NS16C2552 Datasheet, PDF (28/50 Pages) National Semiconductor (TI) – Dual UART with 16-byte/64-byte FIFOs and up to 5 Mbit/s Data Rate

NS16C2552, NS16C2752
SNLS238D – AUGUST 2006 – REVISED APRIL 2013
www.ti.com
Figure 13. TXRDY in DMA Mode 0
Transmit Hardware Flow Control
CTS is a flow control input used to prevent remote receiver FIFO data overflow. The CTS input is monitored to
suspend/resume the local transmitter. The automatic CTS flow control can be enabled to suit specific application.
• Enable auto CTS flow control EFR[7]=1.
Transmit Flow Control Interrupt
• Enable auto CTS flow control EFR[7]=1.
• Enable CTS interrupt IER[7]=1.
An interrupt is generated when CTS pin is de-asserted (logic 1). IIR[5] is set to logic 1. The transmitter suspends
transmission as soon as the stop bit is shifted out. Transmission is resumed after the CTS pin is asserted logic 0,
indicating remote receiver is ready to accept data word.
SOFTWARE XON/XOFF FLOW CONTROL
Software flow control uses programmed Xon or Xoff characters to enable the transmit/receive flow control. The
receiver compares one or two sequentially received data words. If the received character(s) match the
programmed values, the transmitter suspends operation as soon as the current transmitting frame is completed.
When a match occurs, the Xoff (if enabled via IER[5]) flag is set and an interrupt is generated. Following a
transmission suspension, the UART monitors the receive data stream for an Xon character. When a match is
found, the transmission resumes and the IIR[4] flag clears.
Upon reset, the Xon/Xoff characters are cleared to logic 0. The user may write any Xon/Xoff value desired for
software flow control. Different conditions can be set to detect Xon/Xoff characters and suspend/resume
transmissions. When double 8-bit Xon/Xoff characters are selected, the UART compares two consecutively
received characters with two software flow control 8-bit values (Xon1, Xon2, Xoff1, and Xoff2) and controls
transmission accordingly. Under the above described flow control mechanisms, flow control characters are not
placed in the user accessible Rx data buffer or FIFO.
During the flow control operation, when Receive FIFO pointer reaches the upper trigger level, the UART
automatically transmits Xoff1 and Xoff 2 messages via the serial TX line output to the remote modem. When
Receive FIFO pointer position matches the lower trigger level, the UART automatically transmits Xon1 and Xon2
characters.
Care should be taken when designing the software flow control section of the driver. In the case where a local
UART is transmitting and the remote UART initiates flow control, an Xoff character is sent by the remote UART.
Upon receipt the local UART ceases to transmit until such time as the remote UART FIFO has been drained
sufficiently and it signals that it can accept further data by sending an Xon character to the local UART.
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