MT8888C/MT8888C-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, 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.
INTERRUPT IS ACTIVE INDICATING THAT NEW DATA IS IN THE RX DATA REGISTER. THE INTERRUPT IS
IRQ/CP
CLEARED AFTER THE STATUS REGISTER IS READ.
t
REC
MAXIMUM DTMF SIGNAL DURATION NOT DETECTED AS VALID.
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 9 - Description of Timing Events
DTMF Generator
The DTMF transmitter employed in the MT8888C/
MT8888C-1 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 using row and column programmable
dividers and switched capacitor D/A converters. The
row and column tones are mixed and filtered
providing 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 this 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 com-
pensate for high group attenuation on long loops.
The period of each tone consists of 32 equal time
segments. The period of a tone is controlled by
varying the length of these time segments. During
4-96
write operations to the Transmit Data Register the 4
bit data on the bus is latched and converted to 2 of 8
coding for use by the programmable divider circuitry.
This code is used to specify a time segment length,
which will ultimately determine the frequency of the
tone. When the divider reaches the appropriate
count, as determined by the input code, a reset pulse
is issued and the counter starts again. The number
of time segments is fixed at 32, however, by varying
the segment length as described above the
frequency can also be varied. The divider output
clocks another counter, which addresses the
sinewave lookup ROM.
The lookup table contains codes which are used by
the switched capacitor D/A converter to obtain
discrete and highly accurate DC voltage levels. Two
identical circuits are employed to produce row and
column tones, which are then mixed using a low
noise summing amplifier. The oscillator described
needs no “start-up” time as in other DTMF
generators since the crystal oscillator is running
continuously thus providing a high degree of tone
burst accuracy. A bandwidth limiting filter is
incorporated and serves to attenuate distortion
products above 8 kHz. It can be seen from Figure 8
that the distortion products are very low in amplitude.
MITEL [ MITEL NETWORKS CORPORATION ]
