Programmable
threshold
TX buffer maximum
Zero byte
DMA active periods; this
does not represent the
DMA signaling.
Example: DMA is disabled to
show the end of the transfer.
Time
56 spaces
uart-027
Functional Description
In transmit mode, a DMA request is automatically asserted when the TX FIFO is empty. This request is
deasserted when the number of bytes defined by the number of spaces in the UARTi.UART_TLR register
is written by the sDMA. If an insufficient number of characters is written, the DMA request stays active.
Figure 19-8. Transmit FIFO DMA Request Generation (56 Spaces)
The DMA request is again asserted if the FIFO can receive the number of bytes defined by the
UARTi.UART_TLR register.
The threshold can be programmed in a number of ways.
shows a DMA transfer operating with
a space setting of 56 that can arise from using the auto settings in the UARTi.UART_FCR[5:4]
TX_FIFO_TRIG bit field or the UARTi.UART_TLR[3:0] TX_FIFO_TRIG_DMA bit field concatenated with
the TX_FIFO_TRIG bit field.
The setting of 56 spaces in the UART/IrDA/CIR module must correlate with the settings of the sDMA so
that the buffer does not overflow (program the DMA request size of the LH controller to equal the number
of spaces in the UART/IrDA/CIR module).
shows an example with eight spaces to show the buffer level crossing the space threshold.
The LH DMA controller settings must correspond to those of the UART/IrDA/CIR module.
3464
Universal Asynchronous Receiver/Transmitter (UART)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated