CHAPTER 8

SERIAL I/O (SIO) PORT

A serial input/output (SIO) port provides a means for the system to communicate with external devices. This microcontroller has a two-channel serial I/O (SIO) port that shares pins with port 2. This chapter describes the SIO port and explains how to configure it.

8.1SERIAL I/O (SIO) PORT FUNCTIONAL OVERVIEW

The serial I/O port is an asynchronous/synchronous port that has two identical channels. Each channel includes a universal asynchronous receiver and transmitter (UART) and has four modes of operation: one synchronous mode (mode 0) and three asynchronous modes (modes 1, 2, and 3). Each channel consists of a dedicated receiver, transmitter, control logic, two interrupt signals, and a baud-rate generator.

The transmitter and receiver contain buffers and shift registers. The buffers are accessible as spe- cial-function registers (SFRs). Write transmit data to the transmit buffer (SBUFx_TX) and read received data from the receive buffer (SBUFx_RX). Unlike the buffers, the shift registers are internal registers and are not accessible as SFRs. For receptions, data is shifted into the receive shift register, least-significant bit first, via the receive data pin (RXDx). After the last bit (eighth bit for mode 0 or stop bit for modes 1, 2, and 3) is shifted in, the receiver transfers the data from the receive shift register to SBUFx_RX where it can be accessed. For transmissions, data in SBUFx_TX is transferred to the transmit shift register then shifted out through the serial transmit pin (RXDx for mode 0 or TXDx for modes 1, 2, and 3).

Each channel contains a serial port control (SPx_CON) register and a serial port status (SPx_STATUS) register. SPx_CON configures the SIO channel for one of the operating modes and for receptions or transmissions. SPx_STATUS contains status and error flags. These registers are discussed in detail in “Programming the Control Register” on page 8-11 and “Determining Serial Port Status” on page 8-16.

Each SIO channel has two interrupt signals, allowing for interrupt-driven transmit and receive service routines. The receive interrupt (RIx) indicates the receive buffer (SBUFx_RX) contains received data, available for reading. The transmit interrupt (TIx) indicates the transmit buffer (SBUFx_TX) is empty and available for writing.

Each channel contains a 15-bit baud-rate generator. Either the internal peripheral clock or a signal input on the T2CLK pin can provide the clock signal. The baud-rate register (SP x_BAUD) selects the clock source and the baud rate. For synchronous mode 0, the baud-rate generator controls the baud rate output on the serial clock pin (TXDx). For asynchronous modes 1, 2, and 3, the baudrate generator controls the transmit and receive shift clocks.

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8XC196EA USER’S MANUAL

The SIO channel signals, registers, and interrupts are shown in Figures 8-1 and 8-2. The signals and registers are described in the following section.

As shown in Figure 8-1, the RXDx pin is the data pin and the TXDx pin is the clock pin for synchronous mode 0 operation. In this mode, the baud-rate generator drives eight pulses out the TXDx pin and the UART shifts data, least-significant bit first, into or out of the microcontroller via the RXDx pin. The UART samples data when the TXDx pulse is low. “Synchronous Mode (Mode 0)” on page 8-6 describes mode 0 in detail.

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SERIAL I/O (SIO) PORT

Figure 8-2. SIO Block Diagram (Modes 1, 2, and 3)

As shown in Figure 8-2, the RXDx pin is the receive data pin and the TXDx pin is the transmit data pin for asynchronous modes 1, 2, and 3. Either the internal operating frequency (f) or an input signal on the T2CLK pin provides the clock input to the baud-rate generator. “Asynchronous Modes (Modes 1, 2, and 3)” on page 8-7 describes modes 1, 2, and 3 in detail.

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SERIAL I/O (SIO) PORT

Table 8-2. Serial Port Control and Status Registers (Continued)

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