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UM10601
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© NXP B.V. 2012. All rights reserved.
Preliminary user manual
Rev. 1.0 — 7 November 2012
179 of 313
NXP Semiconductors
UM10601
Chapter 15: LPC800 USART0/1/2
15.6.9 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.
Details on how to select the right values for BRG can be found later in this chapter, see
Remark:
If software needs to change the baud rate, 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 registers). 3) Write
the new BRGVAL. 4) Write to the CFG register to set the Enable bit to 1.
15.6.10 USART Interrupt Status register
The read-only INTSTAT register provides a view of those interrupt flags that are currently
enabled. This can simplify software handling of interrupts. See
for detailed
descriptions of the interrupt flags.
Table 166. USART Baud Rate Generator register (BRG, address 0x4006 4020 (USART0),
0x4006 8020 (USART1), 0x4006 C020 (USART2)) 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
Table 167. USART Interrupt Status register (INTSTAT, address 0x4006 4024 (USART0),
0x4006 8024 (USART1), 0x4006 C024(USART2)) bit description
Bit
Symbol
Description
Reset
Value
0
RXRDY
Receiver Ready flag.
0
1
-
Reserved. Read value is undefined, only zero should be
written.
NA
2
TXRDY
Transmitter Ready flag.
1