Silicon Laboratories SI2494/39 manual Uart Serial Interface, Parallel and SPI Interface Operation

AN93

A block diagram of the UART in the serial interface mode is shown in Figure 5.

.

11 Bits

to Data Bus

MUX

TX FIFO

TX Shift Register

TXD (10)

CONTROL

	CTS		RTS		INT
(11)		(8)		(16)

RX FIFO

RX Shift Register

RXD

(9)

Figure 5. UART Serial Interface

2.2.4. Parallel and SPI Interface Operation

Refer to "2.1. Resetting the Device" on page 11 for interface selection. The parallel interface has an 8-bit data bus and a single address bit. The SPI likewise operates with 8-bit data transfers, using a single address bit. When the parallel or SPI interface mode is selected, the modem must be configured for a DTE interface or 8N1 only. The host processor must calculate parity for the MSB. The modem sends bits as received by the host and does not calculate parity. Refer to "Appendix C—Parallel/SPI Interface Software Implementation" on page 290 for detailed parallel or SPI interface application information.

The parallel or SPI interface uses the FIFOs to buffer data in the same way as in UART mode, with the addition of Hardware Interface Registers 0 (HIR0) and Hardware Interface Register 1 (HIR1). The Hardware Interface Registers were formerly called Parallel Interface Registers (PIR0 and PIR1) in older products, because those products would support only a parallel interface. Flow control must be implemented by monitoring REM and TXE in HIR1. There is no protection against FIFO overflow. Data transmitted when the transmit FIFO is full are lost. Figure 6 shows the interaction of the transmit and receive FIFOs with the Hardware (Parallel) Interface Registers in the case of a parallel interface. The arrangement is similar when the SPI interface is selected. Table 21 on page 25 shows a bit map of HIR0 and HIR1.

UART oriented control lines, such as RTS and CTS, are not used in Parallel and SPI Interface mode. They are replaced by bits in the HIR1 register.

SPI and parallel operation only supports 8-bit data words. The longer words that are implied by the \B5 (8P1) & \B6 8X1 commands are not allowed. These commands should not be used.

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