UM10850
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User manual
Rev. 2.4 — 13 September 2016
272 of 464
NXP Semiconductors
UM10850
Chapter 21: LPC5410x USARTs (USART0/1/2/3)
21.6.9 USART Baud Rate Generator register
The Baud Rate Generator is a simple 16-bit integer divider controlled by the BRG register.
The BRG register contains the value used to divide the base clock in order to produce the
clock used for USART internal operations.
A 16-bit value allows producing standard baud rates from 300 baud and lower at the
highest frequency of the device, up to 921,600 baud from a base clock as low as 14.7456
MHz.
Typically, the baud rate clock is 16 times the actual baud rate. This overclocking allows for
centering the data sampling time within a bit cell, and for noise reduction and detection by
taking three samples of incoming data.
Note that in 32 kHz mode, the baud rate generator is still used and must be set to 0 if 9600
baud is required.
For more information on USART clocking, see
and
.
Remark:
In order to change a baud rate after a USART is running, the following sequence
should be used:
1. Make sure the USART is not currently sending or receiving data.
2. Disable the USART by writing a 0 to the Enable bit (0 may be written to the entire
register).
3. Write the new BRGVAL.
4. Write to the CFG register to set the Enable bit to 1.
Table 315. USART Baud Rate Generator register (BRG, offset 0x20) bit description
Bit
Symbol
Description
Reset Value
15:0
BRGVAL
This value is used to divide the USART input clock to determine the baud rate, based on
the input clock from the FRG.
0 = The FRG clock is used directly by the USART function.
1 = The FRG clock is divided by 2 before use by the USART function.
2 = The FRG clock is divided by 3 before use by the USART function.
...
0xFFFF = The FRG clock is divided by 65,536 before use by the USART function.
0
31:16
-
Reserved. Read value is undefined, only zero should be written.
NA