# SVA Methods

## $rose

$rose(boolean expression or signal name)

returns true if the least significant bit of the expression changed to 1. Otherwise, it returns false.

sequence seq_rose; @(posedge clk) $rose(a); endsequence

Sequence seq_rose checks that the signal “a” transitions to a value of 1 on every positive edge of the clock. If the transition does not occur, the assertion will fail.

## $fell

$fell(boolean expression or signal name)

returns true if the least significant bit of the expression changed to 0. Otherwise, it returns false.

sequence seq_fell; @(posedge clk) $fell(a); endsequence

Sequence seq_fell checks that the signal “a” transitions to a value of 0 on every positive edge of the clock. If the transition does not occur, the assertion will fail.

## $stable

$stable(boolean expression or signal name)

returns true if the value of the expression did not change. Otherwise, it returns false.

sequence seq_stable; @(posedge clk) $stable(a); endsequence

Sequence seq_stable checks that the signal “a” is stable on every positive edge of the clock. If there is any transition occurs, the assertion will fail.

## $past

$past(signal_name, number of clock cycles)

provides the value of the signal from the previous clock cycle.

Below Property checks that, in the given positive clock edge, if the “b” is high, then 2 cycles before that, a was high.

property p; @(posedge clk) b |-> ($past(a,2) == 1); endproperty a: assert property(p);

## $past construct with clock gating

The $past construct can be used with a gating signal. on a given clock edge, the gating signal has to be true even before checking for the consequent condition.

$past (signal_name, number of clock cycles, gating signal)

Below Property checks that, in the given positive clock edge, if the “b” is high, then 2 cycles before that, a was high only if the gating signal “c’ is valid on any given positive edge of the clock.

Property p; @(posedge clk) b |-> ($past(a,2,c) == 1); endproperty a: assert property(p);

## Built-in system functions

- $onehot(expression)
- checks that only one bit of the expression can be high on any given clock edge.

- $onehot0(expression)
- checks only one bit of the expression can be high or none of the bits can be high on any given clock edge.

- $isunknown(expression)
- checks if any bit of the expression is X or Z.

- $countones(expression)
- counts the number of bits that are high in a vector.

a_1: assert property( @(posedge clk) $onehot(state) ); a_2: assert property( @(posedge clk) $onehot0(state) ); a_3: assert property( @(posedge clk) $isunknown(bus) ) ; a_4: assert property( @(posedge clk) $countones(bus)> 1 );

Assert statement a_1 checks that the bit vector “state” is one-hot.

Assert statement a_2 checks that the bit vector “state” is zero one-hot.

Assert statement a_3 checks if any bit of the vector “bus” is X or Z.

Assert statement a_4 checks that the number of ones in the vector “bus” is greater than one.

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