Computers transfer data in two ways: parallel and serial. In parallel data transfers, often 8 or more lines (wire conductors) are used to transfer data to a device that is only a few feet away. Examples of parallel transfers are printers and hard disks; each uses cables with many wire strips. Although in such cases a lot of data can be transferred in a short amount of time by using many wires in parallel, the distance cannot be great. To transfer to a device located many meters away, the serial method is used. In serial communication, the data is sent one bit at a time, in contrast to parallel communication, in which the data is sent a byte or more at a time. Serial communication of the 8051 is the topic of this chapter. The 8051 has serial communication capability built into it, thereby making possible fast data transfer using only a few wires.
In this chapter we first discuss the basics of serial communication. In Section 10.2, 8051 interfacing to RS232 connectors via MAX232 line drivers is discussed. Serial port programming of the 8051 is discussed in Section 10.3. The second serial port of DS89C4xO is programmed in Section 10.4. Section 10.5 covers 8051 C programming for serial ports #0 and #1.
SECTION 10.1: BASICS OF SERIAL COMMUNICATION
When a microprocessor communicates with the outside world, it provides the data in byte-sized chunks. In some cases, such as printers, the information is simply grabbed from the 8-bit data bus and presented to the 8-bit data bus of the printer. This can work only if the cable is not too long, since long cables diminish and even distort signals. Furthermore, an 8-bit data path is expensive. For these reasons, serial communication is used for transferring data between two systems located at distances of hundreds of feet to millions of miles apart. Figure 10-1 diagrams serial versus parallel data transfers.
Figure 10-1. Serial versus Parallel Data Transfer
The fact that serial communication uses a single data line instead of the 8-bit data line of parallel communication not only makes it much cheaper but also enables two computers located in two different cities to communicate over the telephone.
For serial data communication to work, the byte of data must be convertedto serial bits using a parallel-in-serial-out shift register; then it can be transmitted over a single data line. This also means that at the receiving end there must be a serial-in-parallel-out shift register to receive the serial data and pack them into a byte. Of course, if data is to be transferred on the telephone line, it must be converted from Os and Is to audio tones, which are sinusoidal-shaped signals. This conversion is performed by a peripheral device called a modem, which stands for "modulator/demodulator."
t , When the distance is short, the digital signal can be transferred as it is on a simple wire and requires no modulation. This is how IBM PC keyboards transfer data to the motherboard. However, for long-distance data transfers using communication lines such as a telephone, serial data communication requires a modem to modulate (convert from Os and 1 s to audio tones) and demodulate (converting from audio tones to Os and 1 s).
Serial data communication uses two methods, asynchronous and synchronous. The synchronous method transfers a block of data (characters) at a time, while the asynchronous method transfers a single byte at a time. It is possible to write software to use either of these methods, but the programs can be tedious and long. For this reason, there are special 1C chips made by many manufacturers for serial data communications. These chips are commonly referred to as UART (universal asynchronous receiver-transmitter) and USART (universal synchronous-asynchronous receiver-transmitter). The 8051 chip has a built-in UART, which is discussed in detail in Section 10.3.
Figure 10-2. Simplex, Half-, and Full-Duplex Transfers
Half- and full-duplex transmission
In data transmission if the data can be transmitted and received, it is a duplex transmission. This is in contrast to simplex transmissions such as with printers, in which the computer only sends data. Duplex transmissions can be half or full duplex, depending on whether or not the data transfer can be simultaneous. If data is transmitted one way at a time, it is referred to as half duplex. If the data
can go both ways at the same time, it is full duplex. Of course, full duplex requires two wire conductors for the data lines (in addition to the signal ground), one for transmission and one for reception, in order to transfer and receive data simultaneously. See Figure 10-2.
Asynchronous serial communication and data framing
The data coming in at the receiving end of the data line in a serial data transfer is all Os and Is; it is difficult to make sense of the data unless the sender and receiver agree on a set of rules, a protocol, on how the data is packed, how many bits constitute a character, and when the data begins and ends.
Start and stop bits
Asynchronous serial data communication is widely used for character-oriented transmissions, while block-oriented data transfers use the synchronous method. In the asynchronous method, each character is placed between start and stop bits. This is called framing. In data framing for asynchronous communications, the data, such as ASCII characters, are packed between a start bit and a stop bit. The start bit is always one bit, but the stop bit can be one or two bits. The start bit is always a 0 (low) and the stop bit(s) is 1 (high). For example, look at Figure 10-3 in which the ASCII character "A" (8-bit binary 0100 0001) is framed between the start bit and a single stop bit. Notice that the LSB is sent out first.
Figure 10-3. Framing ASCII "A" (41H)
Notice in Figure 10-3 that when there is no transfer, the signal is 1 (high), which is referred to as mark. The 0 (low) is referred to as space. Notice that the transmission begins with a start bit followed by DO, which is the LSB, then the rest of the bits until the MSB (D7), and finally, the one stop bit indicating the end of the character "A".
In asynchronous serial communications, peripheral chips and modems can be programmed for data that is 7 or 8 bits wide. This is in addition to the number of stop bits, 1 or 2. While in older systems ASCII characters were 7-bit, in recent years, due to the extended ASCII characters, 8-bit data has become common. In some older systems, due to the slowness of the receiving mechanical device, two stop bits were used to give the device sufficient time to organize itself before transmission of the next byte. In modern PCs however, the use of one stop bit is standard. Assuming that we are transferring a text file of ASCII characters using 1 stop bit, we have a total of 10 bits for each character: 8 bits for the ASCII code, and 1 bit each for the start and stop bits. Therefore, for each 8-bit character there are an extra 2 bits, which gives 20% overhead.
In some systems, the parity bit of the character byte is included in the data frame in order to maintain data integrity. This means that for each character (7- or 8-bit, depending on the system) we have a single parity bit in addition to start and stop bits. The parity bit is odd or even. In the case of an odd-parity bit the number of data bits, including the parity bit, has an odd number of Is. Similarly, in an even-parity bit system the total number of bits, including the parity bit, is even. For example, the ASCII character "A", binary 0100 0001, has 0 for the even-parity bit. UART chips allow programming of the parity bit for odd-, even-, and no-parity options.
Data transfer rate
The rate of data transfer in serial data communication is stated in bps (bits per second). Another widely used terminology for bps is baud rate. However, the baud and bps rates are not necessarily equal. This is due to the fact that baud rate . is the modem terminology and is defined as the number of signal changes per second. In modems a single change of signal, sometimes transfers several bits of data. As far as the conductor wire is concerned, the baud rate and bps are the same, and for this reason in this book we use the terms bps and baud interchangeably.
The data transfer rate of a given computer system depends on communication ports incorporated into that system. For example, the early IBM PC/XT could transfer data at the rate of 100 to 9600 bps. In recent years, however, Pentium-based PCs transfer data at rates as high as 56K bps. It must be noted that in asynchronous serial data communication, the baud rate is generally limited to 100,000 bps.
RS232 standards
To allow compatibility among data communication equipment made by various manufacturers, an interfacing standard called RS232 was set by the Electronics Industries Association (EIA) in 1960. In 1963 it was modified and called RS232A. RS232B and RS232C were issued in 1965 and 1969, respectively. In this book we refer to it simply as RS232. Today, RS232 is the most widely used serial I/O interfacing standard. This standard is used in PCs and numerous types of equipment. However, since the standard was set long before the advent of the TTL logic family, its input and output voltage levels are not TTL compatible. In RS232, a 1 is represented by -3 to -25 V, while a 0 bit is +3 to +25 V, making -3 to +3 undefined. For this reason, to connect any RS232 to a microcontroller system we must use voltage converters such as MAX232 to convert the TTL logic levels to the RS232 voltage levels, and vice versa. MAX232 1C chips are commonly referred to as line drivers. RS232 connection to MAX232 is discussed in Section 10.2.
RS232 pins
Table 10-1 provides the pins and their labels for the RS232 cable, commonly referred to as the DB-25 connector. In labeling, DB-25P refers to the plug connector (male) and DB-25S is for the socket connector (female). See Figure 10-4.
Figure 10-4. RS232 Connector DB-25
Since not all the pins are used in PC cables, IBM introduced the DB-9 version of the serial I/O standard, which uses 9 pins only, as shown in Table 10-2. The DB-9 pins are shown in Figure 10-5.
Data communication
classification
classification
Current terminology classifies data communication equipment as DTE (data terminal equipment) or DCE (data communication equipment). DTE refers to terminals and computers that send and receive data, while DCE refers to communication equipment, such as modems, that are responsible for transferring the data. Notice that all the RS232 pin function definitions of Tables 10-1 and 10-2 are from the DTE point of view.
The simplest connection between a PC and microcontroller requires a minimum of three pins, TxD,
RxD, and ground, as shown in Figure 10-6. Notice in that figure that the RxD and TxD pins are interchanged.
Examining RS232 handshaking signals
To ensure fast and reliable data transmission between two devices, the data transfer must be coordinated. Just as in the case of the printer, because the receiving device in serial data communication may have no room for the data, there must be a way to inform the sender to stop sending data. Many of the pins of the RS-232 connector are used for handshaking signals. Their descriptions are provided below only as a reference and they can be bypassed since they are not supported by the 8051 UARTchip.
- DTR (data terminal ready). When a
terminal (or a PC COM port) is
turned on, after going through a
self-test, it sends out signal DTR to
indicate that it is ready for commu
nication. If there is something
wrong with the COM port, this sig
nal will not be activated. This is an
active-low signal and can be used
to inform the •modem that the com
puter is alive and kicking. This is
an output pin from DTE (PC COM
port) and an input to the modem.
- DSR (data set ready). When DCE
(modem) is turned on and has gone
through the self-test, it asserts DSR
to indicate that it is ready to com
municate. Thus, it is an output
from the modem (DCE) and input
to the PC (DTE). This is an active-
low signal. If for any reason the
modem cannot make a connection
to the telephone, this signal
remains inactive, indicating to the
PC (or terminal) that it cannot
accept or send data.
- RTS (request to send). When the
DTE device (such as a PC) has a
byte to transmit, it asserts RTS to
signal the modem that it has a byte
of data to transmit. RTS is an
active-low output from Ihe DTE
and an input to the modem.
- CTS (clear to send). In response to
RTS, when the modem has room for storing the data it is to receive, it sends out signal CTS to the DTE (PC) to indicate that it can receive the data now. This input signal to the DTE is used by the DTE to start transmission.
- DCD (carrier detect, or DCD, data carrier detect). The modem asserts signal
DCD to inform the DTE (PC) that a valid carrier has been detected and that
contact between it and the other modem is established. Therefore, DCD is an
output from the modern and an input to the PC (DTE).
- RI (ring indicator). An output from the modem (DCE) and an input to a PC
(DTE) indicates that the telephone is ringing. It goes on and off in synchro
nization with the ringing sound. Of the six handshake signals, this is the least
often used, due to the fact that modems take care of answering the phone.
However, if the PC is in charge of answering the phone, this signal can be used.
From the above description, PC and modem communication can be summarized as follows: While signals DTK and DSR are used by the PC and modem, respectively, to indicate that they are alive and well, it is RTS and CTS that actually control the flow of data. When the PC wants to send data it asserts RTS, and in response, if the modem is ready (has room) to accept the data, it sends back CTS. If, for lack of room, the modem does not activate CTS, the PC will deassert DTR and try again. RTS and CTS are also referred to as hardware control flow signals.
This concludes the description of the most important pins of the RS232 handshake signals plus TxD, RxD, and ground. Ground is also referred to as SG (signal ground).
IBM PC/compatible COM ports
IBM PC/compatible computers based on x86 (8086, 286, 386, 486, and Pentium) microprocessors normally have two COM ports. Both COM ports have RS232-type connectors. Many PCs use one each of the DB-25 and DB-9 RS232 connectors. The COM ports are designated as COM 1 and COM 2. At the present time COM 1 is used for the mouse and COM 2 is available for devices such as a modem. We can connect the 8051 serial port to the COM 2 port of a PC for serial communication experiments.With this background in serial communication, we are ready to look at the 8051. In the next section we discuss the physical connection of the 8051 and RS232 connector, and in Section 10.3 we show how to program the 8051 serial communication port.