ISO
2
-CMOS
Burst Mode
In certain telephony applications it is required that
DTMF signals being generated are of a specific
duration determined either by the particular
application or by any one of the exchange transmitter
specifications currently existing. Standard DTMF
signal timing can be accomplished by making use of
the Burst Mode. The transmitter is capable of issuing
symmetric bursts/pauses of predetermined duration.
This burst/pause duration is 51 ms±1 ms which is a
standard interval for autodialer and central office
applications. After the burst/pause has been issued,
the appropriate bit is set in the Status Register
indicating that the transmitter is ready for more data.
The timing described above is available when DTMF
mode has been selected. However, when CP mode
(Call Progress mode) is selected, a second burst/
pause
time of 102 ms ±2 ms is available. This
extended interval is useful when precise tone bursts
of longer than 51 ms duration and 51 ms pause are
desired. Note that when CP mode and Burst mode
have been selected, DTMF tones may be transmitted
only and
not
received.
In applications where a non-standard burst/pause
duration is required, burst mode must be disabled
MT8880C/MT8880C-1
and the transmitter gated on and off by an external
hardware or software timer.
Single Tone Generation
A single tone mode is available whereby individual
tones from the low group or high group can be
generated. This mode can be used for DTMF test
equipment applications, acknowledgment tone
generation and distortion measurements. Refer to
Control Register B description for details.
Distortion Calculations
The MT8880C/C-1 is capable of producing precise
tone bursts with minimal error in frequency (see
Table 1). The internal summing amplifier is followed
by a first-order lowpass switched capacitor filter to
minimize harmonic components and intermodulation
products. The total harmonic distortion for a
single
tone
can be calculated using Equation 1, which is the
ratio of the total power of all the extraneous
frequencies to the power of the fundamental
frequency expressed as a percentage. The Fourier
components of the tone output correspond to V
2f
....
V
nf
as measured on the output waveform. The total
harmonic distortion for a
dual tone
can be calculated
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
DTMF COMPOSITE INPUT SIGNAL.
V
in
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. 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
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.
MAXIMUM DTMF SIGNAL DURATION NOT DETECTED AS VALID.
t
REC
MINIMUM DTMF SIGNAL DURATION REQUIRED FOR VALID RECOGNITION.
t
REC
MINIMUM TIME BETWEEN VALID SEQUENTIAL DTMF SIGNALS.
t
ID
MAXIMUM ALLOWABLE DROPOUT DURING VALID DTMF SIGNAL.
t
DO
TIME TO DETECT VALID FREQUENCIES PRESENT.
t
DP
TIME TO DETECT VALID FREQUENCIES ABSENT.
t
DA
GUARD TIME, TONE PRESENT.
t
GTP
GUARD TIME, TONE ABSENT.
t
GTA
Figure 11 - Description of Timing Events
4-39
MITEL [ MITEL NETWORKS CORPORATION ]
