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Architecture
19.2.3.1 Hardware Flow Control
Hardware flow control is composed of auto-CTS and auto-RTS. Auto-CTS and auto-RTS can be
enabled/disabled independently by programming EFR[7:6]. With auto-CTS, CTS must be active before
the module can transmit data.
Auto-RTS only activates the RTS output when there is enough room in the RX FIFO to receive data
and de-activates the RTS output when the RX FIFO is sufficiently full. The HALT and RESTORE trigger
levels in the TCR determine the levels at which RTS is activated/de-activated.
If both auto-CTS and auto-RTS are enabled, data transmission does not occur unless the receiver FIFO
has empty space. Thus, overrun errors are eliminated during hardware flow control. If not enabled,
overrun errors occur if the transmit data rate exceeds the receive FIFO latency.
19.2.3.1.1 Auto-RTS
Auto-RTS data flow control originates in the receiver block (see functional block diagram). The receiver
FIFO trigger levels used in auto-RTS are stored in the TCR. RTS is active if the RX FIFO level is below
the HALT trigger level in TCR[3:0]. When the receiver FIFO HALT trigger level is reached, RTS is
deasserted. The sending device (for example, another UART) may send an additional byte after the
trigger level is reached because it may not recognize the deassertion of RTS until it has begun sending
the additional byte. RTS is automatically reasserted once the receiver FIFO reaches the RESUME
trigger level programmed via TCR[7:4]. This reassertion requests the sending device to resume
transmission.
19.2.3.1.2 Auto-CTS
The transmitter circuitry checks CTS before sending the next data byte. When CTS is active, the
transmitter sends the next byte. To stop the transmitter from sending the following byte, CTS must be
de-asserted before the middle of the last stop bit that is currently being sent. The auto-CTS function
reduces interrupts to the host system. When auto-CTS flow control is enabled, the CTS state changes
need 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.
19.2.3.2 Software Flow Control
Software flow control is enabled through the enhanced feature register (EFR) and the modem control
register (MCR). Different combinations of software flow control can be enabled by setting different
combinations of EFR[3:0]. There are two other enhanced features relating to software flow control:
•
XON any function (MCR[5]): operation will resume after receiving any character after recognizing the
XOFF character. The XON-any character is written into the RX FIFO even if it is a software flow
character.
•
Special character (EFR[5]): Incoming data is compared to XOFF2. Detection of the special character
sets the XOFF interrupt (IIR[4]) but does not halt transmission. The XOFF interrupt is cleared by a
read of the IIR. The special character is transferred to the RX FIFO.
1684
UART/IrDA/CIR Module
Preliminary
© 2011, Texas Instruments Incorporated
www.ti.com
SPRUGX9 – 15 April 2011
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