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arunb
Joined: 08 Sep 2003
Posts: 492
Location: India
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| Maximum USART speed possibel with 4Mhz Xtal |
 Posted: Sat Jul 31, 2004 11:48 pm |
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Hello,
I am using PIC 16F877 and the CCS compiler. I as wondering what would be the maximum Baud rate possible with a 4 MHz xtal.
thanks
arun |
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Guest
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| Posted: Sun Aug 01, 2004 2:04 am |
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With BRGHOK it is
4000000/16=250kbaud
Normally it is
4000000/64 = 62500
Keep in mind that the baud rate is calculated as either
Fosc/16/(N+1) or
Fosc/64/(N+1) where N is the baud rate generator division factor. So you can not use most of standard baud rates @4MHz. Use a baud rate crystal instead. |
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arunb
Joined: 08 Sep 2003
Posts: 492
Location: India
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| RE: |
| Posted: Sun Aug 01, 2004 6:39 am |
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Hello,
I calculated that the baud rate cannot be more than 19200. Which means if I want higher baud rates I must go in for a 20 Mhz crystal.
What is a baud rate crystal and how can I connect it to the PIC.
Just curious to know, I am using a PCB with parts that have been soldered manually ( the maximum track spacing on the PCB is 0.5 mm) , is it OK to use a 20 Mhz crystal or will it cause problems ??. 1
thanks
arun |
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ckielstra
Joined: 18 Mar 2004
Posts: 3680
Location: The Netherlands
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| Posted: Sun Aug 01, 2004 7:10 am |
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| Quote: | | I calculated that the baud rate cannot be more than 19200. Which means if I want higher baud rates I must go in for a 20 Mhz crystal. |
Your calculations must be wrong. Even without calculations you can check Table 10-3 and 10-3 of the PIC16F877 datasheet where settings for several baudrates and crystals are shown: For a 4MHz crystal it states that the maximum baudrate with BRGH=0 is 62500 baud and with BRGH=1 even 250.000 baud.
| Quote: | | What is a baud rate crystal and how can I connect it to the PIC. |
These are normal crystals with very odd looking frequencies like 3.6864MHz. They are usefull in baudrate generators because dividing this frequency by a power of 2 will give you an exact baudrate value.
Just remember that in specifying the baudrate you are only allowed a maximum error of 3%. For example, trying to create a baudrate of 19k2 with a 4MHz crystal with BRGH=0 requires you to set SPBRG=2 (See table 10-3). The actual baudrate generated will be 20.833baud, being an error of 8.51%!!! This is to large an error. Same settings with a 3.6864MHz crystal give a 0% error. |
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arunb
Joined: 08 Sep 2003
Posts: 492
Location: India
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| RE: |
| Posted: Mon Aug 02, 2004 7:30 am |
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Yes you are right my calculations are wrong !!! Silly of me
Thank you very much
cheers
arun |
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