Issue
156 July 2003
Stealth
Telephone Screener
Mad
Dash for Flash Cash Grand Prize Winner
FRAME
UPDATE
The
frame update routine runs every 25 ms. Rather than calling
it directly from the interrupt routine, it’s called
regularly by the main routine by checking the frame
update flag (FRMFLG) whenever there’s a loop waiting
for an event. FRMFLG is set by the interrupt routine
every 200 samples.
This
routine reads and debounces the inputs, and checks for
an abort condition (see Listing 1). The two switches,
ring detector, VOX, and the line-voltage detector are
debounced using two separate on and off times for each
input. Thus, multiple functions can be based on a single
input. For instance, when the phone rings, the short
debounce-time ringing bit would follow the ring cadence,
and the longer one would indicate the overall ringing
period.
| Listing
1—The debounce routine checks each of the input
conditions (IFLAGS) and sets the status bits (SFLAG1
and SFLAG2) after applying the debounce times from
ONTBL1/2 or OFFTBL1/2. This provides two different
debounce times for each bit. |
INTERRUPTS
The
highest priority is the sample interrupt, which runs
at 8 kHz; it handles the A/D and PWM samples along with
reading and writing to the external EEPROM. The mode
flag determines which action will be taken depending
on the current operating mode.
The
VOX is the most time-intensive function, because each
sample must be squared and summed as it’s acquired.
The routine was carefully optimized so that the interrupt
routine loading is only 64%, leaving plenty of CPU cycles
for the main routine.
Audio
recording and playback, including ADPCM coding, is handled
entirely at the interrupt level, using a 64-byte circular
buffer feeding the I2C EEPROM. Because the I2C bus takes
time to transfer data, only one bus transaction is performed
per interrupt. A state machine steps through each I/O
operation to execute a read or write of the buffer.
Audible
ring-back and beep tones are generated with a pair of
phase accumulators and a sine wave look-up table. Dividing
down the frame counter and toggling the speaker output
pin generate the two-tone ringing sound.
If
the phone line is picked up, the line voltage is sampled
once per frame. The A/D conversion is interleaved between
a pair of audio conversions by setting Timer0 to generate
an interrupt roughly halfway between audio samples.
Thus, the timing of the audio samples isn’t affected.
THE
MICROCONTROLLER
The
PIC16F872 uses a 16.384-MHz ceramic resonator, which
is a convenient multiple of the 8-kHz sample rate (see
Figure 4). It controls an EEPROM via the I2C port and
drives a speaker to generate an alternating tone ringing
sound. In addition, it controls the phone line hook
switch and power to the analog section via a DPDT relay.
|
(Click
here to enlarge)
|
Figure
4—An ultralow-power linear regulator and switchable
pull-down for S1 result in miniscule power consumption
when the MCU is sleeping. |
I
used a low-power relay that requires only 10 mA and
is directly driven by a port pin. The On/Off switch
is driven by an output port pin, which goes high in
Sleep mode to eliminate the pull-up current. The prototype
has an ICSP port for easier programming.
PHONE-LINE
INTERFACE
In
order to connect directly to a telephone line, there
are a number of FCC and Underwriters Laboratories (UL)
safety and performance requirements. To learn more about
these requirements, take a look at the resources listed
at the end of this article. Although the FCC’s technical
requirements are now handled by the Administrative Council
on Terminal Attachments and published by the Telecommunications
Industry Association, I’ll refer to them by their former
name.
Often,
the most difficult requirements are the line-to-line
and line-to-ground surge specifications, which are designed
to protect against a lightning strike (FCC) or a direct
power line short (UL). Because the Screener doesn’t
have external connections, the line-to-ground requirements
are relatively easy: they’re met by using a plastic
box and properly isolating any exposed metal parts.
UL typically allows the battery contacts not be isolated
from the phone line, provided the battery compartment
requires a tool for opening it (e.g., a screwdriver
or coin) and the product has a suitable warning message.
The
line-to-line surges are somewhat trickier because anything
connected to the line could be exposed. The surges are
applied both on and off the hook. Different strategies
were applied here for each case.
When
the Screener is on-hook, the relay will protect anything
downstream from the surge. The only two components that
will be exposed to high voltage are C8 and R11, which
are both rated for 1 kV—the highest surge voltage applied
(see Figure 5). Most common 0.25-W resistors are rated
for 500 V, so two can be used in series rather than
using a more expensive part.
|
(Click
here to enlarge)
|
Figure
5—The phone-line interface consists of the hook
relay, ring detector, and active current sink. A
pair of dual op-amps is used for the antialiasing
and reconstruction filters. The receive gain is
switchable, allowing either the caller’s voice or
a locally connected phone (used for recording a
greeting) to be the right level for the A/D converter. |
In
the off-hook state, a surge suppressor, D2, limits the
surge to 18 V. The circuit must withstand a 1000-V,
25-A, type-B metallic surge without damage; therefore,
I used a special surge-withstand fuse that was designed
to meet FCC requirements (i.e., TIA-968-A section 4.2.3.1,
which was formerly FCC part 68, section 302 (c)(1)).
All of the wiring from the phone jack to D2 must be
able to handle this surge current. Otherwise, surge
testing may result in a dramatic separation of the wiring
from its former connections!
An
active current sink (Q3 and nearby components) controls
the DC-line current with a dynamic resistance of 200
W. This is about two times
the resistance of a typical telephone and was chosen
so the line voltage would be noticeably higher than
normal, making it easier to tell when a phone is picked
up.
When
the Screener first picks up the line, the current sink
doesn’t start drawing current because C11 isn’t charged
yet. If no other phone is picked up, the line voltage
will rapidly rise until it’s limited by D2 (around 18
V). After 10 ms, if it doesn’t go high enough, the Screener
immediately hangs up the line and waits for another
call.
Ring
detection is done using a diac, which is a breakdown
device that triggers at 60 V. This powers a bidirectional
optoisolator driving the PIC’s interrupt input. When
the phone rings, the signal toggles at 40 Hz. The optoisolator
is only used to protect the PIC from lightning surges,
because it isn’t otherwise needed for electrical isolation.
