CPC7695 Datasheet, PDF (16/23 Pages) Clare, Inc. – Line Card Access Switch

CPC7695
To facilitate hot plug insertion and system power up
state control, the LATCH pin has an integrated weak
pull up resistor to the V_DD power rail that will hold a
non-driven LATCH pin at a logic high state. This
enables board designers to use the CPC7695 with
FPGAs and other devices that provide high
impedance outputs during power up and logic
configuration. The weak pull up allows a fan out of up
to 32 when the system’s LATCH control driver has a
logic low minimum sink capability of 4mA.
2.2.2 Hot Plug and Power Up Circuit Design
Considerations
There are six possible start up scenarios that can
occur during power up. They are:
1. All inputs defined at power up & LATCH = 0
2. All inputs defined at power up & LATCH = 1
3. All inputs defined at power up & LATCH = Z
4. All inputs not defined at power up & LATCH = 0
5. All inputs not defined at power up & LATCH = 1
6. All inputs not defined at power up & LATCH = Z
Under all of the start up situations listed above the
CPC7695 will hold all of it’s switches in the all-off state
during power up. When V_DD requirements have been
satisfied the LCAS will complete it’s start up procedure
in one of three conditions.
For start up scenario 1 the CPC7695 will transition
from the all-off state to the state defined by the inputs
when V_DD is valid.
For start up scenarios 2, 3, 5, and 6 the CPC7695 will
power up in the all-off state and remain there until the
LATCH pin is pulled low. This allows for an indefinite
all-off state for boards inserted into a powered system
but are not configured for service or boards that need
to wait for other devices to be configured first.
Start up scenario 4 will start up with all switches in the
all-off state but upon the acceptance of a valid V_DD the
LCAS will revert to any one of the legitimate states
listed in the truth tables and there after may randomly
change states based on input pin leakage currents
and loading. Because the LCAS state after power up
can not be predicted with this start up condition it
should never be utilized.
On designs that do not wish to individually control the
LATCH pins of multi-port cards it is possible to bus
many (or all) of the LATCH pins together to create a
single board level input enable control.
2.3 Switch Logic
2.3.1 Start-up
The CPC7695 uses smart logic to monitor the V_DD
supply. Any time the V_DD is below an internally set
threshold, the smart logic places the control logic to
the all-off state. An internal pullup on the LATCH pin
locks the CPC7695 in the all-off state following
start-up until the LATCH pin is pulled down to a logic
low. Prior to the assertion of a logic low at the LATCH
pin, the switch control inputs must be properly
conditioned.
2.3.2 Switch Timing
The CPC7695 provides, when switching from the
ringing state to the talk state, the ability to control the
release timing of the ringing switches SW3 and SW4
relative to the state of the break switches SW1 and
SW2 using simple TTL logic-level inputs. The two
available techniques are referred to as
make-before-break and break-before-make operation.
When the switch contacts of SW1 and SW2 are closed
(made) before the ringing switch contacts of SW3 and
SW4 are opened (broken), this is referred to as
make-before-break operation. Break-before-make
operation occurs when the ringing contacts of SW3
and SW4 are opened (broken) before the switch
contacts of SW1 and SW2 are closed (made). With
the CPC7695, make-before-break and
break-before-make operations can easily be
accomplished by applying the proper sequence of
logic-level inputs to the device.
The logic sequences for either mode of operation are
given in “Make-Before-Break Operation Logic Table
(Ringing to Talk Transition)” on page 17,
“Break-Before-Make Operation Logic Table (Ringing to Talk
Transition)” on page 17 and “Alternate Break-Before-Make
Operation Logic Table (Ringing to Talk Transition)” on
page 18. Logic states and explanations are shown in
“Truth Tables” on page 14.
2.3.3 Make-Before-Break Operation
To use make-before-break operation, change the logic
inputs from the ringing state directly to the talk state.
Application of the talk state opens the ringing return
switch, SW3, as the break switches SW1 and SW2
close. The ringing switch, SW4, remains closed until
the next zero-crossing of the ringing current. While in
the make-before-break state, ringing potentials in
excess of the CPC7695 protection circuitry thresholds
will be diverted away from the SLIC.
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PRELIMINARY
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