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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
281 of 571
NXP Semiconductors
UM10316
Chapter 19: LPC29xx Universal Asynchronous Receiver/Transmitter
Example:
Suppose the UART operating in type 550 has trigger level in FCR set to 0x2
then if Auto-RTS is enabled the UART will deassert the RTS output as soon as the receive
FIFO contains 8 bytes. The RTS output will be reasserted as soon as the receive FIFO
hits the previous trigger level: 4 bytes.
19.4.8.3 Auto-CTS
The Auto-CTS function is enabled by setting the CTSen bit. If Auto-CTS is enabled the
transmitter circuitry in the TSR module checks CTS input before sending the next data
byte. When CTS is active (low), the transmitter sends the next byte. To stop the
transmitter from sending the following byte, CTS must be released before the middle of
the last stop bit that is currently being sent. In Auto-CTS mode a change of the CTS signal
does not trigger a modem status interrupt unless the CTS Interrupt Enable bit is set, Delta
CTS bit in the MSR will be set though.
lists the conditions for 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 65. Auto-RTS Functional Timing
start
bits0..7
start
bits0..7
stop
start
bits0..7
stop
UART1 TX
CTS1 pin
~ ~
~ ~
~ ~
~ ~
stop
Table 239: Modem status interrupt generation
Enable
Modem
Status
Interrupt
(IER[3])
CTSen
(MCR[7])
CTS
Interrupt
Enable
(IER[7])
Delta CTS
(MSR[0])
Delta DCD or Trailing Edge
RI or
Delta DSR (MSR[3] or
MSR[2] or MSR[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