DTMF RECEIVER

HM 9270C/D

- 7 -

FIGURE 5. TIMING DIAGRAM

A. Short tone bursts: detected. Tone duration is invalid.

B. Tone #n is detected. Tone duration is valid. Decoded

to outputs.

C. End of tone #n is dectected and validated.

D. 3 State outputs disabled (high impedance).

E. Tone #n + 1 is detected. Tone duration is valid. De

coded to outputs.

F. Tristate outputs are enabled. Acceptable drop out of

tone #n + 1 does not negister at outputs.

G. End of tone #n + 1 is detected and validated.

FIGURE 5. TIMING DIAGRAM

the GT output is activated and drives V

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, StD, goes high, signaling

that a recieved tone-pair has been registered. The contents of the output lacth are made available on the 4-bit

output bus by raising the 3-state control input (TOE) to a logic high. The steering circuit works in reverse to

validate the interdigit paues 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. The 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 requiremetns.

FIGURE 6. TYPICAL FILTER

CHARACTERISTIC

Before registration of a decoded tone-pair, the receiver checks

for a valid signal duration (referred to as "character-recogni-

tion-condition"). This check is per-

formed by an external RC time-constant driven by ESt.

A logic high on ESt causes V

C

(see Fig. 5) to rise as the

capacitor discharges. Provided signal-condition is main-

tained (ESt remains high) for the validation period (t

reaches the threshold (V

tone-pair, latching its corresponding 4-bit code (see Fig. 3)

into the output latch. At this point,

STEERING CIRCUIT

D

AB

C

E

F

G

t

REC

INTERDIGIT

PAUSE

t

ID

TONE DROPOUT

t

DO

TONE # n

TONE #n+1

TONE#n+1

t

DP

DA

t

GTP

t

PQ

t

t

GTA

DECODE TONE n-1

DECODED

TONE#n

DECODED TONE # n + 1

HIGH

IMPEDANCE

t

PTE

PTD

t

EVENTS

ESt

DATA

OUTPUTS

Q1-Q4

TOE

St/GT

StD

OUTPUT

0

10

20

30

40

50

60

70

80

0

1K

2K