MT88L89 Data Sheet
8
Zarlink Semiconductor Inc.
Figure 8 - Description of Timing Events
Call Progress Filter
A call progress mode, using the MT88L89, can be selected to allow the detection of various tones, which 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 CP mode has been selected. DTMF signals cannot be
detected if CP mode has been selected (see Table 7). Figure 10 indicates the useful detect bandwidth of the call
progress filter. Frequencies presented to the input, which are within the ‘accept’ bandwidth limits of the filter, are
hard-limited by a high gain comparator with the IRQ
/CP pin serving as the output. The square-wave output obtained
from the schmitt trigger can be analyzed by a microprocessor or counter arrangement to determine the nature of
the call progress tone being detected. Frequencies which are in the ‘reject’ area will not be detected and
consequently the IRQ
/CP pin will remain low.
DTMF Generator
The DTMF transmitter employed in the MT88L89 is capable of generating all sixteen standard DTMF tone pairs
with low distortion and high accuracy. All frequencies are derived from an external 3.579545 MHz crystal. The
sinusoidal waveforms for the individual tones are digitally synthesized by using row and column programmable
dividers and switched capacitor D/A converters. The row and column tones are mixed and filtered to provide a
DTMF signal with low total harmonic distortion and high accuracy. To specify a DTMF signal, data conforming to the
encoding format shown in Table 1 must be written to the transmit Data Register. Note that Table 1 is the same as
the receiver output code. The individual tones which are generated (f
LOW
and f
HIGH
) are referred to as Low Group
and High Group tones. As seen from the table, the low group frequencies are 697, 770, 852 and 941 Hz. The high
group frequencies are 1209, 1336, 1477 and 1633 Hz. Typically, the high group to low group amplitude ratio (twist)
is 2 dB to compensate for high group attenuation on long loops.
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, INFORMATION IN RX DATA REGISTER
RETAINED UNTIL NEXT VALID TONE PAIR.
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, DATA REMAINS UNCHANGED.
F) END OF TONE #n+1 DETECTED, TONE ABSENT DURATION VALID, INFORMATION IN RX DATA REGISTER
RETAINED UNTIL NEXT VALID TONE PAIR.
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
0
-RX
3
4-BIT DECODED DATA IN RECEIVE DATA REGISTER
b3 DELAYED
STEERING IN STATUS REGISTER (BIT 3) INDICATES THAT VALID FREQUENCIES HAVE BEEN
PRESENT/ABSENT FOR THE REQUIRED GUARD TIME THUS CONSTITUTING A VALID SIGNAL. ACTIVE LOW
FOR THE DURATION OF A VALID DTMF SIGNAL.
b2 RECEIVE DATA REGISTER FULL (BIT 2) IN STATUS REGISTER INDICATES THAT VALID DATA IS IN THE
RECEIVE DATA REGISTER. THE BIT IS CLEARED AFTER THE STATUS REGISTER IS READ.
IRQ
/CP INTERRUPT IS ACTIVE INDICATING THAT NEW DATA IS IN THE RX DATA REGISTER. THE INTERRUPT IS
CLEARED AFTER THE STATUS REGISTER IS READ.
t
REC
MAXIMUM DTMF SIGNAL DURATION NOT DETECTED AS VALID. TYPICALLY 20MS.
t
REC
MINIMUM DTMF SIGNAL DURATION REQUIRED FOR VALID RECOGNITION. TYPICALLY 40MS.
t
ID
MINIMUM TIME BETWEEN VALID SEQUENTIAL DTMF SIGNALS. TYPICALLY 40MS.
t
DO
MAXIMUM ALLOWABLE DROPOUT DURING VALID DTMF SIGNAL. TYPICALLY 20MS.
t
DP
TIME TO DETECT VALID FREQUENCIES PRESENT.
t
DA
TIME TO DETECT VALID FREQUENCIES ABSENT.
t
GTP
GUARD TIME, TONE PRESENT.
t
GTA
GUARD TIME, TONE ABSENT.
