UM10208_2
© NXP B.V. 2007. All rights reserved.
User manual
Rev. 02 — 1 June 2007
166 of 362
NXP Semiconductors
UM10208
Chapter 14: LPC2800 UART
The auto-baud interrupts have to be cleared by writing a 1 to the corresponding ACR
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
does impact the measuring of the RXD pin baud rate, but the value of the FDR is not
modified after rate measurement. Also, when auto-baud is used, any write to DLM and
DLL registers should be done before the ACR is written. The minimum and the maximum
baud rates r
min
and r
max
supported are functions of the UART clock and the number of
data bits, stop bits and parity bits:
(1)
3.15.2 Auto-baud modes
When the software is expecting an ”AT" command, it configures the UART with the
expected character format and sets the ACR_Start bit. The initial values in the divisor
latches DLM and DLM don‘t matter. Because of the ”A" or ”a" ASCII coding (”A" = 0x41,
”a" = 0x61), the RXD 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 RSR is
reset. The RSR baud rate is switched to the highest rate.
2. A falling edge on RXD triggers the beginning of the start bit. The rate measuring
counter will start counting UART_CLK 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) 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 input clock.
5. If the Mode bit is 0, the rate counter stops on next falling edge of the RXD pin. If the
Mode bit is 1, the rate counter stops on the next rising edge of the RXD 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 ABEOInt is set in
the IIR, if it is enabled. The RSR will now continue receiving the remaining bits of the
”A/a" character.
r
min
2
UART
_
×
CLK
16
215
×
-------------------------------------------
UART
n
baudrate
UART
_
CLK
16
2
databits
paritybits
stopbits
+
+
+
(
)
×
------------------------------------------------------------------------------------------------------------
≤
≤
r
max
=
=