Isolation(upf)

Isolation is used to clamp the output value of power gated block to fixed value during power-down state. UPF provides set_isolation and set_isolation_control constructs to add isolation strategy in the design.

Syntax for isolation cell :

set_isolation isolation_name
-domain domain_name
<-isolation_power_net net_name -isolation_ground_net net_name-isolation_power_net net_name -isolation_ground_net net_name
-no_isolation>
-clamp_value <01latchz>
-applies_to
[latch : The value of the non-isolated port when the isolation signal becomes active.]

Syntax to specify the control signals for isolation strategy.

set_isolation_control isolation_name
-domain domain_name
-isolation_signal signal_name
[-isolation_sense <highlow> ]
[-location ]

[location : Specifies the location for isolation cell to be placed in the logic hierarchy]
A simulation model for isolation cell :

always @( iso_enable, non_isolated )
begin
if (iso_enable == isolation_sense )
isolated = clamp_value;
else
isolated = non-isolated; // Output of the power gated block.
end

Every time the control signal changes value, the new value is compared with its corresponding isolation sense value.If the control signal is the same as the sense value, the specified clamp value is driven as the isolated signal; otherwise, the non-isolated signal propagates as the isolated value.If power supply to the corresponding isolation elements is turned off or the enable signal is X or Z, the isolated signal is driven to X.

Example :

set_isolation iso3
–domain PDgreen
–isolation_power_net Vbu ----> Back up power net
–clamp_value 0 ----> Clamp isolation output to 0
–applies_to outputs ----> Isolates the outputs

set_isolation_control iso3
–domain PDgreen
–isolation_signal CPU_iso ----> Isolation enable
–isolation_sense low
–location self ----> Iso cell desired location

When isolation signal CPU_iso goes low, isolation cell becomes active clamps the outputs of iso3 to 0 else isolation cell became inactive i.e it propagates non-isolated value as isolated value.