Microcontrollers in Practice (Mitescu M., 2005)

196 16 Remote Relay Controller over Telephone Lines

Table 16.1. DTMF combinations of frequencies for each digit

Table 16.2. The binary codes associated with the digits of the telephone set keypad

Fig. 16.4. Schematic of the multivibrator that generates the dialog tones

the DTMF commands received. This is a classic multivibrator, built with NE555, calibrated to generate a square wave signal with frequency around 1 kHz.

The circuit is controlled by the output line PA5 of the microcontroller. When PA5 = 1 the multivibrator is enabled. The duration of the audible beep transmitted is controlled by software by controlling PA5. One long beep (1 s) is used to indicate that

the last DTMF command is invalid, while three short beeps (0.3 s) indicate a valid command.

16.3 Description of the Software

The structure of the software application, as defined in MAIN.ASM, is entirely similar to the structure of the other HC11 applications described in this book:

The application-specific modules are RING.ASM, which controls the ring detector and the auto-answer circuits, DTMF.ASM, containing the interrupt service routine that handles the data from CM8870, and executes the commands, and BEEP.ASM, which contains the routines for controlling the generation of the dialog tones.

The central element of RING.ASM is the interrupt service routine for TIC3, called on the rising edge of the output of the ring detector circuit. This increments the variable RCNT (Ring Counter) and sets the variable QRING to $FF, to inform the main program that a ring has been detected.

The logic diagram of the task RING.ASM is presented in Fig. 16.5. Besides the variable RCNT, which is compared to the EEPROM constant MAXRING to determine the moment when the controller must open the telephone line, RING.ASM uses two software timers.

The first timer controls the time interval between two successive rings detected. If this interval is longer than 3 seconds, the detection sequence is aborted and RCNT is cleared. The other timer provides a software mechanism to terminate the connection and close the line when no DTMF tones are detected for more than 30 seconds.

The interesting aspect of this piece of software is that it is implemented as a finite state machine. The ring detection and auto-answer machine has three distinct states, encoded by the variable RSTATUS.

State 0 is the idle state, when no ring has been detected yet, and the line is closed. State 1 corresponds to the situation when 1 < RCNT < MAXRING, and state 2 is the open line state, when the controller receives and executes DTMF commands.

198 16 Remote Relay Controller over Telephone Lines

RCNT=0

RSTATUS=0

Close line

Fig. 16.5. Logic diagram of the ring counter and auto-answer program

Each time the program enters this task, it is directed to a different section, according to the value of the variable RSTATUS. The value of RSTATUS is used as an offset in a table containing the starting addresses of the routines associated with the corresponding states:

The transitions from one state to another occur when global variables, controlled by the interrupt service routines or by other tasks, change their values. Here is an example showing how this particular state machine changes state from 0 to 1:

This simple technique can be used to implement pretty complex finite state machines, with dozens of states.

The module DTMF.ASM handles data provided by the interrupt service routine for TIC2, the interrupt generated by the data strobe of CM8870. This routine places the DTMF data in a four-byte buffer DTBUF. When the end of buffer is reached, or when the binary code associated with the key ‘#’ is detected, the flag DBFULL is set to true to inform the program that the data buffer is full, and ready to be interpreted.

The following commands are accepted:

•*10# – Change relay status to OFF

•*11# – Change relay status to ON

•*2x# – Change the number of rings before the call is answered to the value x, ranging from 2 to 9. This parameter is stored in the EEPROM location MAXRING.

•*30# – Close the line and terminate the session.

The content of the buffer is checked against this structure, and if a valid command is recognized, it is executed immediately, and three short beeps are transmitted to the telephone line to acknowledge the command. One long beep indicates invalid data detected in the buffer.

Here is the listing of the interrupt service routine for TIC2:

The information prepared in DTBUF by the interrupt service routine is interpreted in the task DTMF.ASM. The following code fragment shows how the relay is controlled according to the contents of the reception buffer:

The module BEEP.ASM that controls the generation of the dialog tones is structured as a finite state machine, in a way similar to the module RING.ASM. This time, the variable that directs the program to the subsections dedicated to each state is called BSTATUS.

There are two predefined sequences of states, associated with the two types of dialog tones required. The program transits from one state to another when a timer expires, and, after going through all the states in sequence, it returns to the idle state 0.

To start a sequence, another active task must write a value in BSTATUS, according to the initial state in the sequence. In this particular case, the initial states for the two sequences are defined as follows:

•BSTATUS = 1 starts the sequence that generates one long beep (1 sec).

•BSTATUS = 3 start the sequence that generates three 0.3-seconds beeps, separated by a pause of 0.1 seconds.

DTMF.ASM writes directly to BSTATUS to generate the two possible dialog tones:

Appendices

A.1 Glossary of Terms

ADC

Acronym for Analog-to-Digital Converter. A circuit that converts the amplitude of an analog signal into a numeric value, presented as a binary number.

See also: Resolution, S/H, SAR

Address

An unique code that identifies a resource in a microcontroller system.

See also: Address bus, Address space

Address bus

Physical connection between the CPU and the resources accessed, carrying the address information. The number of lines of the address bus determines the maximum number of resources that can be accessed.

See also: Address, Address space

Address space

The range of addresses associated with a resource, or a group of resources.

See also: Address, Address bus

Architecture

In a computer system, this term designates the structure and organization of the resources which determines the way resources are accessed by the CPU.

See also: Harvard, Von Neumann data memory, Program memory, Address bus, Data bus

204 Appendices

ASCII

Acronym for American Standard Code for Information Interchange. This is a 7- bit binary code created to facilitate data communication between computers and peripheral equipment created by different manufacturers.

Asynchronous communication

Communication technique that allows data exchange without the need of a common synchronization clock for the transmitter and receiver(s).

See also: SCI, UART, Baud rate

Baud rate

Data transmission rate in serial communications, measured in bits per second.

See also: Asynchronous communication, SCI, UART

Buffer

Hardware: A device intended to adapt the electrical characteristics of a MCU I/O line to a load, or signal source.

Software: Temporary data storage intended to compensate the differences in communication rates between processes or devices involved in a computer system.

Bus

Physical connection between the CPU and the other resources of a computer system that carries data or addresses.

See also: Address bus, Data bus, Architecture, Harvard, Von Neumann

CPU

Acronym for Central Processing Unit. The core of any computer system that does all data processing, and controls the access to the data and address buses of the system. The CPU includes the instruction decoder, and an arithmetic and logic unit (ALU).

See also: Architecture, Data bus, Address bus, Peripheral interface, Instruction, Program

CRC

Acronym for Cyclic Redundancy Check. A special checksum included in data packets for error detection.

See also: Asynchronous communication, Parity bit