nRF24LE1 Clock output

Questions for programming 8051 microcontrollers in C or assembly using SDCC

nRF24LE1 Clock output

Postby incredible » Mon Nov 03, 2014 2:04 am

For my application, I need to synchronize the 16 MHz clock on the nRF24LE1 with the clock signal on my sensor. The sensor can operate from 6-27 Mhz, so ideally I could just run both chips at 16 MHz.
The problem is there is no "clockout" type of output in the nRF24LE1. Connecting directly to pin XC1 might destabilize the crystal oscillator, warping the frequency / duty cycle or preventing it from starting up in the first place. Also, section 13.3.3 of the nRF24LE1 datasheet ("External 16 MHz clock") says you cannot bypass the crystal and input a clock signal directly to pin XC1, unless you disable the transmitter feature. Why?! Any suggestions? Thank you!
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Re: nRF24LE1 Clock output

Postby brennen » Mon Nov 03, 2014 2:15 pm

I had never actually come across that section in the datasheet. My assumption is that they ran the XC1 and XC2 lines into the separate circuits of the microcontroller core and the on-board L01+. My second assumption is that the buffer that allows using a rail-to-rail clock oscillator only changes the clock input on the microcontroller core, rather than sending that signal over to the on-board L01+. It's a bit lazy, but at least as far as I can tell, they literally just plopped the silicon from the L01+ onto the die of the LE1 and made zero changes to it, other than wiring the SPI pins from the L01+ to the microcontroller core.

I'm not an expert with oscillator circuits, so I would suggest contacting Nordic's technical support about this one. They are usually extremely quick to respond, and are quite helpful. If you do pursue this route, feel free to come back here and let us all know what they say, so that anyone else who runs into this situation might have a solution.
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Re: nRF24LE1 Clock output

Postby incredible » Wed Nov 05, 2014 2:58 am

Thank you for the quick reply! I was surprised that Tech Support also replied very quick, here's what I asked and what they said:

I am using the nRF24LE1 in a design because of its very small (4x4mm) footprint and built-in microcontroller, however I am having trouble understanding how I can synchronize the clock with an external sensor.

The problem is there is no "clock out" pin, so I have to generate a signal with a crystal oscillator and feed it to both the nRF24LE1 and the sensor. According to nRF24LE1 Product Specification v1.6, section 13.3.3 "External 16 MHz clock", a digital signal cannot be used at the same time as the radio transceiver function, however an analog signal CAN be used. What exactly does the datasheet mean by "analog input"? Does it refer to a (clipped) sine wave? Can I use a passive RC filter between a CMOS clock driver and pin XC1 to filter out high frequencies?

The analog clock is a sinus curve. So a filtered square wave should be ok. The problem with supplying a square wave is the harmonics of the clock will disturb the radio.

You can also pull the 16MHz clock from the XC1 pin of the nRF24LE1 oscillator. The output of the oscillator can be used as a clock source for an external sensor / MCU.

best regards,
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