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UM10503
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© NXP B.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 10 December 2015
1142 of 1441
NXP Semiconductors
UM10503
Chapter 40: LPC43xx/LPC43Sxx USART0_2_3
The fractional baud rate generator must be disabled (DIVADDVAL = 0) during auto-baud.
Also, when auto-baud is used, any write to DLM and DLL registers should be done before
ACR register write. The minimum and the maximum baud rates supported by USART are
function of USART_PCLK, number of data bits, stop bits and parity bits.
(7)
40.7.2 Auto-baud modes
When the software is expecting an ”AT" command, it configures the USART with the
expected character format and sets the ACR Start bit. The initial values in the divisor
latches DLM and DLM don‘t care. Because of the ”A" or ”a" ASCII coding (”A" = 0x41,
”a" = 0x61), the USART Rx pin sensed start bit and the LSB of the expected character are
delimited by two falling edges. When the ACR Start bit is set, the auto-baud protocol will
execute the following phases:
1. On ACR Start bit setting, the baud rate measurement counter is reset and the USART
RSR is reset. The RSR baud rate is switched to the highest rate.
2. A falling edge on USART Rx pin triggers the beginning of the start bit. The rate
measuring counter will start counting USART_PCLK cycles.
3. During the receipt of the start bit, 16 pulses are generated on the RSR baud input with
the frequency of the USART input clock, guaranteeing the start bit is stored in the
RSR.
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 USART input clock
(USART_PCLK).
5. If Mode = 0, the rate counter will stop on next falling edge of the USART Rx pin. If
Mode = 1, the rate counter will stop on the next rising edge of the USART Rx pin.
6. The rate counter is loaded into DLM/DLL and the baud rate will be switched to normal
operation. After setting the DLM/DLL, the end of auto-baud interrupt IIR ABEOInt will
be set, if enabled. The RSR 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
=
=