Si3220/25 Si3200/02
46
Rev. 1.3
Not
Recommended
fo
ew
D
esi
gn
s
3.11. Ground Key Detection
Ground key detection detects an alerting signal from the
terminal equipment during the tip open or ring open
linefeed states. The functional blocks required to
implement a ground key detector are shown in
The primary input to the system is the longitudinal
current sense value provided by the voltage/current/
power monitoring circuitry and reported in the ILONG
RAM address. The ILONG value is produced in the ISP
provided the LFS bits in the linefeed register indicate
the device is in the tip open or ring open state.
The longitudinal current (ILONG) is computed as shown
for the transistor references used in the equation (Q1,
Q2,
Q5,
and
Q6—note
that
the
Si3200/2
has
corresponding MOS transistors). The same ILONG
equation applies to the discrete bipolar linefeed as well
as the Si3200/2 linefeed device.
The output of the ISP (ILONG) is the input to a
programmable, digital low-pass filter, which removes
unwanted ac signal components before threshold
detection.
The low-pass filter coefficient is calculated using the
following equation and is entered into the LONGLPF
RAM location:
Where f = the desired cutoff frequency of the filter.
The programmable range of the filter is from 0h (blocks
all signals) to 4000h (unfiltered). A typical value of 10
(0A10h) is sufficient to filter out any unwanted ac
artifacts while allowing the dc information to pass
through the filter.
The output of the low-pass filter is compared to the
programmable threshold, LONGHITH. Hysteresis is
enabled
by
programming
a
second
threshold,
LONGLOTH, to detect when the ground key is released.
The threshold comparator output feeds a programmable
debounce filter.
Table 25. Register and RAM Locations Used for Loop Closure Detection
Parameter
Register/RAM
Mnemonic
Register/RAM Bits
Programmable
Range
LSB Size
Effective
Resolution
Loop Closure Interrupt
Pending
IRQVEC2
LOOPS
Yes/No
N/A
Loop Closure Interrupt Enable
IRQEN2
LOOPE
Yes/No
N/A
Linefeed Shadow
LINEFEED
LFS[2:0]
Monitor only
N/A
Loop Closure Detect Status
LCRRTP
LCR
Monitor only
N/A
Loop Closure Detect Debounce
Interval
LCRDBI
LCRDBI[15:0]
0 to 40.96 s
1.25 ms
Loop Current Sense
ILOOP
ILOOP[15:0]
50.54 to
101.09 mA
3.097 A
500 A1
Loop Closure Threshold
(on-hook to off-hook)
LCROFFHK
LCROFFHK[15:0]
0 to 101.09 mA2
3.097 A
396.4 A
Loop Closure Threshold
(off-hook to on-hook)
LCRONHK
LCRONHK[15:0]
0 to 101.09 mA2
3.097 A
396.4 A
Loop Closure Filter Coefficient
LCRLPF
LCRLPF[15:3]
0 to 4000h
N/A
Loop Closure Mask Interval
LCRMASK
LCRMASK[15:0]
0 to 40.96s
1.25 ms
Notes:
1. The effective ILOOP resolution is approximately 500 A.
2. The usable range for LCRONHK and LCROFFHK is limited to 61 mA. Entering a value > 61 mA will disable threshold
detection.
I
LONG
I
Q1
I
Q6
–
I
Q5
–
I
Q2
+
2
----------------------------------------------------
=
LONGLPF
2
f 4096
800
---------------------------------
2
3
=