Application Information ......
Principles of Decoder Operation
Level Detection
As level and frequency discrimination operations
take place in parallel the audio signal should, under
normal circumstances, be input to both Signal In and
Level In pins via coupling capacitors.
If the input signal level (Level In) is outside the
preset ‘Must/Must Not Decode’ thresholds
(Specification Page), the Universal Call Progress
Decoder will be disabled.
If it is wished to disregard signal input
levels
at the
Level In pin and attempt to decode under all
conditions, the decoder may be permanently enabled
by holding the Level In pin at V
DD
.
The device can disabled by pulling Level In to V
SS
.
N
OTONE
Recognition
The N
OTONE
condition can be recognized using
µProcessor software timing as below.
a. Set the µP timer period to a period greater than
the relevant frequency-band measurement
period (13.16ms or 39.47ms).
b. Each ‘Tone Measurement Complete’ interrupt
from the FX613 must reset the µP timer.
c. With N
OTONE
or white noise at the decoder input,
the FX613 on-chip timer will be continually reset.
ii No ‘Tone Measurement Complete’ interrupt
will occur - the µP timer will run.
ii The µP Timer time-out can be considered as
a
N
OTONE
indication.
Level In
In Limits
Out of Limits
V
DD
V
SS
Timer
Running
Reset
Running/Reset
Reset
IRQ
Enabled
Disabled
Enabled/Disabled
Disabled
Data Out
Enabled
Disabled (frozen to previous bit-5 level
Enabled (dependent upon Quality measurement)
Disabled (frozen to previous bit-5 level)
Frequency Band Discrimination
The input signal is amplified by a self-biased (zero-
crossing) inverting amplifier and then ‘filtered’ to
remove high-frequency noise and jitter.
High (HI) and Low (LO) counters are employed to
determine the input frequency band (HI = 900Hz to
2150Hz, LO = 300Hz to 660Hz).
If the input frequency is in the LO Band, the device
will operate as a LO Band decoder and will remain so
until a HI frequency signal is detected. If the input
frequency is in the HI Band, the device will operate as
a HI Band decoder and will remain so until a LO
frequency signal is detected.
Frequency band monitoring is continuous with the
band selection taking place every 9.8ms. It will
therefore take 9.8ms from Power-Up to set up the
initial correct decode frequency band.
On-Chip Timer Operation
For frequency measurement, the FX613 counts
the number of input cycles in a fixed time period. This
fixed period, measured by the continuous on-chip
timer, is set to 13.16ms for HI Band inputs and
39.47ms for LO Band inputs.
On-Chip Timer Operation ......
When the timer expires the following actions take
place:
a. A HI or LO (“1” or “0”) band indication bit is
latched into Bit-5 of the Serial Output Port.
b. The Frequency Counter count of 5-bits is latched
into the Serial Output Port (Bit-4 [MSB] to Bit-0).
The Serial Output Port Contains 6-bits, if 8 Serial
Clock edges are employed, two extra “0s”, which
should be ignored, will be output last.
c. An interrupt is generated (IRQ) to the
µProcessor. The contents of the Serial Output
Port should be read before the next interrupt is
expected; if not data will be overwritten.
When the Chip Select input is set to “0” the
interrupt is reset.
The On-Chip Timer and Frequency Counter will be
reset in mid-count, and therefore unable to allow a
valid measurement, under the following conditions:
a. A change of decode frequency band.
b. Decoder disabled; signal input level out of
specification or Level Detect input set to V
SS
.
c. Signal Quality Assessment considered ‘Bad’.
d. Input signal frequency outside limits.
4
CMLMICRO [ CML MICROCIRCUITS ]
