VSC3138
Datasheet
F
UNCTIONAL
D
ESCRIPTIONS
Power-On RESET
The VSC3138 device provides a power-on RESET function, which ensures that the matrix is fully disconnected and
powered off. When the device is powered up, the switch draws additional power as each output is programmed. To avoid
an accidental power down, care must be taken to keep power supply excursions above 2.2 VDC by providing adequate
supply decoupling.
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Software Power-Down
Unused outputs may be disabled using the software power-down feature. This is accomplished by programming each
unused output with the power-down code (7FF’h). Programming a valid input address will reactivate the channel. Any
changes in power programming should be executed only as part of an initialization sequence. This will guard against the
effects of switching transients that might result from suddenly changing the power supply current.
CONFIG
The functionality of the CONFIG pin is duplicated in bit 1 of the Switch Control register (address 281’h). Writing to this
bit has the same effect as driving the external pin to the same value. This bit is multiplexed with the external pin through
logic and initializes to the state that uses the external pin. The states are therefore mutually exclusive. To pulse CONFIG
using the register pin, write an active signal and then an inactive signal to register bit 1 of the Switch Control register.
Holding the CONFIG pin “HIGH,” or setting the CONFIG bit to a 1 causes the programming to be active upon asserting
LOAD.
To prevent accidental reprogramming of the switch interconnects, the VSC3138 programming interface provides the
option of disabling this feature through software. Setting the CONFIG Lockout bit [2] in the Switch Setup register
(address 280’h) to a 1 will inhibit the use of both the external CONFIG pin and the CONFIG bit [1] in the Switch Control
register from reprogramming the interconnects.
On power-up, the CONFIG Lockout bit [2] of the Switch Setup register is 0, and consequently, CONFIG is enabled.
Staging Register Readback
When the CONFIG control is used to accumulate a number of program steps and execute them simultaneously, it is
possible to verify the pending switch programming prior to asserting the CONFIG control. Setting the Staging Register
Readback bit [4] of the Switch Configuration register (address 281’h) to 1 and reading the programming information
from the switch will return configuration information about the pending switch programming, rather than the current
connections.
If CONFIG is set to a value of 1, all programming propagates directly to the switch matrix, and the information returned
during a readback will be the same. In this case, the result is the same, regardless of the setting of the Staging Register
Readback bit [4] of the Switch Configuration register.
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G52414 Revision 4.2
March 8, 2006
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