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Re: [Mspgcc-users] clock questions
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From: <wh...@te...> - 2007-03-27 20:04:17
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Reza, Well, both requirements need to be met. Oscillator circuits (such as those integrated onto the microprocessor die) have certain requirements for capacitive and resistive feedback and loading, to ensure reliable start-up and oscillation strength. So does the crystal. It used to be that you had to design your circuits taking into account the oscillator and crystal requirements, so indeed it was MCU-specific. Modern integrated circuits have wide ranges for capacitance (and/or feedback resistor) requirements, so today the crystal requirements are usually the determining factors in choosing the load capacitance values. I have not played with the 16 MHz version of the chip, so let us know if you do a design with it. William P.S.: To the other subscribers on the list: If this thread is getting too out-of-scope for the list, let us know and we'll take this off-list. thereza wrote: > William - > > Thanks for the excellent information. My only confusion is that my adviser > (I'm a grad student) mentioned that the load capacitance of a crystal has to > be equal to what the MCU expects. I had thought that it didn't matter as > long as you got the values equal to what the crystal expects. Who was > right? Or is it MCU specific? > > Also, I've noticed that there is a new 16Mhz version of the chip -- have you > had any experience with it or have any opinions about it? > > Thanks! > reza > > > > wh_fcab wrote: > >>Reza, >> >>Isn't T.I. documentation great? There's tonnes of it, yet you can't find >>certain basic information... :) >> >>Okay, with the XT2 oscillator, you can use either a crystal or resonator >>of up to 4 MHz if your supply voltage is 2.2V or higher. Go up to at >>least >>2.8V and you can use 8 MHz. The resonator should be hooked up between the >>XT2IN and XT2OUT pins. An AT-cut crystal can be hooked up the same way, >>but design your circuit board to provide for connecting two loading >>capacitors to ground, one from each side of the crystal. The crystal >>manufacturer usually indicates the load capacitance to use; in general >>they >>are 12 to 22 pF for an 8 MHz crystal. (Note that the oscillator will run >>at 8 MHz at 2.8V or higher, but the CPU itself needs 3.6V to reliably >>operate >>at 8 MHz; see the graph in the Recommended Operating Conditions chapter >>near >>the end of the 'F13x,14x,14x1 datasheet. >> >>The built-in capacitors you saw reference to are only for use with a 32 >>kHz >>watch crystal, if I remember correctly. >> >>Regarding the DCO/multiplier, you can get a DCO output of around 7 MHz >>that is >>stable enough for 115200 baud communication with the USART. I use a >>multiplier >>of 218 for 7.134324 MHz (well, it's as close to that frequency as the >>crystal is >>good) because it's the highest frequency the 'F149 can run at 3.3V over a >>reasonable temperature range. 8 MHz can be used only at 3.6V. The key >>with the >>'F1xx chips, which lack a hardware FLL, is ensuring you have a good >>*software* >>frequency-locked loop. My colleague and I tweaked our software FLL >>algorithm >>many times (over many years) before getting DCO output that was >>jitter-free >>enough to run 115200 baud without spurious framing errors. >> >>Cheers, >> >> William >> >> >>thereza wrote: >> >>>I hope that this isn't too off topic, if so, is there another mailling >>>list >>>that I can be directed to? >>> >>>I'm trying to switch over to the XT2 on a MSP430F149 but I'm having >>>problems >>>finding documentation on how the crystal should be hooked up. Can I >>>attach >>>a ceramic resonator with built in caps or will it only work with a >>>crystal >>>-- it also seems to imply that it has built-in caps for the xtal -- is >>>this >>>true? >>> >>>Also, with the built in frequency multiplier can I get a stable 8Mhz (or >>>16) >>>with just a watch crystal -- enough for some high speed serial >>>communication >>>via the uart/spi? >>> >>>Thanks, >>>Reza >> >>------------------------------------------------------------------------- >>Take Surveys. Earn Cash. Influence the Future of IT >>Join SourceForge.net's Techsay panel and you'll get the chance to share >>your >>opinions on IT & business topics through brief surveys-and earn cash >>http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV >>_______________________________________________ >>Mspgcc-users mailing list >>Msp...@li... >>https://lists.sourceforge.net/lists/listinfo/mspgcc-users >> >> > > |