I'm little confused about the baud rate calculating equation of LPC2148

In data sheet equation is

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But in every tutorial I could find

equation is

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Second equation sets the baud rate as required.

Am I missing something?

Tutorials that I checked

  1. Link1
  2. Link2
  3. Link3
  • \$\begingroup\$ I have no experience with LPC2148, but equations are same if U0 stand for oversampling which is usually 16 (or 8 with faster baud rates). \$\endgroup\$
    – Rokta
    Oct 11, 2018 at 6:53
  • \$\begingroup\$ @Rokta Here U0 stands for UART0, U0DLL & U0DLM are device latch registers of UART0 \$\endgroup\$
    – Athul
    Oct 11, 2018 at 6:56

1 Answer 1


The only thing that makes sense is that the equation in the datasheet has an error.

That same datasheet says:

The U0DLL and U0DLM registers together form a 16 bit divisor where U0DLL contains the lower 8 bits of the divisor and U0DLM contains the higher 8 bits of the divisor

That suggests it should be 256*DLM+DLL as written in tutorials. In example tables below, it's also treated as 16-bits altogether (upper 8 + lower 8).

Note that April 2012 version of datasheet [1] contains the corrected equation.

[1] https://www.nxp.com/docs/en/user-guide/UM10139.pdf

  • \$\begingroup\$ I gave DLM = 1, DLL = 97, MULVAL = 14 & DIVADDVAL = 1, If equation was the one with 16 in denominator, Calculated baud rate would be 7743, and Equation with 256 in denominator would give 2478. I checked the value in Keil debugger, and the answer is 2478. So equation with 256 is the right one. \$\endgroup\$
    – Athul
    Oct 12, 2018 at 19:15
  • \$\begingroup\$ Can you tell me how you got the denominator as 256 by the info of U0DLL & U0DLM \$\endgroup\$
    – Athul
    Oct 12, 2018 at 19:17
  • \$\begingroup\$ Upper 8 bits means 1<<8 or 2**8, which is 256. \$\endgroup\$
    – domen
    Oct 14, 2018 at 9:03

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