NXP Semiconductors LPC24XX UM10237 - Manual (Page 460)

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User manual Rev. 04 — 26 August 2009 460 of 792
NXP Semiconductors
UM10237
Chapter 17: LPC24XX UART1
4.16 Auto-baud modes
When the software is expecting an ”AT" command, it configures the UART1 with the
expected character format and sets the U1ACR Start bit. The initial values in the divisor
latches U1DLM and U1DLM don‘t care. Because of the ”A" or ”a" ASCII coding
(”A" = 0x41, ”a" = 0x61), the UART1 Rx pin sensed start bit and the LSB of the expected
character are delimited by two falling edges. When the U1ACR Start bit is set, the
auto-baud protocol will execute the following phases:
1. On U1ACR Start bit setting, the baud-rate measurement counter is reset and the
UART1 U1RSR is reset. The U1RSR baud rate is switch to the highest rate.
2. A falling edge on UART1 Rx pin triggers the beginning of the start bit. The rate
measuring counter will start counting pclk cycles.
3. During the receipt of the start bit, 16 pulses are generated on the RSR baud input with
the frequency of the UART1 input clock, guaranteeing the start bit is stored in the
U1RSR.
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 UART1 input clock (pclk).
5. If Mode = 0 then the rate counter will stop on next falling edge of the UART1 Rx pin. If
Mode = 1 then the rate counter will stop on the next rising edge of the UART1 Rx pin.
6. The rate counter is loaded into U1DLM/U1DLL and the baud-rate will be switched to
normal operation. After setting the U1DLM/U1DLL the end of auto-baud interrupt
U1IIR ABEOInt will be set, if enabled. The U1RSR will now continue receiving the
remaining bits of the ”A/a" character.
ratemin
2 P
× CLK
16 215
×
------------------------- UART 1
baudrate
PCLK
16 2 databits paritybits stopbits + + +
( ) ×
------------------------------------------------------------------------------------------------------------ ≤ ≤
ratemax = =