UM10237_2 © NXP B.V. 2008. All rights reserved.
User manual Rev. 02 — 19 December 2008 459 o f 792
NXP Semiconductors UM10237
Chapter 17: LPC24XX Universal Asynchronous Receiver/Transmitter
4.14 Auto-baud
The UART1 auto-baud function can be used to measure the incoming baud-rate based on
the ”AT" protocol (Hayes command). If enabled the auto-baud feature will measure the bit
time of the receive data stream and set the divisor latch registers U1DLM and U1DLL
accordingly.
Auto-baud is started by setting the U1ACR Start bit. Auto-baud can be stopped by clearing
the U1ACR Start bit. The Start bit will clear once auto-baud has finished and reading the
bit will return the status of auto-baud (pending/finished).
Two auto-baud measuring modes are available which can be selected by the U1ACR
Mode bit. In mode 0 the baud-rate is measured on two subsequent falling edges of the
UART1 Rx pin (the falling edge of the start bit and the falling edge of the least significant
bit). In mode 1 the baud-rate is measured between the falling edge and the subsequent
rising edge of the UART1 Rx pin (the length of the start bit).
The U1ACR AutoRestart bit can be used to automatically restart baud-rate measurement
if a time-out occurs (the rate measurement counter overflows). If this bit is set the rate
measurement will restart at the next falling edge of the UART1 Rx pin.
The auto-baud function can generate two interrupts.
•The U1IIR ABTOInt interrupt will get set if the interrupt is enabled (U1IER ABToIntEn
is set and the auto-baud rate measurement counter overflows).
•The U1IIR ABEOInt interrupt will get set if the interrupt is enabled (U1IER ABEOIntEn
is set and the auto-baud has completed successfully).
The auto-baud interrupts have to be cleared by setting the corresponding U1ACR
ABTOIntClr and ABEOIntEn bits.
Typically the fractional baud-rate generator is disabled (DIVADDVAL = 0) during
auto-baud. However, if the fractional baud-rate generator is enabled (DIVADDVAL > 0), it
is going to impact the measuring of UART1 Rx pin baud-rate, but the value of the U1FDR
register is not going to be modified after rate measurement. Also, when auto-baud is used,
any write to U1DLM and U1DLL registers should be done before U1ACR register write.
The minimum and the maximum baudrates supported by UART1 are function of pclk,
number of data bits, stop bits and parity bits.
(4)
9 ABTOIntClr Auto-baud time-out interrupt clear bit (write only
accessible). 0
0 Writing a 0 has no impact.
1 Writing a 1 will clear the corresponding interrupt in the
U1IIR.
31:10 - NA Reserved, user software should not write ones to
reserved bits. The value read from a reserved bit is not
defined.
0
Table 411: Auto-baud Control Register (U1ACR - address 0xE001 0020) bit description
Bit Symbol Value Description Reset value