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Subject: vr_ad with reserved bits but writable
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e_vangelist
Posts: 5
Online: User is Offline
1/26/2008 1:54 AM  

Hi 'e' folks,

I'm working in the verification of a SOC using vr_ad last version:

reg def BLABLA_0 BLA_REGS 0x01 {
    reg_fld unused  : uint(bits:28) : R: 0:cov;
    reg_fld foo : uint(bits:4):RW:0:cov;
    set_static_info() is {
       static_info.compare_mask = 0xFFFFFFFF;  // compare up to 32 bits
       static_info.description = "Control Reg";
 };

In my test, I perform write -read access on my register: the thing I do not understand , Why the write value chosen by bfm unit does not take care of the fact that unused is R (read only).

Actually, the write value is 0x5e006a9 and of course I collect a dut error due to a mismatch because my unused is R on a spec point of view but writable in DUT.

Have you some hint to constrain the ability to :
perform write with appropriate width on such register (R reg_field and RW reg fields)
and perform read with appropriate width when you have W reg fields and RW reg_fields

Cheers 

Fred
thinkverification
Posts: 28
Online: User is Offline
1/26/2008 7:16 AM  
Hey there Fred,

When you define a register field as R (read only) it doesn't affect the data sent to DUT, but only the shadow one (in e). This is done in order to be able to compare DUT vs. ENV and verify that read-only bits are actually read-only, i.e. - ignore the value written to them. If both ENV and DUT were to ignore the write then you woudn't be able to detect bugs related to RO access.

What you probably need is a different attribute called "RESERVED" or "UNUSED". This would mean that the value of the field is a "don't care" when checked. You could implement this by setting the respective compare_mask to 0.

Another option, of course, is to avoid writing non-default values to the unused fields...

Hope this helps :-)

Yaron
Yaron Ilani
VLSI Verification Specialist
www.ThinkVerification.com
e_vangelist
Posts: 5
Online: User is Offline
1/29/2008 11:00 AM  
Thanks Yaron for your input.

I forgot to mention that we tends to choose registers randomly and not necessary perform all the bunch of it.
I went for a reformatting of the written value according to the wr mask: works perfectly !

cheers

by the way, congratulations for your website: it is pleasant and funny!
thinkverification
Posts: 28
Online: User is Offline
1/29/2008 11:45 PM  
Posted By e_vangelist on 1/29/2008 11:00 AM
Thanks Yaron for your input.

I forgot to mention that we tends to choose registers randomly and not necessary perform all the bunch of it.
I went for a reformatting of the written value according to the wr mask: works perfectly !

cheers

by the way, congratulations for your website: it is pleasant and funny!


Thanks!
Yaron Ilani
VLSI Verification Specialist
www.ThinkVerification.com
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Forums > Functional Verification > e > vr_ad with reserved bits but writable


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