Si3220/Si3225
The output of the low-pass filter is compared to the As an example of ground key detection, suppose that
programmable threshold, LONGHITH. Hysteresis is the Si3220/25 has been programmed with the current
enabled by programming
a
second threshold, values shown in Table 25.
LONGLOTH, to detect when the ground key is released.
The threshold comparator output feeds a programmable
debounce filter. The output of the debounce filter
remains in its present state unless the input remains in
the opposite state for the entire period of time
programmed by the loop closure debounce interval,
LONGDBI. If the debounce interval is satisfied, the
LONGHI bit is set to indicate that a valid ground key
event has occurred.
When the Si3220/25 detects a ground key event, the
linefeed automatically transitions from the TIP-OPEN
(or RING-OPEN) state to the FORWARD-ACTIVE (or
REVERSE-ACTIVE) state. However, this automatic
state transition is triggered by the LCR bit becoming
active (i.e., =1), and not by the LONGHI bit.
Table 25. Settings for Ground Key Example
ILIM
21 mA
14 mA
10 mA
7 mA
LCROFFHK
LCRONHK
LONGHITH
LONGLOTH
5 mA
With the settings of Table 25, the behavior of I
LONG
,
LOOP
I
, LCR, LONGHI, and CMHIGH is as shown in
Table 26. The entries under “Loop State” indicate the
condition of the loop, as determined by the equipment
terminating the loop. The entries under “LINEFEED
Setting” indicate the state initially selected by the host
CPU (e.g., TIP-OPEN) and the automatic transition to
the FORWARD-ACTIVE state due to a ground key
event (when RING is connected to GND). The transition
from state #2 to state #3 in Table 26 is the automatic
transition from TIP-OPEN to FWD-ACTIVE in response
to LCR = 1.
While I
is used to generate the LONGHI status bit,
LONG
a transition from TIP-OPEN to the FORWARD-ACTIVE
state (or from the RING-OPEN to the REVERSE-
ACTIVE state) occurs when the RING terminal (or TIP
terminal) is grounded and is based on the LCR bit and
implicitly on exceeding the LCROFFHK threshold.
Table 26. State Transitions During Ground Key Detection
#
1
2
3
4
5
Loop
State
LINEFEED
State
I
I
LCR
LONGHI
CMHIGH
LOOP
LONG
(mA)
(mA)
LOOP
OPEN
LFS = 3
(TIP-OPEN)
0
0
0
1
1
1
0
0
1
1
0
0
0
0
1
0
0
RING-GND
LFS = 3
(TIP-OPEN)
22
22
21
0
–11
–11
0
RING-GND
LFS = 1
(FWD-ACTIVE)
LOOP CLO-
SURE
LFS = 1
(FWD-ACTIVE)
LOOP
OPEN
LFS = 1
(FWD-ACTIVE)
0
44
Rev. 1.0
SILICONIMAGE [ Silicon image ]
