MT8889CE/CE-1 Mitel Semiconductor, MT8889CE/CE-1 Datasheet - Page 6

• Manufacturer Part Number: MT8889CE/CE-1
• Description: 6V, 10mA, integrated DTMF transceiver with adaptive micro interface. For paging systems, repeater systems/mobile radio, credit card systems, personal computers, interconnect dialers
• Manufacturer: Mitel Semiconductor

MT8889C/MT8889C-1
DTMF Generator
The DTMF transmitter employed in the MT8889C/
MT8889C-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
generated (f
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
4-112
EXPLANATION OF EVENTS
A)
B)
C)
D)
E)
F)
EXPLANATION OF SYMBOLS
V
ESt
St/GT
RX
b3
b2
IRQ/CP
t
t
t
t
t
t
t
t
REC
REC
ID
DO
DP
DA
GTP
GTA
in
0
-RX
3
RETAINED UNTIL NEXT VALID TONE PAIR.
RETAINED UNTIL NEXT VALID TONE PAIR.
REQUIRED GUARD TIME THUS CONSTITUTING A VALID SIGNAL. ACTIVE LOW FOR THE DURATION OF A
VALID DTMF SIGNAL.
REGISTER IS READ.
CLEARED AFTER THE STATUS REGISTER IS READ.
TONE BURSTS DETECTED, TONE DURATION INVALID, RX DATA REGISTER NOT UPDATED.
TONE #n DETECTED, TONE DURATION VALID, TONE DECODED AND LATCHED IN RX DATA REGISTER.
END OF TONE #n DETECTED, TONE ABSENT DURATION VALID, INFORMATION IN RX DATA REGISTER
TONE #n+1 DETECTED, TONE DURATION VALID, TONE DECODED AND LATCHED IN RX DATA REGISTER.
ACCEPTABLE DROPOUT OF TONE #n+1, TONE ABSENT DURATION INVALID, DATA REMAINS UNCHANGED.
END OF TONE #n+1 DETECTED, TONE ABSENT DURATION VALID, INFORMATION IN RX DATA REGISTER
DTMF COMPOSITE INPUT SIGNAL.
EARLY STEERING OUTPUT. INDICATES DETECTION OF VALID TONE FREQUENCIES.
STEERING INPUT/GUARD TIME OUTPUT. DRIVES EXTERNAL RC TIMING CIRCUIT.
4-BIT DECODED DATA IN RECEIVE DATA REGISTER
DELAYED STEERING. INDICATES THAT VALID FREQUENCIES HAVE BEEN PRESENT/ABSENT FOR THE
INDICATES THAT VALID DATA IS IN THE RECEIVE DATA REGISTER. THE BIT IS CLEARED AFTER THE STATUS
INTERRUPT IS ACTIVE INDICATING THAT NEW DATA IS IN THE RX DATA REGISTER. THE INTERRUPT IS
MAXIMUM DTMF SIGNAL DURATION NOT DETECTED AS VALID.
MINIMUM DTMF SIGNAL DURATION REQUIRED FOR VALID RECOGNITION.
MINIMUM TIME BETWEEN VALID SEQUENTIAL DTMF SIGNALS.
MAXIMUM ALLOWABLE DROPOUT DURING VALID DTMF SIGNAL.
TIME TO DETECT VALID FREQUENCIES PRESENT.
TIME TO DETECT VALID FREQUENCIES ABSENT.
GUARD TIME, TONE PRESENT.
GUARD TIME, TONE ABSENT.
LOW
and f
HIGH
) are referred to as Low
Figure 9 - Description of Timing Events
are
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
frequency can also be varied. The divider output
clocks
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
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.
incorporated and serves to attenuate distortion
products above 8 kHz. It can be seen from Figure 6
that the distortion products are very low in amplitude.
segment
another
length
counter,
A
bandwidth
as
which
described
limiting
addresses
above
filter is
and
the
the