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UARTn_TXD
UARTn_CTS
Start
Stop
Bits0−7
Start
Stop
Bits 0−7
Start
Stop
Bits 0−7
UARTn_RXD
UARTn_RTS
Start
Start
Start
Stop
Stop
Bits N
Bits N+1
Start
Stop
Peripheral Architecture
1511
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Universal Asynchronous Receiver/Transmitter (UART)
31.2.5.4.1
UARTn_RTS Behavior
UARTn_RTS data flow control originates in the receiver block (see
). When the receiver FIFO
level reaches a trigger level of 1, 4, 8, or 14 (see
), UARTn_RTS is deasserted. 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 UARTn_RTS until after it has
begun sending the additional byte. For trigger level 1, 4, and 8, UARTn_RTS is automatically reasserted
once the receiver FIFO is emptied. For trigger level 14, UARTn_RTS is automatically reasserted once the
receiver FIFO drops below the trigger level.
Figure 31-6. Autoflow Functional Timing Waveforms for UARTn_RTS
(1)
N = Receiver FIFO trigger level.
(2)
The two blocks in dashed lines cover the case where an additional byte is sent.
31.2.5.4.2
UARTn_CTS Behavior
The transmitter checks UARTn_CTS before sending the next data byte. If UARTn_CTS is active, the
transmitter sends the next byte. To stop the transmitter from sending the following byte, UARTn_CTS
must be released before the middle of the last STOP bit that is currently being sent (see
When flow control is enabled, UARTn_CTS level changes do not trigger interrupts because the device
automatically controls its own transmitter. Without autoflow control, the transmitter sends any data present
in the transmitter FIFO and a receiver overrun error may result.
Figure 31-7. Autoflow Functional Timing Waveforms for UARTn_CTS
(1)
When UARTn_CTS is active (low), the transmitter keeps sending serial data out.
(2)
When UARTn_CTS goes high before the middle of the last STOP bit of the current byte, the transmitter
finishes sending the current byte but it does not send the next byte.
(3)
When UARTn_CTS goes from high to low, the transmitter begins sending data again.
31.2.5.5 Loopback Control
The UART can be placed in the diagnostic mode using the LOOP bit in the modem control register (MCR),
which internally connects the UART output back to the UART input. In this mode, the transmit and receive
data paths, the transmitter and receiver interrupts, and the modem control interrupts can be verified
without connecting to another UART.