DTMF

DTMF stands for Dual Tone - Multi Frequency and it is the basis for your telephone system. DTMF is actually the generic term for Touch-Tone (touch-tone is a registered trademark of ATT). Your touch-tone® phone is technically a DTMF generator that produces DTMF tones as you press the buttons.

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How does it work?

When you press the buttons on the keypad, a connection is made that generates two tones at the same time. A "Row" tone and a "Column" tone. These two tones identify the key you pressed to any equipment you are controlling. If the keypad is on your phone, the telephone company's "Central Office" equipment knows what numbers you are dialing by these tones, and will switch your call accordingly. If you are using a DTMF keypad to remotely control equipment, the tones can identify what unit you want to control, as well as which unique function you want it to perform.

1	2	3	697 Hz
4	5	6	770 Hz
7	8	9	852 Hz
*	0	#	941 Hz
1209 Hz	1336 Hz	1477 Hz

When you press the digit 1 on the keypad, you generate the tones 1209 Hz and 697 Hz.

Pressing the digit 2 will generate the tones 1336 Hz and 697 Hz.

Sure, the tone 697 is the same for both digits, but it take two tones to make a digit and the decoding equipment knows the difference between the 1209 Hz that would complete the digit 1, and a 1336 Hz that completes a digit 2.

Circuit Diagram

This is circuit diagram for DTMF decoder. 

Click here to see working video of DTMF circuit.

8051 Microcontroller

The 8051 is a Harvard architecture, single chip microcontroller (µC) series which was developed by Intel in 1980 for use in embedded systems.Intel's original versions were popular in the 1980s and early 1990s. While Intel no longer manufactures the MCS-51, binary compatible derivatives remain popular today. In addition to these physical devices, several companies also offer MCS-51 derivatives as IP cores for use in FPGAs or ASICs designs.

Intel's original MCS-51 family was developed using NMOS technology, but later versions, identified by a letter C in their name (e.g., 80C51) used CMOStechnology and were less power-hungry than their NMOS predecessors. This made them more suitable for battery-powered 

devices.

Important features and applications

i8051 microarchitecture.

The 8051 architecture provides many functions (CPU, RAM, ROM, I/O, interrupt logic, timer, etc.) in a single package

• 8-bit ALU, Accumulator and 8-bit Registers; hence it is an 8-bit microcontroller
• 8-bit data bus – It can access 8 bits of data in one operation
• 16-bit address bus – It can access 2¹⁶ memory locations – 64 KB (65536 locations) each of RAM and ROM
• On-chip RAM – 128 bytes (data memory)
• On-chip ROM – 4 kByte (program memory)
• Four byte bi-directional input/output port
• UART (serial port)
• Two 16-bit Counter/timers
• Two-level interrupt priority
• Power saving mode (on some derivatives)

One particularly useful feature of the 8051 core was the inclusion of a boolean processing engine which allows bit-level boolean logic operations to be carried out directly and efficiently on select internal registers and select RAM locations. This advantageous feature helped cement the 8051's popularity in industrial control applications because it reduced code size by as much as 30%. Another valued feature is the including of four bank selectable working register sets which greatly reduce the amount of time required to complete an interrupt service routine. With a single instruction the 8051 can switch register banks as opposed to the time consuming task of transferring the critical registers to the stack or designated RAM locations. These registers also allowed the 8051 to quickly perform a context switch which is essential for time sensitive real-time applications.

Basic Working Circuit For 8051

click here to see 8051 datasheet