© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
User manual
Rev. 01 — 15 August 2005
124
Philips Semiconductors
UM10139
Volume 1
Chapter 10: UART1
Auto-RTS
The auto-RTS function is enabled by setting the CTSen bit. Auto-RTS data flow control
originates in the U1RBR module and is linked to the programmed receiver FIFO trigger
level. If auto-RTS is enabled, when the receiver FIFO level reaches the programmed
trigger level RTS1 is deasserted (to a high value). The sending UART may send an
additional byte after the trigger level is reached (assuming the sending UART has another
byte to send) because it may not recognize the deassertion of RTS1 until after it has
begun sending the additional byte. RTS1 is automatically reasserted (to a low value) once
the receiver FIFO has reached the previous trigger level. The reassertion of RTS1 signals
the sending UART to continue transmitting data.
If auto-RTS mode is disabled the RTSen bit controls the RTS1 output of the UART1. If
auto-RTS mode is enabled hardware controls the RTS1 output and the actual value of
RTS1 will be copied in the RTSen bit of the UART1. As long as auto-RTS is enabled the
value if the RTSen bit is read-only for software.
Example: Suppose the UART1 operating in type 550 has trigger level in U1FCR set to 0x2
then if auto-RTS is enabled the UART1 will deassert the RTS1 output as soon as the
receive FIFO contains 8 bytes (Table 123 on page 122). The RTS1 output will be
reasserted as soon as the receive FIFO hits the previous trigger level: 4 bytes.
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.
Fig 20. 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
~ ~
~ ~
~ ~
~ ~
~ ~