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Chris Le

UVM Reg Model: access register error using bitbash seq because of predictor

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Hi all,

I got problem with uvm bitbash seq with uvm-1.1d. I found, when bitbash sequence writes a value to DUT, the desired value is not updated immediately (because auto predict is disabled at default). The desired value is only updated by uvm predictor via monitor (takes long time to update this value). Therefore, at the next get() function, the value doesnt return correctly (see the following code of uvm_bitbash_seq, line 165).

159          rg.write(status, val, UVM_FRONTDOOR, map, this);
160          if (status != UVM_IS_OK) begin
161             `uvm_error("uvm_reg_bit_bash_seq", $sformatf("Status was %s when writing to register \"%s\" through map \"%s\".",
162                                         status.name(), rg.get_full_name(), map.get_full_name()));
163          end
164          
165          exp = rg.get() & ~dc_mask;
 

My question is, should I enable auto_predict to update desired value immediately after writing to DUT? Why is auto_predict disabled at default? Currently, I worked around by adding #5ns delay before get() function to wait desired value is updated by predictor via monitor, but I think it is not a good way.

Thanks.

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hi,

there are two questions you ask:

 

1. why is auto_predict=off the default?: normally auto_predict==off is the more versatile mode of operation. it works through a monitor when the real transaction is seen on the bus, it works in passive mode, it works with back-to-back transfers etc.

2. does setting auto-predict after the write help?: simply no. for auto predict to work the driver-sequencer handshake has to wait till the real end of transaction on the bus and it has to be switched on before the read/write operation. 

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7 hours ago, uwes said:

hi,

there are two questions you ask:

 

1. why is auto_predict=off the default?: normally auto_predict==off is the more versatile mode of operation. it works through a monitor when the real transaction is seen on the bus, it works in passive mode, it works with back-to-back transfers etc.

2. does setting auto-predict after the write help?: simply no. for auto predict to work the driver-sequencer handshake has to wait till the real end of transaction on the bus and it has to be switched on before the read/write operation. 

1. Thanks!

2. If I don't set auto_predict then uvm_reg_bitbash_seq will be failed. If you say so, I think that is a bug of UVM 1.1d, because currently, the write() function is finished immediately without waiting the real transaction on bus monitor update. Any suggestion to fix the problem if we don't set auto_predict?

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