10 auto-cts, Nxp semiconductors – NXP Semiconductors LPC24XX UM10237 User Manual

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UM10237_4

User manual

Rev. 04 — 26 August 2009

455 of 792

NXP Semiconductors

UM10237

Chapter 17: LPC24XX UART1

4.10 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.

Table 17–407

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.

Figure 17–69

illustrates the Auto-CTS functional timing.

While starting transmission of the initial character the CTS1 signal is asserted.
Transmission will stall as soon as the pending transmission has completed. The UART will
continue transmitting a 1 bit as long as CTS1 is deasserted (high). As soon as CTS1 gets
deasserted transmission resumes and a start bit is sent followed by the data bits of the
next character.

Table 407. 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

Yes

Fig 69. Auto-CTS Functional Timing

start

bits0..7

start

bits0..7

stop

start

bits0..7

stop

UART1 TX

CTS1 pin

~ ~

stop