Файл: The quintessential PIC microcontroller (S. Katzen, 2000).pdf

12. One Bit at a Time 345

Program 12.12 The USART-based I/O subroutines.

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;* FUNCTION: Transmits one 8-bit byte in asynchronous format *

; *************************************************************

; *************************************************************

; *************************************************************

Some systems may not wish the processor to hang up waiting for a character which is a long time in coming. In such cases an alternative input subroutine, perhaps called getch, could return an ERR of −1 if the return was empty handed. Another approach would be to generate an interrupt each time an incoming character is sensed rather than using a polling technique.

In the case of the CCS C compiler the #use rs232 directive tells the compiler which pins are to be used for RX and TX. The normal C I/O functions, such as printf(), use these pins as their link to the standard

346 The Quintessential PIC Microcontroller

channel. If these pins are specified as PIN_C6 and PIN_C7 then where the part has a built-in USART this will be used instead of a software technique.

There is more to setting up a communication link than establishing a suitable protocol. PIC devices have normal logic voltage and current levels which are not intended for connections greater than 30 cm (1 ). Although with care14 distances considerably in excess of this can be employed, in situations with relatively fast bit rates di erent signalling techniques have to be used.

In the era of electromechanical TTYs the de facto 20 mA loop standard was in common use. This uses zero and 20 mA current to signal logic 0 and logic 1 respectively. Use of current means that line attenuation is not a problem (as current out must equal current in) and this level of current was su cient to directly activate the receiver solenoid relay.

Current sources are realized by using high voltages in series with a large resistance. The latter gives long time constants, which, while adequate in the area of 110 baud rates, did not transfer well to the introduction of electronic terminals, UARTs and modems. RS-23215 was introduced in 1969 as the standard interface for connecting an item of Data Terminal Equipment (DTE), such as a terminal, to approved Data Circuit terminating Equipment (DCE), typically a modem. Thus, not only did it define signalling levels, as shown in Fig. 12.20(a), but also various control and handshake lines, some of which are shown in Figs. 12.20(d) and 12.21. For example the modem would signal back to the DTE that a telephone link had been opened with the remote DTE by activating the Clear To Send (CTS) handshake signal. Two data lines plus a ground line are needed for a full duplex transmission circuit.

The RS-232 standard has a range of 15 m (50 ) at a maximum rate of 20 kbaud, which it achieves by mapping logic 0 (often called a space) to typically +12 V and logic 1 (often called a mark) to typically −12 V. The receiver can distinguish levels down to ±5 V. The RS-423 standard (1978) in Fig. 12.20(b) is similar but can manage 1.2 km (6000 ) at up to 80 kbaud and 10 Mbaud at 12 m (40 ) with up to ten receivers.

Both RS-232 and RS-423 are unbalanced (or single-ended) standards, where the receiver measures the potential between signal line and ground reference. Even though the transmitter and receiver grounds are usually connected through the transmission line return, the impedance over a long distance may support a significant di erence in the two ground potentials, which will degrade noise immunity. Furthermore, any noise induced from outside will a ect signal lines di erently from the ground return due to their dissimilar electrical characteristics – hence the name unbalanced.

14Or sometimes ignorance!

15Defined in USA as the Electronics Industries Association EIA 232-E standard and as the V24 interface by the CCITT in Europe.

348 The Quintessential PIC Microcontroller

The RS-422 (1978) and RS-485 (1983) standards are described as balanced. Here each signal link comprises two conductors, normally twisted around each other, known as twisted pair. The logic level is represented as the di erence of potential across the conductors, not the di erence from ground. Calling the conductors A and B, then logic 0 is represented as A<B and logic 1 by A>B. A di erence of more than ±200 mV at the receiver is su cient to establish the logic level and the transmitter will typically generate a ∆V = ±5 V. As the A and B conductors have the same characteristics and are tightly wound together they represent similar targets for induced noise. As the same noise voltage appears in both conductors and the receiver only distinguishes di erences, rejecting commonmode voltages up to ±7 V, then the noise immunity of these balanced links is clearly superior to unbalanced schemes. Commercial twistedpair cables, used in Local Area Networks (LANs), often carry three or four pairs of conductors, each link having a di erent twist pitch to reduce induction between links.

The main di erence between the RS-422 and RS-485 standards is the provision in the latter case for multiple transmitters as well as receivers to implement multi-drop LANs. As only one transmitter can be active at any one time, an RS-485 transmitter bu er must have an enable input, to select the master device. The single RS-422 transmitter has no need to be disabled.

RS-232 was originally designed for DTE-modem interconnection, although its use is now much more varied – see Fig. 12.21. Figure 12.20(d) shows a simple Frequency Shift Keying (FSK) full duplex system with the mark/space of one channel being represented by the tones 1070/1270 Hz and the other by 2025/2225 Hz; frequencies which fit well inside the normal telephone link bandwidth of 300 – 3400 Hz. Handshake lines DCD (Data Carrier Detect), CTS (Clear TO Send) and RTS (Ready To Send) are used to control the sequence of operations prior to and terminating the communication of data.

Many modem schemes currently use Phase Shift Keying (PSK) where typically at least eight di erent phases in 45◦ steps of a single tone are used to encode 3-binary bit code groups (tri-bits) in any one time slot. In this way the baud rate may be increased with the same signalling rate, albeit at the expense of noise immunity, as witnessed by the steady increase in PC-based home telephone internet data rates in recent years up to 56 kbaud.

As an example, Fig. 12.21 shows the connection between a PIC and the serial port of a PC – or any device with an asynchronous RS-232 port. The Maxim MAX233 dual RS-232 transceiver translates from +12 V to 0 V (logic 0) and −12 V to +5 V (logic 1). If handshake lines are not being used, as is usual in simple links, the PC can be ‘fooled’ into treating the interface as ready to accept data by linking as shown in the diagram. For