Formal Definition
Timing Check Tasks are for verification of timing properties of designs and for reporting timing violations.
Simplified Syntax
$setup (data_event, reference_event, limit[, notifier]) ;
$skew (reference_event, data_event, limit[,notifier]) ;
$hold (reference_event, data_event, limit[,notifier]) ;
$recovery (reference_event, data_event, limit, [notifier]) ;
$setuphold (reference_event, data_event, setup_limit, hold_limit, [notifier]) ;
$width (reference_event, limit, threshold [,notifier]) ;
$period (reference_event, limit[,notifier]) ;
$nochange (reference_event, data_event, start_edge_offset, end_edge_offset [,notifier]) ;
Description
Timing check tasks are invoked every time critical events occur within given time limits. See the table below with descriptions of all arguments:
| Argument | Description | Type |
| Reference_event | The transition at a control signal that establishes the reference time for tracking timing violations on the data_event | Module input or inout that is scalar or vector net |
| Data_event | The signal change that initiates the timing check and is monitored for violations. | Module input or inout that is scalar or vector net |
| Limit | A time limit used to detect timing violations on the data_event | Constant expression or specparam |
| Threshold | The largest pulse width that is ignored by the timing check $width | Constant expression or specparam |
| Setup_limit | A time limit used to detect timing violations on the data_event for $setup. | Constant expression or specparam |
| Hold_limit | A time limit used to detect timing violations on the data_event for $hold. | Constant expression or specparam |
| Notifier | An optional argument that "notifies" the simulator when a timing violation occurs | Register |
$setup checks setup time. When modeling synchronous circuits, flip-flops need time to force a correct value. Data cannot change within the setup time because flip-flops cannot detect the new value. If data changes within a given time limit, $setup reports a timing violation. If a data event and reference event occur at the same time there is no violation. The$setup first checks timing data then records a new data value. The formula to report a timing violation is as shown below:
(time of reference event) - (time of data event) < limit
Notice that the limit argument has to be a positive number.
$skew checks the following:
(time of data event) - (time of reference event) > limit
$skew can be used to check synchronicity of clocks inside a circuit. If different clocks are used in a design and are synchronized, $skew will report a timing violation when the active edge of one of them occurs outside the time limit allowed for the other clock to occur.
When the data event and the reference event occur at the same time, $skew will not report a timing violation.
$hold will report a timing violation if the following formula is true:
(time of data event) - (time of reference event) < limit
$hold simply checks that data is stable in the specified interval of time after the edge of the clock. In flip-flops, data should remain stable for a given time after the active edge of the clock to allow for propagation of data.
Also, a violation will be reported if the data event and the reference event occur at the same time.
$recovery responds when the following formula is true:
(time of data event) - (time of reference event) < limit
The 'reference_event' must be an edge-triggered event: posedge or negedge. A timing violation occurs if the time interval between an edge-triggered reference event and a data event exceeds the 'limit'. If a reference event and data event occur at the same time, a timing violation is reported. If a 'reference_event' argument is specified without edge specification, an error is reported.
$setuphold checks setup and hold timing violations. This task combines the functionality of $setup and $hold in one task. The following formula has to be applied:
setup_limit + hold_limit > 0
'reference_event' have to be one of the following:
- $hold lower bound event
- $setup upper bound event
'data_event' have to be one of the following:
- $hold upper bound event
- $setup lower bound event
In $width both limit and threshold have to be positive numbers. The 'reference_event' must be the edge specification, otherwise an error will be reported. The 'data_event' is not specified directly, but by default means 'reference_event' with opposite edge. A timing violation occurs with the following formula:
threshold < (time of data event) - (time of reference event) < limit
$width reports when width of the active-edge is too small. In FF case it is very important to ensure that the width of an active-edge is sufficient and FF will work properly.
The $period checks that a period of signal is sufficiently long. The reference_event has to be an edge specification. The data_event is not specified directly and by default, is the same as a reference_event. The $period reports a timing violation when the following formula comes true:
(time of data event) - (time of reference event) < limit
The $nochange checks if the data signal is stable in an interval of start_edge_offset and end_edge_offset. If the signal has changed, a timing violation is reported. The reference_event argument can be posedge or negedge but the edge control specifiers are disallowed.
Examples
Example 1
module setup (data1, data2, q);
input data1, data2;
output q;
and (q, data1, data2);
specify
specparam tsetup = 7, delay = 10 ;
(data1 => q) = 10 ;
$setup(data1, posedge data2, tsetup);
endspecify
endmodule
Example 2
module two_clocks (clk1, clk2, q);
input clk1, clk2;
output q;
specify
specparam tskew = 7;
$skew(posedge clk1, posedge clk2, tskew);
endspecify
endmodule
Example 3
module hold (data1, data2, q);
input data1, data2;
output q;
and (q, data1, data2);
specify
specparam thold = 7, delay = 10 ;
(data1 => q) = 10 ;
$hold(posedge data2, data1, thold);
endspecify
endmodule
Example 4
module recovery (in1, out1);
input in1 ;
output out1 ;
assign out1 = in1 ? 1'b1 : 1'bz ;
specify
specparam trecovery = 10;
$recovery(posedge in1, out1, trecovery);
endspecify
endmodule
Example 5
module setuphold (data1, data2, q);
input data1, data2;
output q;
and (q, data1, data2);
specify
specparam tsetup = 7,
thold = 7,
delay = 10 ;
(data1 => q) = 10 ;
$setuphold(posedge data2, data1, tsetup, thold);
endspecify
endmodule
Example 6
module width (data1, data2, q);
input data1, data2;
output q;
and (q, data1, data2);
specify
specparam twidth = 10,
delay = 10 ;
(data2 => q) = 10 ;
$width(posedge data2, twidth);
endspecify
endmodule
Example 7
module dff (clk, q);
input clk;
output q;
buf (q, clk);
specify
specparam tperiod = 100 ;
$period(posedge clk, tperiod);
endspecify
endmodule
Example 8
module nochange (data1, data2, q);
input data1, data2;
output q;
and (q, data1, data2);
specify
specparam tstart = -5,
tend = 5 ;
$nochange(posedge data2, data1, tstart, tend);
endspecify
endmodule
Important Notes
- All timing check system tasks should be invoked within specify blocks.
