Connect 8051 based microcontroller to SCI port

This application note describes how to configure the UART of a high-speed microcontroller or ultra-high-speed flash microcontroller to communicate with an SCI-enabled device. It begins with a brief discussion of the differences between SCI and UART modules and ends with a practical example of how to configure an 8051-based Dallas Semiconductor microcontroller UART to communicate with a SCI module.

introduce

The Serial Communications Interface (SCI) is a high-speed serial I/O port that allows synchronous or asynchronous communication between devices. It allows the microcontroller to be connected to a variety of similarly functional peripherals, as well as the standard RS-232 interface. The exact implementation of SCI varies by device manufacturer; many devices support features such as full-duplex communication in asynchronous mode, parity checking, error detection, and programmable character length in <> to <> bits.

All 8051-based Dallas Semiconductor microcontrollers are capable of communicating with SCI-capable devices, even if the SCI functionality is not explicitly listed in the microcontroller's feature list. All of our microcontrollers contain one to three 8051-type UARTs and can be configured to operate in most common SCI modes.

This application note describes how to configure the UART of a high-speed microcontroller or ultra-high-speed flash microcontroller to communicate with a SCI-enabled device. It begins with a brief discussion of the differences between SCI and UART modules and ends with a practical example of how to configure an 8051-based Dallas Semiconductor microcontroller UART to communicate with a SCI module. A code example is provided that demonstrates how to initialize the microcontroller and perform simple tests to ensure the device is communicating correctly.

Characteristics of SCI

As mentioned above, SCI is a high-speed serial interface. It has many similarities to the 8051-style UART on Dallas Semiconductor 8051-based microcontrollers. The following is a list of SCI functions in UART and their counterparts. Users should note that not all SCI modules support all features listed, so users should carefully read the data sheets of SCI-enabled devices to understand how to use them.

example

Most SCI modules support asynchronous communication formats, many of which are exclusive. The example here demonstrates how to configure a Dallas Semiconductor 8051-based microcontroller to communicate asynchronously with an SCI-enabled device. In this case, we configure the microcontroller to communicate with the target SCI configured with the following characteristics:

10-bit asynchronous mode; 1 start, 8 data, 1 stop bit

Baud rate: 19200 bps

In order to communicate with this device we will make the following decisions to set up the Dallas Semiconductor microcontroller:

Use serial port 0 for communication

The external clock source is 22.1184MHz

The serial port will be configured in 10-bit asynchronous mode; 1 start, 8 data, 1 stop bit (this is serial port mode 1.

The baud rate generator clock source will be Timer 1 in auto-reload mode (Timer Mode 2).

Since all Dallas Semiconductor 8051-based microcontroller timers default to native divide-by-12 operating mode, this example has the advantage of being applicable to all Dallas Semiconductor devices regardless of the core's clock divisor. This is because the DS5000FP (divide by 12), the DS80C320 (divide by 4), and the DS89C450 (divide by 1) all use the same serial port timings if the timer's higher speed option is not selected. For details on UART operation, see the Serial I/O section of the appropriate user guide.

Since the SCI determines the format of the data, the Dallas Semiconductor microcontroller must next be initialized to the correct baud rate. The 8-bit auto-reload mode (Timer Mode 2) generates the baud rate via a user-selectable timer overflow driven by an external clock source. This adds considerable flexibility to the design and simplifies development since the baud rate can be easily selected in software, allowing multiple baud rates from the same clock source. The formula for determining the baud rate is as follows:

where osc_frequency is the frequency of the external clock source in MHz, TH1 is the 1-bit reload value placed in the Timer 8 MSB SFR, and SMOD_0 (PCON.7) is the serial port 0 multiplier enable bit. Alternatively, if the baud rate and oscillator frequency are known, the value of the 8-bit reload number TH1 can be solved for using the following formula:

Assuming the external clock source is 22.1184MHz, a TH1 value of FDh will produce a target baud rate of 19200 and clear the multiplier bit. For more information on baud rate selection, see the Serial I/O section of the appropriate user guide.

The following short assembly code example demonstrates how to initialize serial port 0 to communicate with an SCI module configured in 10-bit asynchronous mode at 19200 bps. On successful operation, it will echo any characters received. This functionality can be easily removed, making it a universal shell for any user requiring SCI communications applications.

;SCI emulation example

; Simple transmit test to demonstrate how to configure 8051 UART to

; emulate an SCI module. Test code embedded in this example echoes back

; received characters.

org 0h ;Reset vector.

ljmp start

org 23h ;Serial port 0 vector.

ljmp SP0_ISR

org 100h ;Start of code.

start: ;Initialize Serial Port 0 for mode 1, 19200 baud

MOV TMOD, #020h ;Set timer 1 for mode 2 (8-bit auto reload)

MOV SCON0, #050h ;SP0 10-bit asynchronous mode with receive enabled

;Now select the reload value based on baud rate and xtal frequency.

MOV TH1, #0FDh ;19200 baud at 22.11 MHz

;MOV TH1, #0FDh ;9600 baud at 11.059 MHz

;MOV TH1, #0FAh ;9600 baud at 22.11 MHz

SETB TR1 ;Serial port is initialized, now start timer

;Enable Interrupts

MOV IE, #90h ;This example supports interrupt-driven communications, so

                  ; enable global and serial port 0 interrupts.

;Test code in receive interrupt routine echoes back any received characters

; when combined with the loop here.

loop: sjmp loop

SP0_ISR: ;Serial port 0 Interrupt Service Routine

jb RI0, RIO_INT ;Determine if receiver/transmitter was cause of interrupt.

TIO_INT: ;Interrupt was caused by transmission.

;

; Placeholder for transmitter routine

;

CLRTI0

RETI

RIO_INT: ;Interrupt was caused by reception

;

; Placeholder for receiver routine

;

MOV A, SBUF0 ;Test code that echoes back received character

MOV SBUF0, A ; Remove for real code.

CLRRI0

RETI

Summarize

The UART in Dallas Semiconductor's 8051-based microcontrollers can be easily configured to interface with SCI modules in many devices. This popular serial interface can operate in a variety of modes, but the most common is the 11/232-bit asynchronous mode used in RS-10 communications. Allowing Dallas Semiconductor microcontrollers to connect to SCI modules increases overall system flexibility as they can be connected to a wider range of embedded systems.

Although this example focuses on asynchronous mode of operation, the Dallas Semiconductor microcontroller can also be configured to interface with an SCI operating in synchronous mode. The SCI module's similarity to the 8051 UART allows this interface to be accomplished with minimal effort. For more information about synchronous mode (Serial Port Mode 0), see the Serial I/O section of the appropriate user guide.