• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 98 subscribers
  • Views 739 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

External Oscillator ?

mscand
Prodigy 70 points
Other Parts Discussed in Thread: SN74LVC1GX04

Hi everyone:

I am new to C55 processors and this is my first post.

The C5505/15 system guide suggests that an external 12MHz oscillator is required.

A full oscillator IC consumes many mAmps thereby offsetting the power saving features of the c5515.  What are you using for the external oscillator?

Can one plug in a 12Mhz crystal + 2cap instead? But I only see 1 CLKIN pin in the datasheet.

Any advise is greatly appreciated.

Just so you know,  I need to change CPU clock speed during run time while maintaining an uninterrupted  clock rate of at least 12Mhz.

Thanks,

Marco

  • 0
    TI__Genius 13045 points

    Marco,

    Which document are you looking at?  There should be number like SPRUxxx or SPRSxxx on the first page.

    If you look at the data manual for the C5515 (http://www-s.ti.com/sc/techlit/sprs645) section 3.3 it shows a few options for clocking.  The only 12 MHz requirement I see is if you need to use USB.  Can you clarify your requirements?

    Regards.

  • 0 in reply to TommyG
    Prodigy 70 points

    Tommy,

    Thanks for the feedback:

    I refer to SPRUFP0A - Section 4: "System Clock Generator" of C5505's System User Guide. There is a diagram on page 37 showing the onboard oscillator which accepts only a 32.768Khz crystal while the alternative is an external oscillator that goes into CLKIN (single pin).

    In the Datasheet that you mentioned on page: 17 it is said that CLKIN is an input Pin for an external clock of frequency  11.2896, 12, or 12.288 MHz that can be used instead of the 32.768 oscillator. That's the external clock I am talking about.

    I would be happy to use the onboard 32.768KHz oscillator however I have the following problem:

    In order to change the PLL multiplier during runtime you need to switch the clock generator to bypass mode for 4ms (time needed by the PLL to lock phase). According to SPRUFP0 during those 4 milliseconds the CPU runs off the 32.768 KHz frequency.

    I cannot have my CPU running at 32KHz for 4ms in my design every thime I need to change the CPU speed.

    I am sure I am missing something. If you could point me in the right direction I would appreciate.


    Marco

  • 0 in reply to mscand
    TI__Genius 13045 points

    Marco,

    I see your point.  Your analysis is correct that your only options are external oscillator or 32 KHz clock from RTC. 

    I have a few questions to better understand your requirements:
    1. What is the minimum frequency your system will tolerate? 
    2. What other operating frequencies does your system require?
    3. Why is the current of an external oscillator so critical? 
    4. What are your power requirements? 

    Regards.

  • 0 in reply to TommyG
    Prodigy 70 points

    Tommy:

    1. The C5515 needs to read .wav data from NAND or microSD and send it to an external DAC via I2S. A minimum clock of 12Mhz is required to drive the DAC as slave and to run the I2S peripheral. Also the audio stream goes with a 44.1KHz frequency and we cannot afford to go below that.

    2. During read from SD a peak clock frequency of 50Mhz is required by the MMC/SD peripheral and we plan to read data in large 128KB packages to minimize power consumption from the SD card.
    After the 128KB package is downloaded to the on-chip RAM, the PLL should be lowered back to 12Mhz. The duty cycle should be 1/50 @50Mhz 49/50 @12Mhz every second.

    3. We are very tight with battery life and we are trying to avoid additional IC's due to severe footprint constraints as well. The oscillator ICs I found on DigiKey use 5 to 30mW of power which I would like to avoid.

    4. The whole system needs to consume less than 30mW. With the DAC using 12mW, the NAND/SDcard 3mW and the remaining 15mW are for the DSP.
    The C5515 CPU power consumption is very competitive: we were hoping we could use C5515 with a simple external crystal.

    Regards,
    Marco

     

  • 0 in reply to mscand
    Intellectual 520 points

    Hi Marco,

    I have just tested a custom board with TMX320C5515AZCH12 and TLV320AIC3204IRHBT both clocked by an external SN74LVC1GX04 crystal oscillator driver and 12MHz crystal.

    With a 2.5V supply the SN74LVC1GX04 is taking 1.5mA so 3.75mW.

    Circuit is the Pierce Oscillator shown in Figure 3 in the SN74LVC1GX04 data sheet.  Circuit values are the same except it uses 22pF capacitors. (32pF caps don't allow for strays).  Crystal is a Euro Quartz 12.0000MHz MT-30/50/20/16pF.

    Data Sheet from: http://focus.ti.com/docs/prod/folders/print/sn74lvc1gx04.html

    Regards, Andrew.

  • 0 in reply to A.Wilson
    Prodigy 70 points

    Thanks, Andrew.

    Good information