MHz? Is this software controlled?

Brian,

"Is there a way to check the speed that my device is operating at normally?"
You should be able to just probe the clock line and use an o-scope to read the frequency.

"Is there a way to change/increase the speed to 20MHz? Is this software controlled?"
SCLK is an input pin, meaning that it is driven externally. Your master is the one likely generating the clock signal.

-Bobby

Bobby,
Thanks for the response.  It answers part of the question that came in about in parallel to receiving your response.  Can you help to clarify regarding the additional details:
" I found the schematic for the MIPI (LM8335) and see that the processor is running off a 8 MHz crystal. I suppose then that if I change it to a 20 MHz crystal (and drop the 20pF caps to about 8 pF) the MIPI data/bus should be running at 20 MHz. Right?"

Thank you!

Hi Brian,

" I found the schematic for the MIPI (LM8335) and see that the processor is running off a 8 MHz crystal. I suppose then that if I change it to a 20 MHz crystal (and drop the 20pF caps to about 8 pF) the MIPI data/bus should be running at 20 MHz. Right?"

Well, I believe you still have to configure the code to set the clock speed to 20MHz but from what I can see that probably won't work.

Note: This is outside my analog background so take what I say with some salt
The crystal is used for timing/sampling/running digital operations. If you have a 20MHz clock then assuming every operation is done on EITHER a rising or falling edge (one and not both) then each digital operation takes 50 nanoseconds. If it takes multiple pulses to perform a logic high and then a logic low on the MIPI clock output then you will no longer be able to generate the 20MHz clock signal for the MIPI.

For this reason I am skeptical that a 20MHz processor could generate a 20MHz signal. If the code were very optimized and each edge on the clock output could be done in 5 clock pulses (10 for rising and falling) then the max clock frequency you would get on the MIPI clock would be 2 MHz not 20MHz.

Thanks,

-Bobby

Hi Bobby,
I'm stuck on grasping this completely and hoping you can help me going in the right direction.
I'll try to simplify the questions on my end for easier interpretation:
- Can LM8335 support 20MHz with an external clock?  I think you confirmed it, but want to make sure this is what is meant.

- Do you suggest the change to 8pF (from 20pF) and change to 20MHz crystal as the only required changes to make this work (or to validate it)?

- Is your concern at running at 20MHz, isolated to how the EVM is set up, OR is this applicable for systems in general?

Just want to help our colleague along on this portion of the evaluation.  If you prefer, please email me directly.

Thanks

"Can LM8335 support 20MHz with an external clock?"

LM8335 supports up to 26MHz.

The way this question is phrased makes it seem like you want to put an external clock pulse on the SCLK line rather than drive the SCLK from a processor. This implies you want to constantly keep the clock pulsing the the SCLK line... RFFE's idle condition is a return to zero (RTZ) so this isn't allowed.

"Do you suggest the change to 8pF (from 20pF) ?"

This is completely dependent on the clock crystal and the clock driver circuit.

"change to 20MHz crystal as the only required changes to make this work (or to validate it)?"

Using a 20MHz crystal does not tell me if the processor can drive the SCLK up to 20MHz. This would be a question for the manufacture of the processor.

"Is your concern at running at 20MHz, isolated to how the EVM is set up, OR is this applicable for systems in general?"

My concern is on the processor end, particularly, can it drive 20MHz on the SCLK in order to operate at 20MHz. If yes, then our device will run at 20MHz, if no then our processor will not run at 20MHz.

-Bobby