Intel® Stratix® 10 Hard Processor System Technical Reference Manual

ID 683222
Date 11/28/2022
Public

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21.4.3.3. Automatic CTS mode

Automatic CTS mode becomes active when the following conditions occur: †

  • AFCE (MCR.AFCE bit is set)
  • FIFO buffers are enabled (through FIFO buffer control register IIR_FCR.FIFOE) bit

When automatic CTS is enabled (active), the UART transmitter is disabled whenever the cts_n input becomes inactive (high). This prevents overflowing the FIFO buffer of the receiving UART. †

If the cts_n input is not deactivated before the middle of the last stop bit, another character is transmitted before the transmitter is disabled. While the transmitter is disabled, you can continue to write and even overflow to the transmit FIFO buffer. †

Automatic CTS mode requires the following sequence:

  1. The UART status register are read to verify that the transmit FIFO buffer is full (UART status register USR.TFNF set to zero). †
  2. The current FIFO buffer level is read via the transmit FIFO level (TFL) register. †
  3. Programmable THRE interrupt mode must be enabled to access the FIFO buffer full status from the LSR. †

When using the FIFO buffer full status, software can poll this before each write to the transmit FIFO buffer. When the cts_n input becomes active (low) again, transmission resumes. If the FIFO buffers are disabled with the FCR.FIFOE bit, automatic flow control is also disabled regardless of any other settings. When auto CTS is not implemented or disabled, the transmitter is unaffected by cts_n.†

Figure 129. Automatic CTS Timing