SEQUENCES
Properties can be created based on a single event or a sequence of events. The sequence of events are distributed over time. The sequence can be defined as follows.
Within 3 clocks from assertion of frame_ , the device is expected to claim the transfer by asserting devsel_. This can be represented using sequences as follows. The intent of this example is only to demonstrate a basic sequence. In future examples we will see complex usage of sequences.
sequence frame_assert
$fall(!frame_);
endsequence
sequence device_sel
##[1:3] $fall(devsel_);
endsequence
property transfer_init;
@(posedge clk) frame_assert |-> device_sel;
endproperty
assert property (transfer_init);
Assertions are classified as
- Concurrent Assertions
- Immediate Assertions
Concurrent assertions are clock cycle based, whereas Immediate assertions are event based.
e.g. 1. PCI parity error assertion. If parity errors are seen error should be flagged.
//Concurrent assertion example
pci_error_chk: assert property (@(posedge clk) perr);
//Immediate assertion example
always_comb
begin
pci_error2_chk: assert (perr);
end
As shown in the above code examples, the first property pci_error_chk is evaluated every posedge of clock. It is a declarative piece of code.
The second property pci_error2_chk is evaluated as soon as there is a change in value of perr, and is used as procedural coding in a always block.
