![](http://datasheet.mmic.net.cn/Silicon-Laboratories-Inc/SI3225-G-GQ_datasheet_102098/SI3225-G-GQ_68.png)
Si3220/25 Si3200/02
68
Rev. 1.3
Not
Recommended
fo
r N
ew
D
esi
gn
s
3.22. Pulse Metering Generation
The Si3220 offers an additional tone generator to
generate tones above the audio frequency band. This
oscillator generates billing tones that are typically
12 kHz or 16 kHz. The generator follows the same
sample rate for computation is 64 kHz instead of 8 kHz.
The equation is as follows:
where Full Scale VPK =0.5 V.
The pulse metering oscillator has a volume envelope
(linear ramp) on the on/off transitions of the oscillator.
The ramp is controlled by the value in the PMRAMP
RAM address, and the sinusoidal generator output is
multiplied by this volume before it is sent to the pulse
metering DAC. The volume value is incremented by the
value in PMRAMP at an 8 kHz rate. The volume will
ramp from 0 to 7FFF in increments of PMRAMP to allow
the value of PMRAMP to set the slope of the ramp. The
clip detector stops the ramp once the signal seen at the
transmit path exceeds the amplitude threshold set by
PMAMPTH, which provides an automatic gain control
(AGC) function to prevent the audio signal from clipping.
When the pulse metering signal is turned off, the
volume ramps down to 0 by decrementing according to
the
value
of
PMRAMP.
illustrates
the
functional blocks involved in pulse metering generation,
and
Table 38 presents the required register and RAM
locations that must be set to generate pulse metering
signals.
Coeff
2
f
64000 Hz
--------------------------
cos
=
PMFREQ
coeff
2
14
1
–
=
PMAMPL
1
4
--- 1coeff
–
1
coeff
+
------------------------
2
15
1
–
Desired V
PK
FullScale V
PK
----------------------------------------
=
Table 38. Register and RAM Locations Used for Pulse Metering Generation
Parameter
Register/RAM
Mnemonic
Register/RAM
Bits
Description/Range
(LSB Size)
Pulse Metering Frequency
Coefficient
PMFREQ
PMFREQ[15:3]
Sets oscillator frequency
Pulse Metering Amplitude
Coefficient
PMAMPL
PMAMPL[15:0]
Sets oscillator amplitude
Pulse Metering Attack/Decay
Ramp Rate
PMRAMP
PMRAMP[15:0]
0 to PMAMPL
(full amplitude)
Pulse Metering Active Timer
PMTALO/PMTAHI
PULSETA[15:0]
0 to 8.19 s (125 s)
Pulse Metering Inactive Timer
PMTILO/PMTIHI
PULSETI[15:0]
0 to 8.19 s (125 s)
Pulse Metering, Control
Interrupt
IRQVEC1, IRQEN1
PULSTAE,
PULSTIE,
PULSTAS,
PULSTIS
Interrupt Status and control
registers
Pulse Metering AGC
Amplitude Threshold
PMAMPTH
PMAMPTH[15:0]
0 to 500 mV
PM Waveform Present
PMCON
ENSYNC
Indicates signal present
PM Active Timer Enable
PMCON
TAEN
Enable/disable
PM Inactive Timer Enable
PMCON
TIEN
Enable/disable
Pulse Metering Enable
PMCON
PULSE1
Enable/disable