EM357-RTR datasheet(96/241 Pages) SILABS

96 / 241 page

Rev 1.3

8.6.3. FIFOs

Characters transmitted and received by the UART are buffered in the transmit and receive FIFOs that are both 4

entries deep (see Figure 8.4). When software writes a character to the SC1_DATA register, it is pushed onto the

transmit FIFO. Similarly, when software reads from the SC1_DATA register, the character returned is pulled from

the receive FIFO. If the transmit and receive DMA channels are used, the DMA channels also write to and read

from the transmit and receive FIFOs.

Figure 8.4. UART FIFOs

8.6.4. RTS/CTS Flow control

RTS/CTS flow control, also called hardware flow control, uses two signals (nRTS and nCTS) in addition to received

and transmitted data (see Figure 8.5). Flow control is used by a data receiver to prevent buffer overflow, by

signaling an external device when it is and is not allowed to transmit.

Figure 8.5. RTS/CTS Flow Control Connections

The UART RTS/CTS flow control options are selected by the SC_UARTFLOW and SC_UARTAUTO bits in the

SC1_UARTCFG register (see Table 8.12). Whenever the SC_UARTFLOW bit is set, the UART will not start

transmitting a character unless nCTS is low (asserted). If nCTS transitions to the high state (deasserts) while a

character is being transmitted, transmission of that character continues until it is complete.

If the SC_UARTAUTO bit is set, nRTS is controlled automatically by hardware: nRTS is put into the low state

(asserted) when the receive FIFO has room for at least two characters, otherwise is it in the high state

(unasserted). If SC_UARTAUTO is clear, software controls the nRTS output by setting or clearing the

SC_UARTRTS bit in the SC1_UARTCFG register. Software control of nRTS is useful if the external serial device

cannot stop transmitting characters promptly when nRTS is set to the high state (deasserted).

Receive Shift Register

SC1_DATA (read)

Transmit Shift Register

SC1_DATA (write)

RXD

TXD

Parity/Frame Errors

SC1_UARTSTAT

CPU and DMA

Channel Access

Other Device

UART Receiver

UART Transmitter

EM350

UART Transmitter

UART Receiver

RXD

TXD

nRTS

nCTS

TXD

RXD

nCTS

nRTS

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