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Magic Blue Smoke
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    Magic Blue Smoke is a blog dedicated to discussing the challenges of low power ASIC Design

    I have worked in the VLSI industry for 14 years as a digital IC designer. My recent work has been focused on low-power challenges associated with multi-voltage/
    multi-supply designs. The goal of this blog is to open a free exchange of ideas with regards to low power. Please participate!

    - Godwin Maben

Most Commonly Used/Required UPF 1801 Constructs

Posted by Godwin Maben on March 2nd, 2012

 

Often times many designers keep asking one question ; Does all EDA tools support UPF2.0 Commands. In theory all the commands in UPF are referred to as IEEE 1801 commands and all the supported commands by the tool can be accessed through solvnet article  (https://solvnet.synopsys.com/retrieve/021264.html)

In my opinion in addition to the standard UPF commands used so far, there are around 4-5 new UPF constructs that are really useful to describe Power Intent more Precisely

      1.   Supply Sets and its related Commands :   Supply set is a collection of supply nets each of which serves a specific function (power, ground, nwell, pwell, deepnewell, and deeppwell) for the set.

     2.    Set_isolation with source/sink/diff_supply : This option to set_isolation command eliminates   the need to come up with the list of elements/ports/pins for isolation.

     3.    Set_port_attributes : Specifies information relevant to ports on the interface of the power domains. This information is used to determine isolation and guard requirements for the port.

     4.    Add_power_state : Adds state information to a supply set. It defines a state for a supply set and specifies the state of a specific supply net in the supply set for the defined state

     5.    Create_logic_port/create_logic_net/connect_logic_net : Some ports if referred only in UPF, can also be created in UPF , instead of manually creating these ports/nets/pins in RTL.

     6.    Query_* commands:

More details about these commands are available in the above mentioned solv-net article. Will  discuss about these commands with some example in my next post.

Posted in Uncategorized | 2 Comments »

Low Power Static Checks

Posted by Godwin Maben on August 12th, 2010

There seem to be some confusion about types of checks that need to be performed on a Low Power Design. In the Low Power static check world, following 3 types of analysis can be done on a design, in addition to various other checks.

(a) Critique Check

    Power State Table is Golden here and design structure is validated for correctness based on this table. For example “ Compare the ISO/LS elements present in the design with the ISO/
LS requirement inferred by analyzing the PST”

(b) Power Intent Check

Design is validated as per the Power Intent provided by user, here Power State Is ignored and mostly user written rules/policies are considered while checking, design structure is validated against user written policies  for correctness.

(c) Low Power Architecture Checks

Here design is analyzed for any architectural failures with respect to the requirement specified in the power intent. For example: Checking Power Up and Power Down Sequences of various power domains, checking for reachability of control and clock paths to the design.

Posted in low power general | 3 Comments »

Special cells on Feed Through Nets

Posted by Godwin Maben on August 10th, 2010

How do we minimize or reduce area in a Low Power Design, especially when it come down to using special cells such as isolation cells, level shifter cells…etc

Here is one scenario, where in “especially on a final sign-off netlist, can we get rid of these or will there be any electrical violations due to this”

 

image

Again this is after considering all the physical requirements, since these special cells are inserted very early in the design cycle, should we be visiting them again before signing off? Just a thought!

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Supply Set Usage

Posted by Godwin Maben on August 3rd, 2010

We are so much used to having explicit supply nets and ports in the design as well as in UPF,  its hard to visualize how a hardware logic designer would code the UPF using supply sets.   For those of you who are not familiar with supply sets, here is a quick preview of the same

Supply sets refers to collection of supply nets which substitutes a complete supply source, and can be used to power up an element of a design. Each of these supply nets provide a function such as power/ground/different well supplies….etc

There are predefined supply sets, which can be referred to by any supply set through a specific supply set handle such as primary/default_retention….etc.

Also any supply net explicitly created can be referred in any supply set and function of this supply net could be totally different in each supply sets where its referred. These supply sets can be referred easily by specifying chip_top/module_a/PD1.my_supply_set, which refers to supply set that’s available under the scope chip_top/module_a, belonging to power domain PD1.

One of the main advantage of supply set is that , initially it can be a place holder with just a function attribute ,which can be later modified/updated as and when more information is available.

for example:

Initially we can start with

create_supply_set my_supply_set –function {power} –function {ground}

later it can be updated to

create_supply_set my_supply_set –function {power VDD} –function {ground VSS} –update

Being involved with every phase of design cycle, I can imagine, how this can be mis-coded/mis-interpreted at different phase of design cycle. This could be a real nightmare to debug, if anything is either misinterpreted by the tool or mis-coded by designer.

will cover more on this topic later with examples.

Posted in low power general | 1 Comment »

Multi-Voltage/Power Gated design and LVS

Posted by Godwin Maben on August 2nd, 2010

 

Some interesting observation while running LVS on a power gated or a MV design. Here is quick preview on the problem description

 

image

As shown in the picture above if LVS is run on a Verilog netlist generated without bulk pin connections, bulk connections may not be correct from electrical perspective.

More on how typically designers handle this in my next post.

Posted in low power general | 1 Comment »

Clock Gating State Retention

Posted by Godwin Maben on June 3rd, 2010

Recently came across this request for clock gating retention latch. Here are some details on why these are required and what it means to the design.

Clock gating is the most common low power saving technique in use for a long time.  Latch based clock gating logic is typically used to avoid any glitches even during entry and exit from/to sleep mode.  In a power gated design , usually we stop the clock at in-active phase before retaining states and entering sleep mode and same for wake up mode.  Here one of the main challenge is the validity of the enable signal at wakeup, which is typically provided by the restored states in the registers propagating through cloud of logic to the clock gating latches, which stay open during in-active phase.

In a power gated design, where retention registers are not used, this propagation mechanism may not work and we might have to use some kind of state retention for the clock gating latches, which retains the clock gating state when powered down. In one of the design, this is incorporated using retention latch(similar to retention flop) inside the latch based clock gating cell.

It may not be required to use these special cells if retention registers happen to exist in the design, which controls the clock enable signal state. Its also not required if some logic is built in to ensure controllability of the clock during inactive phase while entering and exiting the hibernation mode.

Posted in low power general | 5 Comments »

Interesting Low Power Sessions at SNUG Sanjose 2010

Posted by Godwin Maben on March 26th, 2010

 

Monday, March 29, 2010
11:00 AM – 12:30 PM

MA1 Tutorial : Implementation

PrimeRail and IC Compiler: In-Design Rail Analysis for Faster Power Network Design Closure

MA2 User – Constraints and Power Challenges in Verification : Verification

Formal Methods to Verify the Power Manager for an Embedded Multiprocessor Cluster

Monday, March 29, 2010
1:45 PM – 3:15 PM

MB3 Tutorial : AMS

HSIMplus CircuitCheck for Low Power Transistor Level Error Detection

Power Correlation with Silicon – A PrimeTime PX Evaluation

Tuesday, March 30, 2010
10:15 AM – 11:45 AM

TA1 User –

Reusable UPF for Multi-Voltage Design & Handling Analog Macros in Power Subsystem

TA2 Tutorial : Verification

Low Power Verification

Tuesday, March 30, 2010
1:00 PM – 2:30 PM

TB1 User – Implementation

LeSa Lowers Leakage

Tuesday, March 30, 2010
2:50 PM – 4:50 PM

Clock Power Reduction-Analysis Metrics and Power Reduction Techniques

Low Power Multi-Voltage Design Implementation Methodology using the IEEE 1801 (UPF) Standard

Wednesday, March 31, 2010
10:15 AM – 11:45 AM

WA1 Tutorial : Implementation

Energy Efficient Processor Implementation with Synopsys’ Eclypse Low Power Solution

WB5 Tutorial : IP

Extreme Low-Power Datapath Design with DesignWare minPower Components

Posted in low power general | 1 Comment »

Architectural Error Example

Posted by Godwin Maben on March 18th, 2010

Again sorry for the long break in writing, wish I could  write at least one post per week,

Recently based on some silicon debugging, we realized verification did not cover some aspect of the power down function that lead to chip failure. Later realized that, this is  being mentioned some where in VMM LP manual, thought of sharing this here.

MOS transistors are dependent on their gate-source voltage difference to determine whether they are “on” or “off”, also known as “conducting” or “non-conducting”. This mechanism is used in power gating. The Gate voltage is such that the transistor is non-conducting. This is done by issuing logic “1” to the Gate, which charges it up to the Vdd level of the driver. In general, this is the same voltage level as the power switch. The gate-source difference kicks in to turn the transistor off. However, when the Vdd of the power gating signal’s driver “dips”, the off island makes an “on” excursion and come back. On the other side, the power gate can also become more resistive. Similarly, an on island with a footer can suddenly become more resistive or make an on excursion.

This phenomena is quite dangerous, as it will lead to a current spike and a further collapse of rails. Although the profile looks like a power integrity issue and may well be caused by bad implementation of the power grid, frequently the cause is an over-scheduling of power state changes instead of staggering them in time. This in turn causes fluctuations in the power supply. The Power Management Unit must take the stability of the power supplies into account before moving rails in voltage value.

Will talk on the verification plan for this scenario in my next post.

Posted in low power general | 1 Comment »

Isolation Cell Usage Tips Continued

Posted by Godwin Maben on January 28th, 2010

There were many questions on why output isolation is preferred over input isolation logic, sorry could not get time to respond to all the queries related to this. Here is my view point on this

Output signal isolation method is usually a preferred choice than the input isolation method as former leads to fewer isolation cells and gives better control over how the enable signal can be  propagated to the isolation logic. 

Input isolation will be a better choice if there are very few independent domains. In such situation isolation enable signal management is very straight-forward.  Here number of power domain is not the main component, its the sequence in which they are powered down and up. In this case isolation cell is only required when the power island is active. During the power down period, the power island is no longer functional and no power is supplied to the island. Therefore, neither the floating inputs nor the input transient state matters to the power island, which is overall good from total power perspective.

For this method, probably regular standard cells (NAND and NOR) can be used as isolation cells rather than requiring a custom cell.

Posted in low power general | 1 Comment »

Generating Partial UPF Automatically

Posted by Godwin Maben on December 17th, 2009

Sorry guys, got tied up with many projects and could not blog for almost 4 weeks.

I know we spend so much time in writing power intent of a design and validating whether its correct or not. In that process on a recent project, I did some analysis on how some of the intent generation can be pseudo automated.

Used the MV static checker, MVRC to auto generate some policies based on the xover analysis and it helped quite a bit and was amazed at how fast I was able to generate these constraints from MVRC.

For example, on one of the design, finding out what need to be isolated and excluded from isolation was not a trivial task due to multi-fanout nature of the ports/pins. Looked at the xover analysis within MVRC and used this feature to auto-generate some of the isolation policies. It was less than 30 lines of TCL code within MVRC, which made my life easier in generating some part of power intent.

just an example on what I did within MVRC

set f1 [open ${source_island}_${dest_island}.xover w]
set xs [get_crossovers -source $source_island -dest $dest_island]
foreach x $xs {
    set src_port  [get_crossover_info -object $x -boundary_source]
    set src_port [regsub -all {{} $src_port {}]
    set src_port [regsub -all {}} $src_port {}]
    lappend src_ports_list $src_port }}
}

if {$src_ports_list!=""} {puts $f1 "set_isolation ${source_island}_${dest_island}_ISO -domain ${source_island} -isolation_power_net $domain_vdd_net -isolation_ground_net VSS -clamp_value 0 -elements "$src_ports_list""}

This may not be complete, but idea is very similar to one given above.

We can debate on whether should a sign-off MV tool be used for this or not?

Happy Holidays.

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