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CC2511 crystal selection

Marius Boarta
Prodigy 30 points
Other Parts Discussed in Thread: CC2511

Hello,

In CC2511 datasheet it is one application circuit that uses 3rd Overtone crystal oscillator. It uses some extra inductance and capacitor.

My question: is it possible to use 3rd overtone and have USB and RF?

I have one like this. I've placed the capacitors and inductance as in datadheet. It is seen as HS RCOSC and not as XOSC

The frequency is as defined in datasheet Fref = 48Mhz/2;
CLKSPD is forced to 1 because CLKCON.OSC=1 so the system frequency wil be Fref/2 => 12MHz

I've set-up the serial channel for 9600 baud based on this calculation and it works (also the timer looks ok)

Can I use USB in this case?

I've read that USB cannot be used with HS RCOSC. I'm not a specialist in Crystals but I was expecting that 3rd Overtone can be used instead of a fundamental crystal. Am I mistaking?

Thanks'

Can someone tell me if this one CFPS-39  can be used with cc2511?

  • 0
    TI__Expert 3940 points
    Marius, My understanding is that the third overtone crystal attaches to the XOSC-Q1/Q2 pins. Changing from a fundamental crystal to a third overtone doesn't affect the RC oscillator. So, yes, the USB and RF cannot be used with the RC oscillator, but since the third overtone crystal is the XOSC oscillator, this will work fine.
  • 0 in reply to GrantC
    Prodigy 30 points

    Hello Grant,

    Thanks' for your reply.

    The problem I have is that using the current 3rd overtone 48mhz crystal when I power up the controller and clear the CLKCON->OSC bit I get this:
    CLKCON->OSC = 1 (=> HS RC oscillator)
    CLKCON->CLKSPD = 1 (CLKCON.OSC=1 Calibrated HS RCOSC used as clock source for system clock) =>System clock speed = fRef /2
    SLEEP->XOSC_STB=0
    SLEEP->HFRC_STB=1 (High speed RC oscillator (HS RCOSC))

    So in the end the system clock will be set to 12MHz (verified trough the SCI baudrate calculation)
    If I use a fundamental crystal instead of the one I currently use (but different value then needed - 26MHz for example) then values are ok:
    CLKCON->OSC = 0
    CLKCON->CLKSPD = 0
    SLEEP->XOSC_STB=1
    SLEEP->HFRC_STB=0

    So it's really working working with 3rd overtone?
    Can be a mistake in my HW design? I've used the application circuit from the datasheet and all calculations shows that it realy oscilates at 48MHz but as HS RCOSC and not XOSC.
    On the crystal it is written 48.000 AEC08E. Can be that this is not suited as 3rd overtone crystal for this application?

  • 0 in reply to Marius Boarta
    TI__Expert 3940 points

    Marius,

    It appears that the CC2511 believes the crystal isn't stable, and isn't switching over.  Just to be sure that the software is being done correctly, you might refer to the clk_xosc.c example in http://www.ti.com/litv/zip/swrc117.  Otherwise, it does appear that there is probably something awry with the crystal.  I'm not familiar with your crystal;  do you have a data sheet for it?