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12

M-8888

Rev. 1

Explanation of Events

(A) Tone bursts detected, tone duration invalid, RX Data Register not updated.

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

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

(D) Tone #n + 1 detected, tone duration valid, tone decoded and latched in RX Data Register.

(E) Acceptable dropout of tone #n + 1, tone absent duration invalid, RX Data Register remain latched.

(F) End of tone #n + 1 detected, tone absent duration valid, RX Data Register remain latched until next valid tone.

Explanation of Symbols

V

IN

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.

RX

4-bit decoded data in receive data register.

b3

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

required guard time, thus constituting a valid DTMF signal.

b2

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

IRQ /CP

Interrupt is active indicating that new data is in the RX data register. The interrupt is cleared

after the status register is ready.

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.