UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 311 of 841
NXP Semiconductors
UM10360
Chapter 14: LPC176x/5x UART0/2/3
14.4.10.2 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:
1. On UnACR Start bit setting, the baud rate measurement counter is reset and the
UARTn UnRSR is reset. The UnRSR baud rate is switch to the highest rate.
2. A falling edge on UARTn Rx pin triggers the beginning of the start bit. The rate
measuring counter will start counting pclk cycles optionally pre-scaled by the
fractional baud-rate generator.
3. During the receipt of the start bit, 16 pulses are generated on the RSR baud input with
the frequency of the (fractional baud-rate pre-scaled) UARTn input clock,
guaranteeing the start bit is stored in the UnRSR.
4. During the receipt of the start bit (and the character LSB for mode = 0) the rate
counter will continue incrementing with the pre-scaled UARTn input clock (pclk).
5. If Mode = 0 then the rate counter will stop on next falling edge of the UARTn Rx pin. If
Mode = 1 then the rate counter will stop on the next rising edge of the UARTn Rx pin.
6. The rate counter is loaded into UnDLM/UnDLL and the baud-rate will be switched to
normal operation. After setting the UnDLM/UnDLL the end of auto-baud interrupt
UnIIR ABEOInt will be set, if enabled. The UnRSR will now continue receiving the
remaining bits of the “A/a” character.
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