Intel CONTROLLERS 413808 - Manual (Page 665)

Intel ® 413808 and 413812 I/O Controllers in TPER Mode
October 2007 Developer’s Manual
Order Number: 317805-001US 665
UARTs—Intel ® 413808 and 413812
13.3.4 Autoflow Control
Autoflow Control uses the Clear-to-Send ( CTS# ) and Request-to-Send ( RTS# ) signals
to automatically control the flow of data between the UART and external modem. When
autoflow is enabled, the remote device is not allowed to send data unless the UART
asserts nRTS low. When the UART deasserts RTS# while the remote device is sending
data, the remote device is allowed to send one additional byte after RTS# is
deasserted. An overflow could occur when the remote device violates this rule.
Likewise, the UART is not allowed to transmit data unless the remote device asserts
CTS# low. This feature increases system efficiency and eliminates the possibility of a
Receive FIFO Overflow error due to long Interrupt latency.
Autoflow mode can be used in two ways:
Full autoflow, automating both CTS# and
RTS# , and half autoflow, automating only CTS# . Full Autoflow is enabled by writing
a 1 to bits 1 and 5 of the Modem Control register (MCR). Auto-CTS-Only mode is
enabled by writing a 1 to bit 5 and a 0 to bit 1 of the MCR register.
13.3.4.1 RTS Autoflow
When in full autoflow mode, RTS# is asserted when the UART FIFO is ready to receive
data from the remote transmitter. This occurs when the amount of data in the Receive
FIFO is below the programmable threshold value. When the amount of data in the
Receive FIFO reaches the programmable threshold, RTS# is deasserted. It is asserted
once again when enough bytes are removed from the FIFO to lower the data level
below the threshold.
13.3.4.2 CTS Autoflow
When in Full or Half-Autoflow mode, CTS# is asserted by the remote receiver when the
receiver is ready to receive data from the UART. The UART checks CTS# before sending
the next byte of data and does not transmit the byte until CTS# is low. When CTS#
goes high while the transfer of a byte is in progress, the transmitter completes this
byte.
Note:
Autoflow mode can be used only in conjunction with FIFO mode.