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Newbie muldrow
Newbie
5,570 Views
Registered: ‎01-30-2013

Spartan 6 (SC6SLX16-3) INIT_B goes to 1.9V during serial slave mode config

While probing on a board during power-up configuration (Spartan-6 FPGA configured by XCF32P using Master SelectMap),
 the INIT_B signal is seen to transition from +3.3V to +1.8V at the end of configuration (when CONFIG_DONE goes from 0V to +3.3V).  It was expected that INIT_B would stay at +3.3V since it has a 4.75K pullup on it per Xilinx recommendations.  This is on a

board that has had problems with internal shorts, so I'm trying to rule that out for management.  I believe what is happening is

a weak pulldown is engaged on the INIT_B at end of configuration (answer record AR# 35002 seems to hint at this), but was hoping someone could confirm this.  Per my calculations, the weak pulldown would need to be about 2.5K, but I couldn't find spec for this in Spartan-6 datasheet.

 

The following related forum seems to ask same kind of question, but I didn't see a conclusive answer in it:

Spartan 6 (SC6SLX16-3) INIT_B goes to 1.9V during serial slave mode config

 

Thanks, GMM

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2 Replies
Instructor
Instructor
5,562 Views
Registered: ‎08-14-2007

Re: Spartan 6 (SC6SLX16-3) INIT_B goes to 1.9V during serial slave mode config

If your design doesn't use the INIT_B pin after configuration it takes on the same defaults as other Unused IO.  Unless you change the bitgen settings, the default is weak pull-down.  ds162 Table 4 lists pulldown current for a 3.3V bank as 200 to 550 uA.  So with a 4.7K pullup to 3.3V, the output voltage should go to somewhere between 2.36V (@ 200uA) and 0.715V (@550uA).  So basically what you're seeing is normal.  If you don't want this behavior you can add the INIT_B pin to your design so it doesn't get pulled down, or you can change the bitgen settings to apply weak pullup istead of pulldown to unused IO.

-- Gabor
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Newbie muldrow
Newbie
5,334 Views
Registered: ‎01-30-2013

Re: Spartan 6 (SC6SLX16-3) INIT_B goes to 1.9V during serial slave mode config

OK, thanks. That explains it

GMM

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