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I want know UART crystal used in it why it is of 11.05MHZ, and why we use it.

tangrs
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Mamta Varma
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    No, we use 14.7456 MHz for the UARTs in our 8051 based µC. – Turbo J Jan 29 '17 at 06:08
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    Get a calculator, and divide 11.0592 MHz by 3 a few times, and by 2 a few times. Notice anything? UARTs run at the standard Baud rates of 300, 600, 1200, 2400 ... and so on, going up in factors of 2. Notice anything? – Neil_UK Jan 29 '17 at 06:13
  • Older parts that didn't have the luxury of PLLs to multiply frequency had to be satisfied with just dividing. – Spehro Pefhany Jan 29 '17 at 12:18

1 Answers1

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This small chart should help a lot:

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There are any number of crystal frequencies that can be divided down by simple integer values to support standard UART Baud rates. Since many more low cost microcontrollers these days support higher frequencies of operation (such as 25MHz) I like to design with the 22.1184MHz crystal to achieve results a in this table.

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Another popular crystal frequency for those MCUs that are rated for up to 20MHz operation is the 19.6608MHz frequency. This frequency however does not exactly support the 115.2K baud rate made very popular by the advent of the standard PC architecture when IBM first came out with their PC. Those early PCs used a frequency of 18.432MHz or 1.8432MHz for their UARTs. Using this latter frequency with a 20MHz rated CPU does mean that you give up more processing performance than using the 19.6608MHz frequency.

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Michael Karas
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  • why were 9600, 19200 or 4800 baud rate chosen in the first place? Why not choosing a baud rate of 10000 then 20000... or even 7000, 14000.... – Jacquelyn.Marquardt Jul 14 '20 at 17:29
  • @Jacquelyn.Marquardt Like many things -- historical reasons. Some early Teletype machine was able to achieve 75 baud, a subsequent model doubled that to 150, then early modems doubled that to 300, and we've been doubling ever since. – gwideman Aug 18 '21 at 08:06