NXP Semiconductors UM10237

User manual Rev. 02 — 19 December 2008 434 o f 792

Chapter 16: LPC24XX Universal Asynchronous Receiver/Transmitter

16.4.10.1 Auto-baud

The UARTn 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 UnDLM and UnDLL

accordingly.

Auto-baud is started by setting the UnACR Start bit. Auto-baud can be stopped by clearing

the UnACR 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 UnACR

Mode bit. In mode 0 the baud-rate is measured on two subsequent falling edges of the

UARTn 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 UARTn Rx pin (the length of the start bit).

The UnACR 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 UARTn Rx pin.

The auto-baud function can generate two interrupts.

•The UnIIR ABTOInt interrupt will get set if the interrupt is enabled (UnIER ABToIntEn

is set and the auto-baud rate measurement counter overflows).

•The UnIIR ABEOInt interrupt will get set if the interrupt is enabled (UnIER ABEOIntEn

is set and the auto-baud has completed successfully).

The auto-baud interrupts have to be cleared by setting the corresponding UnACR

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 UARTn Rx pin baud-rate, but the value of the UnFDR

any write to UnDLM and UnDLL registers should be done before UnACR register write.

The minimum and the maximum baudrates supported by UARTn are function of pclk,

number of data bits, stop bits and parity bits.

(1)

16.4.10.2 Auto-baud modes

When the software is expecting an ”AT" command, it configures the UARTn with the

expected character format and sets the UnACR Start bit. The initial values in the divisor

latches UnDLM and UnDLM don‘t care. Because of the ”A" or ”a" ASCII coding

(”A" = 0x41, ”a" = 0x61), the UARTn Rx pin sensed start bit and the LSB of the expected

character are delimited by two falling edges. When the UnACR Start bit is set, the

auto-baud protocol will execute the following phases:

ratemin 2P×CLK

16 215

-------------------------UARTnbaudrate PCLK

16 2 databits paritybits stopbits++ +()×

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≤≤ ratemax==