M-8870
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
ESt
St/GT
Q1 - Q4
StD
OE
t
REC
t
REC
t
ID
t
DO
t
DP
t
DA
T
GTP
T
GTA
Rev. 3
DTMF composite input signal.
Early steering output. Indicates detection of valid tone frequencies.
Steering input/guard time output. Drives external RC timing circuit.
4-bit decoded tone output.
Delayed steering output. Indicates that valid frequencies have been present/
absent for the required guardtime, thus constituting a valid signal.
Output enable (input). A low level shifts Q1 - Q4 to its high impedance state.
Maximum DTMF signal duration not detected as valid.
Minimum DTMF signal duration required for valid recognition.
Minimum time between valid DTMF signals.
Maximum allowable dropout during valid DTMF signal.
Time to detect the presence of valid DTMF signals.
Time to detect the absence of valid DTMF signals.
Guard time, tone present.
Guard time, tone absent.
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