UM10429
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© NXP B.V. 2010. All rights reserved.
User manual
Rev. 1 — 20 October 2010
81 of 258
NXP Semiconductors
UM10429
Chapter 10: LPC1102 Universal Asynchronous Transmitter (UART)
10.5.12 Auto-baud modes
When the software is expecting an ”AT" command, it configures the UART with the
expected character format and sets the U0ACR Start bit. The initial values in the divisor
latches U0DLM and U0DLM don‘t care. Because of the ”A" or ”a" ASCII coding
(”A" = 0x41, ”a" = 0x61), the UART Rx pin sensed start bit and the LSB of the expected
character are delimited by two falling edges. When the U0ACR Start bit is set, the
auto-baud protocol will execute the following phases:
1. On U0ACR Start bit setting, the baud rate measurement counter is reset and the
UART U0RSR is reset. The U0RSR baud rate is switched to the highest rate.
2. A falling edge on UART Rx pin triggers the beginning of the start bit. The rate
measuring counter will start counting UART_PCLK cycles.
3. During the receipt of the start bit, 16 pulses are generated on the RSR baud input with
the frequency of the UART input clock, guaranteeing the start bit is stored in the
U0RSR.
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 UART input clock
(UART_PCLK).
5. If Mode = 0, the rate counter will stop on next falling edge of the UART Rx pin. If
Mode = 1, the rate counter will stop on the next rising edge of the UART Rx pin.
6. The rate counter is loaded into U0DLM/U0DLL and the baud rate will be switched to
normal operation. After setting the U0DLM/U0DLL, the end of auto-baud interrupt
U0IIR ABEOInt will be set, if enabled. The U0RSR will now continue receiving the
remaining bits of the ”A/a" character.
ratemin
2
P
×
CLK
16
215
×
-------------------------
UART
baudrate
PCLK
16
2
databits
paritybits
stopbits
+
+
+
(
)
×
------------------------------------------------------------------------------------------------------------
≤
≤
ratemax
=
=
