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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
302 of 808
NXP Semiconductors
UM10360
Chapter 15: LPC17xx UART1
Example:
Suppose the UART1 operating in ‘550 mode has trigger level in U1FCR set to
0x2 then if Auto-RTS is enabled the UART1 will de-assert the RTS1 output as soon as the
receive FIFO contains 8 bytes (
). The RTS1 output will be
reasserted as soon as the receive FIFO hits the previous trigger level: 4 bytes.
15.4.9.2 Auto-CTS
The Auto-CTS function is enabled by setting the CTSen bit. If Auto-CTS is enabled the
transmitter circuitry in the U1TSR module checks CTS1 input before sending the next data
byte. When CTS1 is active (low), the transmitter sends the next byte. To stop the
transmitter from sending the following byte, CTS1 must be released before the middle of
the last stop bit that is currently being sent. In Auto-CTS mode a change of the CTS1
signal does not trigger a modem status interrupt unless the CTS Interrupt Enable bit is set,
Delta CTS bit in the U1MSR will be set though.
generating a Modem Status interrupt.
The auto-CTS function reduces interrupts to the host system. When flow control is
enabled, a CTS1 state change does not trigger host interrupts because the device
automatically controls its own transmitter. Without Auto-CTS, the transmitter sends any
data present in the transmit FIFO and a receiver overrun error can result.
illustrates the Auto-CTS functional timing.
Fig 46. Auto-RTS Functional Timing
start
byte N
stop
start
bits0..7
stop
start
bits0..7
stop
N-1
N
N-1
N-1
N-2
N-2
M+2
M+1
M
M-1
UART1 Rx
RTS1 pin
UART1 Rx
FIFO level
UART1 Rx
FIFO read
~ ~
~ ~
~ ~
~ ~
~ ~
Table 279: Modem status interrupt generation
Enable
Modem
Status
Interrupt
(U1ER[3]
)
CTSen
(U1MCR[7])
CTS
Interrupt
Enable
(U1IER[7])
Delta CTS
(U1MSR[0])
Delta DCD or Trailing Edge
RI or
Delta DSR (U1MSR[3] or
U1MSR[2] or U1MSR[1])
Modem
Status
Interrupt
0
x
x
x
x
No
1
0
x
0
0
No
1
0
x
1
x
Yes
1
0
x
x
1
Yes
1
1
0
x
0
No
1
1
0
x
1
Yes
1
1
1
0
0
No
1
1
1
1
x
Yes
1
1
1
x
1
Yes