CM 8870 DTMF Decoder

Description

The CM8870 provides full DTMF receiver capability by integrating both the band split filter and digital decoder functions in to a single 80-pin DIP, or 20 pin PLCC package .the CM8870 is manufactured using state -of -the -art CMOS process technology for low power consumption (35 mW, MAX) and precise data handling .the filter section uses a switched capacitor technique for both high and low group filter and dial tone rejection. The CM8870 decoder uses digital counting technique for the detection and decoding of all 16 DTMF tone pairs in to a 4 bit code. The DTMF receiver minimizes external component count by providing an on chip differential input amplifier, clock generator and a latched three state interface bus. The on-chip clock generator requires only a low cost TV crystal or ceramic resonator as an external component .The CM8870 DTMF integrated receiver provides the designer engineer with not only low power consumption, but high performance in a small 18 pin DIP, or 20-pin PLCC package configuration. the CM8870 internal architecture of a band -split filter section separates the high and low tones of the received pair, forward by a digital decode section which verifies both the frequency and duration of the received tones before passing resultant 4-bit code to the out put bus.

Filter Section

Separations of the low group and high group a tone is achieved by applying the dual-tone signal to the inputs of two 9^th order switched capacitor band pass filter .The bandwidth of this filter corresponds to the bands enclosing the low group and high group tones. The filter section also incorporates notches at 350HZ and 440HZwhich provides excellent dial tone rejection .Each filter output is followed by single order switched capacitor section which smoothes the signals prior to limiting.

Signal limiting is performed by high gain comparators. These comparators are provided with a hysteresis to prevent detection of unwanted low level signal and noise. The output of the comparators provides full-rail logic swings at the frequencies of the incoming tones.

Decoder section

The CM8870 decoder uses a digital counting technique to determine the frequencies of the limited tones and to verify that these tones correspond to standard DTMF frequencies. A complex averaging algorithm is used to protect against tone simulation by extraneous signals (such as voice) while providing tolerance to small frequencies variation. The averaging algorithm has been developed to ensure an optimum combination of immunity to “talk-off” and tolerance to the presence of interfering signals (third tones) and noise. When the detector recognizes the simultaneous presence of two valid tones (known as “signal condition”), it rises the

“Early steering” flag (Est.).any subsequent loss of signal condition will cause Est. to fall.

Steering circuit

Before the registration of a decoded tone pair, the receiver checks for valid signal duration (refer to as “character-recognition-condition”). This is check is performed by an external RC time constant driven by Est. Logic high on Est. causes Vc to rise the capacitor discharges .Providing signal condition is maintained (Est remains high) for the validation period (t gtp), Vc reaches the threshold (V tst) of the steering logic to register the tone pair, thus latching its corresponding 4-bit code into the out put latch. At this point, the GT output is activated and drives Vc to Vdd. GT continuous to drive high as long as Est. it remains, signaling that a received tone pair has been registered the contents of the output latch are made available on the 4-bit out put bus by raising the 3-state control in put (TOE) to logic high. The steering circuit works in reverse to validate the inter digit pause between signals. Thus, as well rejecting signals two short to be considered valid, the receiver will tolerate signal interruption (drop outs) too short to be considered a valid pause. This capability together with the capability of selecting the steering time constant externally, allows the designer to tailor performance to meet a wide Varity of system requirement.

Guard Time Adjustment

In situations which do not require independent selection of receive and pause, the simple steering circuit of fig shown in appendix is applicable. Component values are chosen according to the fallowing formula.

 trec=tdp +tgtp

 tgtp=0.67RC

The value of tdp is a parameter of the device and trec is the minimum signal duration to be recognized by the receiver’s value for C of 0.1microfarads is recommended for most applications, leaving R to be selected by the designer. For example, a suitable value of R for a trec of 40ms would be 300K. A typical circuit using this steering configuration is shown in figure. The timing requirements for the most telecommunication applications are satisfied with this circuit. Different steering arrangements may be used to select independently the guard-times for tone-present (tgtp) and tone absent (tgta). This may be necessary to meet system specifications which place both accept and reject limits on both tone duration and inter digit pause.

Guard time adjustment also allows the designer to tailor system parameter such as talk-off and noise immunity. Increasing trec improves take-off performance, since it reduces the probability that tones simulated by speech will maintain signal condition for long enough to be registered. On the other hand, a relative short trec with along tdd would be appropriate for

 Extremely noisy environments where fast acquisition time and immunity to drop-outs would be requirements.

Input Configuration

The input arrangement of the CM8870 provides a differential input operational amplifier as well as a bias source (Vref) which is used to bias the inputs at mid-rail.

Provision is made for connection of a feedback resistor to the op-amp output (GS) for adjustment of gain. With the op-amp connected for unity gain and Vref biasing the input at ½ Vdd.

Clock Circuit

The internal clock circuit is completed with the addition of a standard television color burst crystal or ceramic resonator having a resonant frequency of 3.579545 MHz. the CM8870 in a PLCC package has a buffered oscillator output that can be used to drive clock inputs of other devices such as a microprocessor or other CM887X’s. Multiple CM 8870s can be connected, such that only one crystal or resonator is required.

Fig.4.4

4.5. Relays

Definition

The Relay is an automatic control element whose output variable undergoes a change by leaps and bounds when its input variable (electric, magnetic, sound, light, heat) reaches a set point.

Introduction

The relay is a device that acts upon the same fundamental principle as the solenoid .The difference between a relay and a solenoid is that a relay does not have a movable core (plunger) while the solenoid does. Where multiple relays are used, several circuits may be controlled at once. Relays are electrically operated control switches, and are classified

according to their use as POWER RELAYS or CONTROL RELAYS. Power relays are called CONTACTORS; control relays are usually known simply as relays.

The function of contactor is to use a relatively small amount of electrical power to control the switching of a large amount of power. The contactor permits you to control power at other locations in the equipment, and the heavy power cables need be run only through the power relay contacts. Only lightweight control wires are connected from the control switches to the relay coil. Safety is also an important reason for using power relays, since high power circuits can be switched remotely without danger to the operator. Control relays, as their name implies, are frequently used in the control of low power circuits or other relays, although they also have many other uses. In automatic relay circuits, a small electric signal may set off a chain reaction of successively acting relays, which then perform various functions.

Classification of Relays

Relays can be classified into many different categories according to their working principle, physical dimensions, protective features, contact loads and product applications.

Relays depending on their working principle

Electromagnetic Relays

Relays in which the relative movements of their mechanical components produce preset responses under the effect of the current in the input circuit are called electromagnetic relays.

Relays in this category include DC electromagnetic relays, AC electromagnetic relays, magnetic-latching relays, polarized relays, and reed relays.

DC electromagnetic relays

Relays whose control current in the input circuit is DC.

AC electromagnetic relays

Relays whose control current in the input circuit is AC.

Magnetic-latching relays

Relays, after the magnetic steel is introduced into the magnetic loop, even the relay coil is de-energized the armature iron steel still maintains its state as that when the coil is energized, with two steady states.

Polarized relays

DC relays whose change of state depends on the polarity of the input exciting variable.

Reed relays

Relays that rely on the movements of the reed which is built in the tube and has dual functions as contact reed and armature iron magnetic circuit for connecting, breaking or switching circuits.

Solid State relays

Relays whose input and output functions are performed by electronic elements without mechanical movement components.

Time Relays

Relays whose controlled circuit connects or breaks when the output part is delayed or timed to a preset time after the input signal is added or erased.

Temperature Relays

Relays that get into motion when the outside temperature reaches a present point.

Wind-Velocity Relays

When the wind velocity reaches a certain point, the controlled circuit will connect or break off.

Acceleration Relays

When the acceleration of the moving object reaches a preset point, the controlled circuit will connect or break off.

Relays in other categories

Including photo relays, sound relays, and heat relays.

Relays According To Physical Dimensions

Description Definition

1. Min relays Relays whose maximum edge is no

larger than10mm.

2. Super Min Relays Relays whose max edge is larger than 10mm but not larger than 25mm.

3. Compact Relays Relays whose max edge is larger than 25mm but not larger than 50mm.

Relays according to contact load

Description Definition

1. Micro Power Relays Relays whose current is smaller than 0.2A.

2. Low power Relays Relays whose current range is in range of 0.2 – 2A.

3. Medium-power Relays Relays whose current is in range of 2 – 10A.

4. High power Relays Relays whose current is larger than 10A

Relays according to protective features

Description Definition

1. Sealed Relays Relays whose contact and coil are sealed in a metal case by welding or other methods and therefore enjoy low leakage rates.

2. Plastic cased Relays Relays whose contact and coil are sealed in Plastic case by gluing and have somewhat Higher leakage rates.

3. Dust proof Relays Relays whose contact and coil are sealed in a Case For protection purposes.

4. Open Relays Relays whose contact and coil are not Protected with a case.

Relays according applications

Description Definition

1. Communication relays Relays with contact load ranging from low level to medium current and therefore enjoying comparatively low ambient conditions for use.

2. Machine tool relays Relays used on machine tools with high contact load power and long service life.

3. House hold appliance relays: Relays used in house hold appliances must meet a high safety standard.

4. Automobile relays: Relays used in automobiles with high switching load power and high impact and vibration resistance.