MT8889CS datasheet(4/18 Pages) MITEL

4 / 18 page

MT8889C/MT8889C-1

4-110

Following the filter section is a decoder employing

digital

counting

techniques

determine

the

frequencies of the incoming tones and to verify that

they correspond to standard DTMF frequencies. A

complex averaging algorithm protects against tone

simulation by extraneous signals such as voice while

providing tolerance to small frequency deviations

and variations. This averaging algorithm has been

developed to ensure an optimum combination of

immunity to talk-off and tolerance to the presence of

interfering frequencies (third tones) and noise. When

the detector recognizes the presence of two valid

tones (this is referred to as the “signal condition” in

some industry specifications) the “Early Steering”

(ESt)

output

will

active

state.

Any

subsequent loss of signal condition will cause ESt to

assume an inactive state.

Steering Circuit

Before registration of a decoded tone pair, the

receiver checks for a valid signal duration (referred

to as character recognition condition). This check is

performed by an external RC time constant driven by

ESt. A logic high on ESt causes vc (see Figure 5) to

rise as the capacitor discharges. Provided that the

signal condition is maintained (ESt remains high) for

the validation period (tGTP), vc reaches the threshold

(VTSt) of the steering logic to register the tone pair,

latching its corresponding 4-bit code (see Table 1)

into the Receive Data Register. At this point the GT

output is activated and drives vc to VDD. GT

continues to drive high as long as ESt remains high.

Finally, after a short delay to allow the output latch to

settle, the delayed steering output flag goes high,

signalling that a received tone pair has been

registered. The status of the delayed steering flag

can be monitored by checking the appropriate bit in

the status register. If Interrupt mode has been

selected, the IRQ/CP pin will pull

low when

the

delayed steering flag is active.

The contents of the output latch are updated on an

active delayed steering transition. This data is

presented to the four bit bidirectional data bus when

the Receive Data Register is read. The steering

circuit works in reverse to validate the interdigit

pause between signals. Thus, as well as rejecting

signals too short to be considered valid, the receiver

will tolerate signal interruptions (drop out) too short

to be considered a valid pause. This facility, together

with the capability of selecting the steering time

constants externally, allows the designer to tailor

performance to meet a wide variety of system

requirements.

Figure 5 - Basic Steering Circuit

Guard Time Adjustment

The simple steering circuit shown in Figure 5 is

adequate for most applications. Component values

are chosen according to the following inequalities

(see Figure 7):

tREC ≥ tDPmax + tGTPmax - tDAmin

tREC ≤ tDPmin + tGTPmin - tDAmax

tID ≥ tDAmax + tGTAmax - tDPmin

tDO ≤ tDAmin + tGTAmin - tDPmax

The value of tDP is a device parameter (see AC

Electrical Characteristics) and tREC is the minimum

signal duration to be recognized by the receiver. A

value for C1 of 0.1 µF is recommended for most

Figure 6 - Guard Time Adjustment

VDD

St/GT

ESt

MT8889C/

tGTA = (R1C1) In (VDD / VTSt)

tGTP = (R1C1) In [VDD / (VDD-VTSt)]

MT8889C-1

VDD

St/GT

ESt

VDD

St/GT

ESt

tGTA = (R1C1) In (VDD/VTSt)

tGTP = (RPC1) In [VDD / (VDD-VTSt)]

RP = (R1R2) / (R1 + R2)

tGTA = (RpC1) In (VDD/VTSt)

tGTP = (R1C1) In [VDD / (VDD-VTSt)]

RP = (R1R2) / (R1 + R2)

a) decreasing tGTP; (tGTP < tGTA)

b) decreasing tGTA; (tGTP > tGTA)

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