Provided that the signal condition is maintained (ESt remains

corresponding 4-bit code (see Table 2) into the receive data reg-

ister.

StGT 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, signaling that a received

tone pair has been registered. It is possible to monitor the status

of the delayed steering flag 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 de-

layed steering transition. This data is presented to the 4-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 in-

terruptions (dropout) too short to be considered a valid pause.

This capability, together with the ability to select the steering

time constants externally, allows the designer to tailor perfor-

mance to meet a wide variety of system requirements.

Guard Time Adjustment: The simple steering circuit shown in

Figure 5 is adequate for most applications. Component values

are chosen according to the formula:

signal duration to be recognized by the receiver. A value for C1

of 0.1

µF is recommended for most applications, leaving R1 to

be selected by the designer. Different steering arrangements

may be used to select independently the guard times for tone

meet system specifications that place both accept and reject

limits on both tone duration and interdigit pause. Guard time ad-

justment also allows the designer to tailor system parameters

talkoff performance since it reduces the probability that tones

simulated by speech will maintain signal condition long enough

where fast acquisition time and immunity to tone dropouts are

required. Design information for guard time adjustment is shown

in Figure 6.

Call Progress Filter

A call progress (CP) mode can be selected, allowing the detec-

tion of various tones that identify the progress of a telephone call

on the network. The call progress tone input and DTMF input are

common; however, call progress tones can only be detected

when the CP mode has been selected. DTMF signals cannot be

Page 3

M-8880

40-406-00012, Rev. G

www.clare.com

Digit

D3

D2

D1

D0

697

1209

10001

697

1336

20010

697

1477

30011

770

1209

40100

770

1336

50101

770

1477

60110

852

1209

70111

852

1336

81000

852

1477

91001

941

1336

01010

941

1209

*

1011

941

1477

#1100

697

1633

A

1101

770

1633

B

1110

852

1633

C

1111

941

1633

D

0000

0 = logic low, 1 = logic high

Table 2 Tone Encoding/Decoding

Figure 5 Basic Steering Circuit

Figure 6 Guard Time Adjustment