Writing Isolation Strategy in UPF _ VLSI COMMUNITY

7/26/2019 Writing Isolation Strategy in UPF _ VLSI COMMUNITY

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6/10/2016 Writing Isolation Strategy in UPF | VLSI COMMUNITY

http://www.vlsicommunity.com/2014/07/writing-isolation-strategy-in-upf/

hhharishhh

Jul 27, 2014

Low

Power

Design

0

set_isolationset_isolation_control

map_isolation_cell

Isolation

strategy is required to instruct

implementation tool to insert

isolation cells where a less-ON signal

drives a more-ON logic. Though tools

can infer this power domain relations

from the PST state table, the

implementation tools doesnt

implement them without an isolation

strategy specified. Also the PST

states do not provide the tools

sufficient information available for

inserting isolation cells like the clamp

value, supplies, and so many other

aspects. However the designer can

easily report the violationswhen the

isolation strategy you have defined is

not in line with the PST. An isolation

strategy can be written by using

following commands,

set_isolation

command in UPF defines the name of

the Isolation strategy along with the

signals to be isolated and their clamp
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value. Clamp value is either 0 or 1

which determines the type of

isolation cell to be used.

Example,

[blockquote align=center]

set_isolation ISO_VDDA_TO_VDDB

-domain VDDA

-source VDDA_SUPPLY

-diff_supply_only TRUE

-isolation_power_net VDDB

-isolation_ground_net VSS

-name_suffix

ISO_LOW_VDDA_TO_VDDB

-clamp_value 0

[/blockquote][space_20]

In the example we defined a strategy

named ISO_VDDA_TO_VDDB. As the

name suggests this strategy is

defined for the signals that cross

from the domain VDDA to VDDB. Theoption -domain specifies the name

of the domain which is to be isolated

hence VDDA. VDDA_SUPPLY is the

supply set of the power domain

VDDA. By specifying the source

option you indicate that the isolation


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strategy applies to pnly signal nets

whose driver is powered by the

supply specified in the -source

option. If you need to apply isolation

only when the supplies of the source

and the sink are different then weneed to provide TRUE for the option

diff_supply_only. The supply to be

used for the isolation cell can be

either the destination domain supply

or the always on supply. If the

destination domain is also an

ON/OFF domain then the destinationsupply is better than the always on

supply because we could save some

power when the destination domain

is in OFF state. The option -

name_suffix helps the designer to

specify the suffix to the instance

name of isolation cell implementedby the tool which helps for easy

identification and debug. Finally the

clamp_value which determines the

type of isolation cell whether its

clamp high or clamp low type.

There are many ways the designer

can customize the strategy lets see

few of them,

1. Designer can specify if the isolation

strategy to be applied for Inputs,

outputs or both. This can be achieved

by using -applies_to option. In the


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following example the strategy is

applied on only output ports of the

domain,



-domain VDDA

-source VDDA_SUPPLY




-name_suffix


-clamp_value 0

-applies_to outputs


2. In some scenarios, the designer

may specify only few ports of a

domain to be isolated. In that case

the option -elements can be

helpful. The elements option takes alist of port with absolute hierarchy

path and applies isolation only to

those ports. In the example bellow,

the strategy is applied only to ports

fmg/write_req_out and

fmg/read_ack_out


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-domain VDDA

-source VDDA_SUPPLY




-name_suffix


-clamp_value 0

-elements { fmg/write_req_out

fmg/read_ack_out}


3. Control signals like clocks should

not be routed through relatively less

ON domain than the sinks because

the isolation cell when enabled can

block its propagation. Designer can

explicitly specify tool not to isolate

specific signals using -no_isolation

option


set_isolation

NOISO_VDDA_TO_VDDB

-domain VDDA


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-no_isolation

-elements {fmg/core_clk}


4. Isolation cell can be forced on a

signal by using -force_isolation

option. When the option force

isolation is used the tool implements

the isolation even when its not

required or redundant. Also the tool

do not optimize away this isolation

even when there is no change to the

design functionality.


set_isolation

FORCE_ISO_VDDA_TO_VDDB

-domain VDDA

-force_isolation

-elements {gmf/php_ack_out}


set_isolation_control command is

used to specify the isolation enable

signal to the already defined isolation

strategy. The isolation enable signal

controls the enabling and disabling of

the isolation cell.

Example,


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set_isolation_control ISO_VDDA_TO_VDDB

-domain VDDA

-isolation_signal

pwr_ctrlr/iso_enable_vdda_to_vddb

-location parent

-isolation_sense low


In the example we defined theisolation control

signal pwr_ctrlr/iso_enable_vdda_to_vddb

to the already defined isolation

strategy ISO_VDDA_TO_VDDB. The

option -isolation_signal can take a

port or pin or a net as the isolation

control. The option -domain specifies the domain to which the

isolation strategy applies. If you are

wondering if this is redundant since

we already specified the domain in

the strategy definition, yes it is. That

is how the IEEE standard 1801-2009

defined isolation strategy. However inIEEE 1801-2013 standard the

command set_isolation_control is

deprecated by enhancing the

command set_isolation. As of today

synopsys low power tools still

support set_isolation_control


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command.

Isolation sense together with the

clamp value determines the type of

isolation cell to be used for the

strategy implementation. Isolation

sense is the value (either high or low)

of the isolation control signal which

enables the isolation cell to isolate

the signal.

AND style: When the isolation sense

is low and the clamp value is 0 then

the AND gate can be used as anisolation cell. Hence an AND style

isolation cell is used for the

implementation.

OR style:When the isolation sense is

high and the clamp value is 1 then an

OR gate can be used as an isolation

cell. Hence an OR style isolation cell

is used for the implementation.

The location of the isolation cell

placement (with respect to the power

domains) can be controlled by the

designer using -location. Allowed

values for the option -location in

synopsys tools are self, parent and

fanout.

Self: Tool implements the isolation

cell in power domain that is specified

in the -domain option of the isolation


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strategy.

Parent: The tool places the isolation

cell in the parent hierarchy.

fanout: In this case the isolation cell

in the fanout logic domain. This

option is more useful when the signal

fansout to more than one power

domain so that a separate isolation

cell is placed in each of the

destination power domain.


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map_isolation_cell:This command is

used to specify the library cell to be

used for an isolation strategy. The

cells specified in the

map_isolation_cell command must

match with the clamp value and theisolation sense.


map_isolation_cell

ISO_VDDA_TO_VDDB

-lib_cells {isolowsrc2hr isolowsrc3ur}


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