MT8880C/MT8880C-1
EVENTS
t
REC
V
in
t
DP
ESt
A
ISO
2
-CMOS
B
t
REC
TONE #n
t
ID
C
D
t
DO
TONE
#n + 1
E
F
TONE
#n + 1
t
DA
t
GTP
t
GTA
V
TSt
t
PStRX
St/GT
RX
0
-RX
3
DECODED TONE # (n-1)
#n
t
PStb3
# (n + 1)
b3
b2
Read
Status
Register
IRQ/CP
Figure 9 - Receiver Timing Diagram
of time segments is fixed at 32, however, by varying
the segment length as described above the tone
output signal frequency will 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 10
that the distortion products are very low in amplitude.
Scaling Information
10 dB/Div
Start Frequency = 0 Hz
Stop Frequency = 3400 Hz
Marker Frequency = 697 Hz and
1209 Hz
Figure 10 - Spectrum Plot
4-38
MITEL [ MITEL NETWORKS CORPORATION ]
