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2005-11-11 by seangra
Assuming that I want to be able run at maximum speed, what oscillator/crystal should I choose? Normally the choice is dictated by the divider for the UART, but in this case (from what I understand) with the prescaler you can get just about any baud rate you want from whatever source frequency you choose. So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are there that influence which frequency should be used?
2005-11-11 by Tom Walsh
seangra wrote: >Assuming that I want to be able run at maximum speed, what >oscillator/crystal should I choose? > >Normally the choice is dictated by the divider for the UART, but in >this case (from what I understand) with the prescaler you can get just >about any baud rate you want from whatever source frequency you choose. > >So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are there >that influence which frequency should be used? > > > You could clue us in on what specific processor the question is about. The UART clock is a bit different in some of the processors. TomW -- Tom Walsh - WN3L - Embedded Systems Consultant http://openhardware.net, http://cyberiansoftware.com "Windows? No thanks, I have work to do..." ----------------------------------------------------
2005-11-11 by Richard Duits
I am not an expert in this, but I think the higher the multiplier (= a lower external crystal freq) the more jitter you get on the CCLK. So if you want a very stable CCLK you choose the highest supported external crystal frequency you can find. Richard Duits. seangra wrote:
> Assuming that I want to be able run at maximum speed, what > oscillator/crystal should I choose? > > Normally the choice is dictated by the divider for the UART, but in > this case (from what I understand) with the prescaler you can get just > about any baud rate you want from whatever source frequency you choose. > > So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are there > that influence which frequency should be used? > > > > > > SPONSORED LINKS > Microprocessor > <http://groups.yahoo.com/gads?t=ms&k=Microprocessor&w1=Microprocessor&w2=Microcontrollers&w3=Pic+microcontrollers&w4=8051+microprocessor&c=4&s=93&.sig=tsVC-J9hJ5qyXg0WPR0l6g> > Microcontrollers > <http://groups.yahoo.com/gads?t=ms&k=Microcontrollers&w1=Microprocessor&w2=Microcontrollers&w3=Pic+microcontrollers&w4=8051+microprocessor&c=4&s=93&.sig=DvJVNqC_pqRTm8Xq01nxwg> > Pic microcontrollers > <http://groups.yahoo.com/gads?t=ms&k=Pic+microcontrollers&w1=Microprocessor&w2=Microcontrollers&w3=Pic+microcontrollers&w4=8051+microprocessor&c=4&s=93&.sig=TpkoX4KofDJ7c6LyBvUqVQ> > > 8051 microprocessor > <http://groups.yahoo.com/gads?t=ms&k=8051+microprocessor&w1=Microprocessor&w2=Microcontrollers&w3=Pic+microcontrollers&w4=8051+microprocessor&c=4&s=93&.sig=1Ipf1Fjfbd_HVIlekkDP-A> > > > > ------------------------------------------------------------------------ > YAHOO! GROUPS LINKS > > * Visit your group "lpc2000 > <http://groups.yahoo.com/group/lpc2000>" on the web. > > * To unsubscribe from this group, send an email to: > lpc2000-unsubscribe@yahoogroups.com > <mailto:lpc2000-unsubscribe@yahoogroups.com?subject=Unsubscribe> > > * Your use of Yahoo! Groups is subject to the Yahoo! Terms of > Service <http://docs.yahoo.com/info/terms/>. > > > ------------------------------------------------------------------------ >
2005-11-11 by seangra
> You could clue us in on what specific processor the question is > about. The UART clock is a bit different in some of the uCs. Sorry, my bad. It's LPC2148. > I am not an expert in this, but I think the higher the multiplier > (= a lower external crystal freq) the more jitter you get on the > CCLK. So if you want a very stable CCLK you choose the highest > supported external crystal frequency you can find. What effect does jitter on the CCLK line have? Original question: > Assuming that I want to be able run at maximum speed, what > oscillator/crystal should I choose? > > Normally the choice is dictated by the divider for the UART, > but in this case (from what I understand) with the prescaler you > can get just about any baud rate you want from whatever > source frequency you choose. > > So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are > there that influence which frequency should be used? [Non-text portions of this message have been removed]
2005-11-11 by Tom Walsh
seangra wrote: >>You could clue us in on what specific processor the question is >>about. The UART clock is a bit different in some of the uCs. >> >> > >Sorry, my bad. > >It's LPC2148. > > > >>I am not an expert in this, but I think the higher the multiplier >>(= a lower external crystal freq) the more jitter you get on the >>CCLK. So if you want a very stable CCLK you choose the highest >>supported external crystal frequency you can find. >> >> > > > No, not really. You have to be aware of PPM spec of the crystal. Parts Per Million is a statement of the accuracy of the crystal at a given frequency (Million being Million Cyles or MHz, Parts being the cycles of drift). But higher frequency 5PPM is no more / less accurate than 5PPM at a lower frequency... It does compute into a percentage of error though. >What effect does jitter on the CCLK line have? > >Original question: > > >>Assuming that I want to be able run at maximum speed, what >>oscillator/crystal should I choose? >> >>Normally the choice is dictated by the divider for the UART, >>but in this case (from what I understand) with the prescaler you >>can get just about any baud rate you want from whatever >>source frequency you choose. >> >>So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are >>there that influence which frequency should be used? >> >> > > > For me, the primary choice of frequency was that of the bootloader. 14.7456MHz crystal was one of the recommended frequencies and it would give me a 58MHz clockrate for the CPU. I also have some MAX3100 SPI UARTs on my board, they needed a 3.686MHz clock, by using an external TTL oscillator to drive both the X1 input of the processor and drive the input to a divide by 4 counter (74HC93), I was able to satisfy the CPU and the UARTs with one clock source. TomW -- Tom Walsh - WN3L - Embedded Systems Consultant http://openhardware.net, http://cyberiansoftware.com "Windows? No thanks, I have work to do..." ---------------------------------------------------- [Non-text portions of this message have been removed]
2005-11-11 by seangra
--- In lpc2000@yahoogroups.com, Tom Walsh <tom@o...> wrote: > >>I am not an expert in this, but I think the higher the multiplier > >>(= a lower external crystal freq) the more jitter you get on the > >>CCLK. So if you want a very stable CCLK you choose the highest > >>supported external crystal frequency you can find. > No, not really. You have to be aware of PPM spec of the crystal. > Parts Per Million is a statement of the accuracy of the crystal at > a given frequency (Million being Million Cyles or MHz, Parts being > the cycles of drift). But higher frequency 5PPM is no more / less > accurate than 5PPM at a lower frequency... It does compute into a > percentage of error though. That explains the vast difference in price between different crystals all at the same frequency. > For me, the primary choice of frequency was that of the > bootloader. 14.7456MHz crystal was one of the recommended > frequencies and it would give me a 58MHz clockrate for the > CPU. I also have some MAX3100 SPI UARTs on my board, they > needed a 3.686MHz clock, by using an external TTL oscillator > to drive both the X1 input of the processor and drive the > input to a divide by 4 counter (74HC93), I was able to > satisfy the CPU and the UARTs with one clock source. Doesn't the 74HC93 cost more than a 2nd crystal? Do the serial busses' operational frequency depend on the "input clock" from the external oscillator? The spec says (for SPI) that it can operate at maximum 1/8th the "input clock" (which I'm assuming is Fosc)? I just re-read the user manual (not the spec manual) and if I want to use USB, then pretty much my only choice for external clock is going to be 12MHz if I want full speed, right? If I want to play, 16MHz for either 48MHz or O/C to 64MHz, or 24MHz for 48Mhz or O/C to 72MHz (probably a bad idea!). So if I want max performance out of my serial bus, then I want a higher Fosc, which essentially limits me to a lower CCLK? Sorry for the newb questions.
> >Original question: > > > >>Assuming that I want to be able run at maximum speed, what > >>oscillator/crystal should I choose? > >> > >>Normally the choice is dictated by the divider for the UART, > >>but in this case (from what I understand) with the prescaler you > >>can get just about any baud rate you want from whatever > >>source frequency you choose. > >> > >>So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are > >>there that influence which frequency should be used? > >> > >> (For an LPC2148)
2005-11-15 by seangra
Anyone have a response to this? If I use the LPC2148 am I essentially stuck with a 12MHz external crystal if I want to use USB and have the uC running at full speed? --- In lpc2000@yahoogroups.com, "seangra" <sgraham@o...> wrote: > > --- In lpc2000@yahoogroups.com, Tom Walsh <tom@o...> wrote: > > > >>I am not an expert in this, but I think the higher the multiplier > > >>(= a lower external crystal freq) the more jitter you get on the > > >>CCLK. So if you want a very stable CCLK you choose the highest > > >>supported external crystal frequency you can find. > > > No, not really. You have to be aware of PPM spec of the crystal. > > Parts Per Million is a statement of the accuracy of the crystal at > > a given frequency (Million being Million Cyles or MHz, Parts being > > the cycles of drift). But higher frequency 5PPM is no more / less > > accurate than 5PPM at a lower frequency... It does compute into a > > percentage of error though. > > That explains the vast difference in price between different > crystals all at the same frequency. > > > For me, the primary choice of frequency was that of the > > bootloader. 14.7456MHz crystal was one of the recommended > > frequencies and it would give me a 58MHz clockrate for the > > CPU. I also have some MAX3100 SPI UARTs on my board, they > > needed a 3.686MHz clock, by using an external TTL oscillator > > to drive both the X1 input of the processor and drive the > > input to a divide by 4 counter (74HC93), I was able to > > satisfy the CPU and the UARTs with one clock source. > > Doesn't the 74HC93 cost more than a 2nd crystal? > > Do the serial busses' operational frequency depend on the "input > clock" from the external oscillator? The spec says (for SPI) that > it can operate at maximum 1/8th the "input clock" (which I'm > assuming is Fosc)? > > I just re-read the user manual (not the spec manual) and if I want > to use USB, then pretty much my only choice for external clock is > going to be 12MHz if I want full speed, right? If I want to play, > 16MHz for either 48MHz or O/C to 64MHz, or 24MHz for 48Mhz or O/C to > 72MHz (probably a bad idea!). > > So if I want max performance out of my serial bus, then I want a > higher Fosc, which essentially limits me to a lower CCLK? > > Sorry for the newb questions. > > > > >Original question: > > > > > >>Assuming that I want to be able run at maximum speed, what > > >>oscillator/crystal should I choose? > > >> > > >>Normally the choice is dictated by the divider for the UART, > > >>but in this case (from what I understand) with the prescaler you
> > >>can get just about any baud rate you want from whatever > > >>source frequency you choose. > > >> > > >>So if 10*6 or 20*3 or 15*4 all yield 60MHz, what factors are > > >>there that influence which frequency should be used? > > >> > > >> (For an LPC2148) >
2005-11-15 by Joel Winarske
seangra wrote: > Anyone have a response to this? > > If I use the LPC2148 am I essentially stuck with a 12MHz external > crystal if I want to use USB and have the uC running at full speed? There are two PLLs in the LPC2148. If you run an external crystal of 12MHz, the two PLL's allow you to run the CPU at 60MHz, and USB at 48MHz. Utilizing the Fractional divider in the UART peripheral, you can achieve nearly any required baud rate. Does that limit your app? Joel
2005-11-15 by seangra
Thanks for the response, yes, I realize the fractional divider can give me any baud rate (well, within tolerance). More specifically what I am trying to get at is for the choice of the external crystal. If I want to use USB then I have to use either 12MHz, 16MHz or 24MHz external crystals, right? I.e. it's not possible to use USB with a 10MHz or 15MHz or 20MHz external crystal? If that is the case then I think that the LPC2148 datasheet should be updated to mention that. The only place that it is mentioned (directly) at all (that I found) is in the user guide, and even that's just under an example, not as a direction. --- In lpc2000@yahoogroups.com, "Joel Winarske" <joelw@i...> wrote: > > seangra wrote: > > > Anyone have a response to this? > > > > If I use the LPC2148 am I essentially stuck with a 12MHz external > > crystal if I want to use USB and have the uC running at full speed? > > There are two PLLs in the LPC2148. If you run an external crystal of 12MHz, > the two PLL's allow you to run the CPU at 60MHz, and USB at 48MHz. > > Utilizing the Fractional divider in the UART peripheral, you can achieve
> nearly any required baud rate. > > Does that limit your app? > > > Joel >