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 Disable Scan Shift Enable in Functional Mode 

Last post Tue, Feb 19 2013 10:10 AM by bmiller. 3 replies.
Started by Terry2000 15 Feb 2013 03:37 AM. Topic has 3 replies and 1302 views
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  • Fri, Feb 15 2013 3:37 AM

    • Terry2000
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    • Joined on Tue, Dec 18 2012
    • Posts 11
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    Disable Scan Shift Enable in Functional Mode Reply

    Hello,

    I'm trying to disable shift enable in function mode.  

    This is the VHDL code:
     i_scan_shift_enable <= '0' when scan_mode_in = '0' else scan_shift_enable;
       

    So "scan_shift_enable" is driven from a pad and  "i_scan_shift_enable" drives the flops.

    However I can't seem to keep net i_scan_shift_enable preserved and even if I did would it be a valid pin for the DFT tools to hookup to.

    How is this generally done? Is it necessary to instantiate tech cell, preserve and hookup to the output pin?  

    • Post Points: 20
  • Fri, Feb 15 2013 7:05 AM

    • bmiller
    • Top 200 Contributor
    • Joined on Tue, Oct 14 2008
    • Ottawa, Ontario
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    Re: Disable Scan Shift Enable in Functional Mode Reply

     Hi,

     I have seen tech cells instantiated for this purpuse, but it isn't absolutely necessary.

    Using a hierarchical pin for this purpose is quite useful.  If you can make i_scan_shift_enable a hierarchical pin, then you can define your shift_enable as follows:

        define_dft shift_enable -name SE -hookup_pin [find / -pin <hier_path>/i_scan_shift_enable] -hookup_polarity non_inverted [find / -port scan_shift_enable]

     When connecting scan chains, RC will know to connect to the <hier_path>/i_scan_shift_enable pin.  But, when writing out interface files for Encounter Test, RC will know that the shift_enable port is scan_shift_enable.

     You are correct that you cannot rely on net names to be preserved throughout the flow.  But, you can generally rely on hierarchical pins to remain throughout the flow (at least through synthesis), as long as you prevent them from being ungrouped.

    Good luck!

     

    • Post Points: 20
  • Mon, Feb 18 2013 12:40 AM

    • Terry2000
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    • Joined on Tue, Dec 18 2012
    • Posts 11
    • Points 275
    Re: Disable Scan Shift Enable in Functional Mode Reply
    Hello,

    Many thanks for your reply, it was very helpful.

    So I created a small hierarchal block for the shift enable gate and used this command as your suggestion.

    define_dft shift_enable -name SE -active high -hookup_pin [find / -pin hlt5000_toplevel/u_scan_shift_gate/scan_shift_enable_gate] -hookup_polarity non_inverted [find / -port scan_shift_enable]

    I am using clock gating and use the following command to hook-up SE.

    set_attribute lp_clock_gating_test_signal SE /designs/ hlt5000_toplevel

    After using your suggested “define_dft shift_enable” command the design failed DFT rules for any flops with clock gating.

    I then change the clock gating hook-up to:

    set_attribute lp_clock_gating_test_signal scan_mode_in /designs/hlt5000_toplevel

    And this passed DFT rules and all looked good except I suffered a small drop off in faults coverage.

    Is it possible to use an internal shift enable signal clock gating control.  I tried various commands on the clock gating control (adding paths etc) without success.  

    Best Regards  
    • Post Points: 20
  • Tue, Feb 19 2013 10:10 AM

    • bmiller
    • Top 200 Contributor
    • Joined on Tue, Oct 14 2008
    • Ottawa, Ontario
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    Re: Disable Scan Shift Enable in Functional Mode Reply

     Hi,

     Have you checked that SE got connected to the clock gaters cells correctly?

     Is there actually a controllable path from the port scan_shift_enable to the clock gater enable cells?  If not, that is the reason for the failure.  You can check the controllablity of this path by using the dft_trace_back command:

     Usage: dft_trace_back [-mode <integer>] [-polarity] [-continue] [-print] <pin|port>

        [-mode <integer>]:
            trace back one level under the specified mode (0-3). 0: no constant propagation, 1:
            tied-constant propagation, 2: tied-constant and test-mode propagation, 3: tied-constant,
            test-mode and shift-enable propagation [default: 3]
        [-polarity]:
            also return whether the polarity changes through the trace
        [-continue]:
            continue the trace back until PI, sequential gate, complex instance or constant is reached
        [-print]:
            print the status and pin at every trace back
        <pin|port>:
            pin or port to start the back-trace from

     Use -mode 3 to propagate shift_enable.

    You could also try temporarily using a primary input for SE (rather than using -hookup_pin) just to prove to yourself that controlling the clock gaters with SE is ok.  But, I believe this to be a better solution than using scan_mode.

     

    • Post Points: 5
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Started by Terry2000 at 15 Feb 2013 03:37 AM. Topic has 3 replies.