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12. One Bit at a Time 335

enabled and programmed in Mode 11 (Slave idle – see Table 12.1) these pins conform to the specified relatively slow (300 ns maximum) rise and fall times. A normal port line has transition times of the order of 10 ns. The slower transition times give less cross talk between bus lines and less transmission line reflections at electrical discontinuities.

As in the case for the SPI protocol, many C compilers targeted to the PIC have built-in functions to implement the I2C protocol and avoid bit banging user-defined functions.

To illustrate the technique, consider Program 12.9 which replicates the assembly-level coding of Programs 12.7 and 12.8 using the CCS compiler.

i2c_start();

Generates the Master Start condition.

i2c_stop();

Generates the Master Stop condition.

i2c_read();

Reads a byte over the bus. If an optional parameter of 0 is used then the Master will not Acknowledge the received data.

Program 12.9 Interfacing to the MAX518 in C.

12. One Bit at a Time 337

two Stop bits. This requires a transmission rate of 110 bits per second, or 110 baud.10

The first purely electronic terminals required only one Stop bit, and could print at 30 characters per second, giving a rate of 300 baud. Traditionally communication channels use multiples of 300; eg. 1200, 2400, 4800, 9600…. PC serial ports can run up to 19,200 baud. However, this ×300 rate is not necessary as long as receiver and transmitter are running at the same nominal rate.

Typically a receiver on detecting an incoming datum will try and sample each bit at approximately mid point. This means that a frequency drift of ±0.5 bit time can be tolerated in the space of ten bits. Thus the receiver and transmitter local clocks must be within ±5%. The two will be resynchronized at the start of each datum.

Although not the most e cient of techniques, the asynchronous protocol outlined here has the major advantage of being an international standard. There are several variants; for instance the word can typically be from five to nine bits long. In our example the word length is eight bits with the eighth bit being used to provide a limited error checking capability. This parity bit is set in our example so that the number of 1s in the word is always odd. This can be checked at the receiver (see SAQ 3.9 on page 73) to detect a single bit error.

The original teleprinter code developed by Emile Baudot in 1875 is only five bits long.11 Here the string "PIC" is coded as 10110 00110 01110. Although limited in capability, its key advantage over Morse code (Samual Morse, 1840) was its fixed length (compare with ·– – ···– ·–·) which considerably simplifies the design of the transmitter and receiver. However, Morse code is more e cient as the number of bits is approximately inversely proportionally to a letter’s statistical frequency of use.

The 7-bit ASCII code of Table 1.1 on page 5, first adopted in 1963, was the first code specifically developed for computer communication systems. In 8-bit systems the extra 128 code patterns is usually utilized to add a selection of accented, mathematical and graphic symbols rather than for parity. However, parity can be accommodated by using a 9-bit word format.

For our example we have adopted a format of one Start, eight data with no parity and one Stop bit. Using a bit banging approach, as we have already done for our SPI and I2C protocols, is straightforward provided

10Strictly the baud rate is a measure of information rate. For a simple baseband system this is equal to the bit rate. However, this equality is not always true. For example, a

telephone modem can use a di-bit modulation scheme where groups of bits two at a time give a carrier tone phase shift of 0◦, 90◦, 180◦ and 270◦ phase shift for the patterns 00, 01, 10, 11 respectively. In this case the baud rate is four times the bit rate.

11Actually the first binary coded alphanumeric code was devised by Francis Bacon in around 1600. It too was a 5-bit code.

338 The Quintessential PIC Microcontroller

that we have an accurate 12 -bit delay. For example, for a 4800 baud link this would be 104 µs. As the delay is so short we can use an in-line approach using a macro in the same manner as in Program 12.6 rather than the subroutine approach of Program 6.8 on page 159.

Program 12.10 A baud-rate delay macro showing a half 9600 baud period delay at 20 MHz evocation.

include "p16F84a.inc" #define XTAL d’20’ #define BAUD d’9600’

#define N (XTAL*d’980000’)/(8*BAUD)

The macro shown in Program 12.10 is designed to give a suitable 12 - bit delay for a range of baud rates from 1200 through 9600 and crystal frequencies of 4 through 20 MHz. Both BAUD and XTAL constants are defined in the program head by the programmer; the example given in the listing showing a baud rate of 9600 and crystal frequency of 20 MHz.

The kernel of our macro is the decrement loop: