M-8870

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7

Rev. 3

Timing Diagram

Explanation of Events

(A) Tone bursts detected, tone duration invalid, outputs not updated.

(B) Tone #n detected, tone duration valid, tone decoded and latched in outputs.

(C) End of tone #n detected, tone absent duration valid, outputs remain latched until next valid tone.

(D) Outputs switched to high impedance state.

(E) Tone #n + 1 detected, tone duration valid, tone decoded and latched in outputs (currently high impedance).

(F) Acceptable dropout of tone #n + 1, tone absent duration invalid, outputs remain latched.

(G) End of tone #n + 1 detected, tone absent duration valid, outputs remain latched until next valid tone.

Explanation of Symbols

VIN

DTMF composite input signal.

ESt

Early steering output. Indicates detection of valid tone frequencies.

St/GT

Steering input/guard time output. Drives external RC timing circuit.

Q1 - Q4

4-bit decoded tone output.

StD

Delayed steering output. Indicates that valid frequencies have been present/

absent for the required guardtime, thus constituting a valid signal.

OE

Output enable (input). A low level shifts Q1 - Q4 to its high impedance state.

t

REC

Maximum DTMF signal duration not detected as valid.

t

REC

Minimum DTMF signal duration required for valid recognition.

t

ID

Minimum time between valid DTMF signals.

t

DO

Maximum allowable dropout during valid DTMF signal.

t

DP

Time to detect the presence of valid DTMF signals.

t

DA

Time to detect the absence of valid DTMF signals.

T

GTP

Guard time, tone present.

T

GTA

Guard time, tone absent.