cgc setup and hold checks

Why clock gaters:
    Clock gating is the well known technique to reduce dynamic power consumption.

i) If done properly: Saving the dynamic power of flops
ii) if not done properly: Glitches may lead to meta-stability issues.

Adv of using CGC:

1. Its worth mentioning that clock gating does not have much significance on individual flops. But imagine a scenario of writing a 64bit register based on an enable. The power dissipated by all 64 flops can be greatly reduced by using a single clock gate cell common to all 64 flops, which amounts to a very significant power saving.

2. On a side note, clock gating can greatly help save area in the scenario described by getting rid of the large mux'ed feedback path which would otherwise be necessary to meet logic requirements.

Drawback of Simple AND as CGC:

Think of a clock gate as "simple and" with an enable gating the clock. The reason you do this is to stop unnecessary toggles on the clock pin of flops. Even if the output doesn't toggle, the internal flop circuitry dissipates unnecessary power. Power saving can be achieved by simply gating the clock with an enable.

1. Here's the catch, if the enable is asynchronous to the clock and gates the clock during its active phase, you can end up with a clipped clock, which effects the duty cycle.

2. This scenario can lead to timing violations on the flop and downstream logic. If clock clipping happens very close to the active edge of the clock, there might even be a clock width violation.

How to mitigate this problem:

To prevent violations, its best to sync the enable signal with respect to the clock it is gating. This is achieved by using a latch which is transparent only during the inactive phase of the clock. For example, to gate the clock to its low state, use an active low latch to sync the enable and gate the clock with the sync'ed version as described by the code below:

Code:

always_latch
   if(!clk) en_syn <= en;

assign gated_clk = clk & en_syn;

always_ff @(posedge gated_clk) ....

Many ASIC vendors, supply clock-gate cells. Which is internally a combination of the latch and the gate described above. These gates can be instantiated in the design or better yet inserted during synthesis (if the RTL is coded following the tool requirements of cg insertions).

The clock_gating_setup_time is essentially the setup requirement of the latch and is available for STA delay annotation in the dbs read by PT.

How to model:

-ve latch followed by AND gate:
En is input to -ve latch, CLK is the clock of the -ve latch
Output of latch and CLK are going to AND
Output of AND is going to set of flops

Let us assume EN pin is gating clock pin (CLK)

The gating check is performed on pins (EN) that gate a clock signal.

When Clock gating Checks required:

Clock gating checks need to be done where the clock is gated with a data or enable signal. The basic idea here is to check whether the enable signal is toggling only when the clock is in its inactive phase. If enable toggles in the clock's active phase, it will result in glitch in the gate output clock.

AND/NAND gate is inactive when clock is in the LOW phase. i.e., at this time gate output will not depen on the other inputs. So AND/NAND gate is having a HIGH clock gating check.  Similarly, for OR/NOT gate is inactive when clock is in the HIGH phase, so it has LOW clock gating check.

Setup violation:
         Similar to normal flop, here, D input is EN and clock is CLK. And we are checking setup violation of Latch.

The clock gating setup check is used to   ensure   the   controlling   data signals   are stable before the clock is active. i.e., EN signal is stable enough before the -ve level (as it is -ve latch) of clock signal.

This check is performed on combinational gates through which the clock signals are  propagated.

The   arrival   time   of   the   leading   edge   of the clock pin is checked against both levels of any data signals gating the clock.

If clock gating setup failure leads to:  A clock   gating   setup failure can cause either a glitch at the leading edge of the clock pulse, or a clipped clock pulse.

How to fix setup violations in PrimeTime:

          Let us assume setup violation is occurring in the below path

 start point:  FF1 clock

 end point: EN input of CGC

Fix1:

1. Reduce data path delay:

2. Clock push:

           - adding delay in the clock path (inserting buffers at CLK input pin of CGC)

When this clock push can be done?

      As a general phenomenon, clock push can be done when there is setup margin at the next stage.
As output of CGC is the start point for setup of flops, so our clock push should not result in setup violtaion at the startpoints of those timing paths.


##Getting slack of next stage flops (from endpoint of current path under fix) as gated clock is the start point for those flops:
set f [open slc_cg.txt w ]
set fo [get_cells [all_fanout -from   CGC_cell/clk -flat -endpoints_only -only_cells]]

foreach_in_collection fo1 $fo {
set slack [get_attribute [get_timing_path -from [get_attribute $fo1 full_name ]/clk -exclude [get_ports *] ] slack]
puts $f "[get_attribute $fo1 full_name] $slack "
}
close $f

ECO:

insert_buffer  path_till_CGC/CGC_cell/clk_in          clk_buffer_1x_drive

Hold check:
The clock gating hold check is used to ensure that the controlling data signals   are   stable while the clock is active. 

The arrival time of the trailing edge of the clock pin is checked against both   levels   of   any
data signal gating the clipped clock pulse.

References:
http://tech.tdzire.com/clock-gating-checks-and-clock-gating-cell/


How to define CGC constraints:

EXAMPLES
         The   following example specifies a setup time of 0.2 and a hold time of 0.4 for all gates in the clock network of clock CK1.

                 pt_shell> set_clock_gating_check -setup 0.2 -hold 0.4 [get_clocks CK1]

         The following example specifies a setup time of 0.5 on the gate and1.

                 pt_shell> set_clock_gating_check -setup 0.5 [get_cells and1]

set_clock_gating_check

NAME          set_clock_gating_check                     Specifies   the   value   of   setup   and hold time for clock gating                     checks. SYNTAX          string set_clock_gating_check                     [-setup setup_value]                     [-hold hold_value]                     [-rise | -fall]                     [-high | -low]                     [object_list]          float       setup_value          float       hold_value          list object_list ARGUMENTS          -setup setup_value                     Specifies the clock gating setup time.   The default is 0.0.          -hold hold_value                     Specifies the clock gating hold time.   The default is 0.0.          -rise   Indicates that only rising delays are   to   be   constrained.    By                     default,   if   neither -rise nor -fall are specified, both rising                     and falling delays are constrained.          -fall   Indicates that only falling delays are to   be   constrained.    By                     default,   if   neither -rise nor -fall are specified, both rising                     and falling delays are constrained.          -high   Indicate that the check is to be performed on the high level   of                     the   clock.   By default, PrimeTime determines whether to use the                     high or low level of the clock using information from the cell's                     logic.   That   is,   for AND and NAND gates PrimeTime performs the                     check on the high level; for OR and NOR gates, on the low level.                     For some complex cells (for example, MUX, OR-AND) PrimeTime can-                     not determine which to use, and does not perform   checks   unless                     you   specify   either -high or -low.   If the user-specified value                     differs from that derived by PrimeTime, the user-specified value                     takes precedence, and a warning message is issued.   Unlike setup                     or hold time, this option sets the attribute only on the   speci-                     fied   pin   or cell and does not affect the transitive fanout pin                     or cell.   If you specify -high or -low   you   must   also   specify                     object_list;   in that case, object_list must not contain a clock                     or a port.          -low    Indicates that the check is to be performed on the low level   of                     the   clock.   By default, PrimeTime determines whether to use the                     high or low clock level using information from the cell's logic.                     That   is, for AND and NAND gates PrimeTime performs the check on                     the high level; for OR and NOR gates, on the low edge.   For some                     complex cells (for example, MUX, OR-AND) PrimeTime cannot deter-                     mine which to use, and does not perform checks unless you   spec-                     ify   either   -high or -low.   If the user-specified value differs                     from that derived by PrimeTime, the user-specified   value   takes                     precedence,   and   a   warning message is issued.   Unlike setup or                     hold time, this option sets the attribute only on the   specified                     pin   or   cell   and   does not affect the transitive fanout pin or                     cell.   If you specify   -high   or   -low   you   must   also   specify                     object_list;   in that case, object_list must not contain a clock                     or a port.          object_list                     Specifies a list of objects in the current design for which   the                     clock   gating check is to be applied. The objects can be clocks,                     ports, pins, or cells. If a cell is specified, all input pins of                     that   cell   are affected.   If a pin, cell, or port is specified,                     all gates in the transitive fanout are affected.   If a clock   is                     specified, the clock gating check is applied to all gating gates                     driven by that clock.    If you specify -high or   -low   you   must                     also   specify   object_list;   in   that case, object_list must not                     contain a clock or a port.   By default, if   object_list   is   not                     specified,   the   clock   gating   check   is applied to the current                     design. DESCRIPTION          The set_clock_gating_check command specifies a setup or hold time clock          gating   check to be used for clocks, ports, pins, or cells.   The gating          check is performed on pins that gate a clock signal. Removing CGC checks: remove_clock_gating_check Captures clock-gating checks. SYNTAX string remove_clock_gating_check [-setup] [-hold] [-rise] [-fall] [-high | -low] [object_list] list object_list ARGUMENTS -setup Indicates the removal of the clock-gating constraint on the setup time only. If you do not specify either the -setup or -hold option, both setup and hold constraints are removed. -hold Indicates the removal of the clock-gating constraint on the hold time only. If you do not specify either the -setup or -hold option, both setup and hold constraints are removed. -rise Indicates the removal of the clock-gating constaint on the rising delays only. If you do not specify either the -rise or -fall option, constraints on both rising and falling delays are removed. -fall Indicates the removal of the clock-gating constaint on the falling delays only. If you do not specify either the -rise nor -fall option, constraints on both rising and falling delays are removed. -high Remove the high specification from the obejct list, previously set up by set_clock_gating_check command. This option has to be either high or low.. -low Remove the low specification from the obejct list, previously set up by set_clock_gating_check command. This option has to be either high or low. object_list Specifies a list of objects in the current design for which to remove the clock gating check. The objects can be clocks, ports, pins, or cells. If you specify a cell, all input pins of that cell are affected. If you do not specify any objects, the clock-gating check is removed from the current design. DESCRIPTION This command is available only if you invoke the pt_shell with the -constraints option. The remove_clock_gating_check command removes clock gating checks for design objects set by set_clock_gating_check. EXAMPLES The following example removes the setup requirement (for rising and falling delays) on all gates in the clock network involved with clock CK1 path. ptc_shell> remove_clock_gating_check -setup [get_clocks CK1] The following example removes the hold requirement on the rising delay of gate and1. ptc_shell> remove_clock_gating_check -hold -rise [get_cells and1] An alternative way to remove information set by set_clock_gating_check is to use the reset_design command.