TL16C752

3.3-V DUAL UART WITH 64-BYTE FIFO

SLLS305 – MAY 1999

5

POST OFFICE BOX 655303

functional description (continued)

trigger levels

The TL16C752 provides independent selectable and programmable trigger levels for both receiver and

transmitter DMA and interrupt generation. After reset, both transmitter and receiver FIFOs are disabled and so,

in effect, the trigger level is the default value of one byte. The selectable trigger levels are available via the FCR.

The programmable trigger levels are available via the TLR.

hardware flow control

Hardware flow control is composed on 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 UART can transmit data.

Auto-RTS only activates the RTS output when there is enough room in the FIFO to receive data and deactivates

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/deactivated.

If both auto-CTS and auto-RTS are enabled, when RTS is connected to CTS, 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.

auto-RTS

Auto-RTS data flow control originates in the receiver block (see functional block diagram). Figure 1 shows RTS

functional timing. 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 (e.g., another 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 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 allows the

sending device to resume transmission.

RX

RTS

IOR

Start

Byte N

Stop

Start

Byte N+1

Stop

Start

1

2

N

N+1

NOTES:

1. N = receiver FIFO trigger level

2. The two blocks in dashed lines cover the case where an additional byte is sent as described in Auto-RTS.

Figure 1. RTS Functional Timing