October
2004, Issue 171
Telephone
Message Watchdog
An
Intelligent Call-Forwarding System
by
Jingxi Zhang, Yang Zhang, & Huifang Ni
MCU
OPERATION
We
mixed H8S/2600 assembly and C languages. Figure 3 is
the program flow chart. When the device is powered on,
the MCU starts a reset process and sets up the on-chip
peripheral I/O modules, including 16-bit timer units,
an ADC, a DMA controller, D/A converters, and I/O pins.
The MCU then enters Sleep mode.
|
(Click
here to enlarge)
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Figure
3—The MCU is in Sleeping mode after initialization.
The line loop current sensor wakes it up and it
starts to detect the DTMF signal. After the loop
current drops, the MCU starts dialing the pager
number and delivers the message. |
A
loop current sensor interrupt (IRQ7) wakes up the MCU
when the answering machine loop current increases in
case an incoming call has arrived. The ring signal pattern
is on for 2 s and pauses for 4 s. To check if the answering
machine is turned on, the IRQ is disabled temporarily,
and the loop-current condition is continuously monitored.
Only when a loop current stays on after the ring on-off
period does the MCU enter DTMF Tone Detection mode.
In
DTMF Tone Detection mode, the ADC continuously samples
the line signal, and the DMA control feeds the converted
data into one of the DTMF detection buffers. For each
14-ms frame, all eight Goertzel filters—four for the
low-frequency group and four for the high-frequency
group—are executed. If any of these filters detects
a significant tone, a validation test is performed.
The validation test includes the process of verifying
that both the high- and low-frequency tones are simultaneously
present. It also verifies that only one tone is present
in the low-frequency group and only one tone is present
in the high-frequency group. Finally, the test verifies
that the dual-tone duration lasts at least three 14-ms
frames (greater than 40 ms, as required by the FCC).
The pause between the DTMF tones also should be verified
in order to recognize two separate DTMF numbers with
the same value. If the validation test passes, the number
is captured and stored in a memory location.
When
the caller terminates the call, the loop current sensor
generates another interrupt at IRQ7. The MCU stops DTMF
Detection mode and starts Message Forwarding mode, during
which the line condition is checked first. If the line
is free, the MCU sets the output pin (PC0) to logic
high. This action engages the telephone line. Then the
DTMF tone generator sends your pager number through
the DAC to the telephone interface. Then the MCU listens
to the incoming audio signal for the pager’s beep. A
few seconds after the beep, the captured phone number
is sent to the output by the DTMF tone generator. Finally,
the MCU frees the phone line and returns to Sleep mode
and waits for the next calling process.
