What is "bleed through"?

i working through the arduino shiftout tutorial.  in code sample 2.2 2 byte 1 one on  lines 137 , 138 find

      //zero data pin after shift prevent bleed through
      digitalwrite(mydatapin, 0);


i can't see change in program behaviour if comment out line 138 why there?
anyway, "bleed through"?

thanks

that line should not needed, clock-a-bit-out code is:

code: [select]

    //sets pin high or low depending on pinstate
    digitalwrite(mydatapin, pinstate);
    //register shifts bits on upstroke of clock pin 
    digitalwrite(myclockpin, 1);
    //zero data pin after shift prevent bleed through
    digitalwrite(mydatapin, 0);

on 74hc595 data on ser (pin 14) of chip needs presented @ least  150ns before srclk (pin 11) goes high. nothing data pin before or after should make difference. (*)


'bleed through' odd term use here, quick search turned example of used term describe problem having led's being driven via 74hc595 - myth might have started there.

anyway... line nothing.

if hardware bit flakey (poor power supply, no decoupling etc.) line might have effect, wouldn't 'fix'

yours,
  tonywilk


(*) p.s.
if line had set data pin high, might make sense 'pre-set' data line:
usually, data pin can driven low faster goes high - on fast processor time between writing data bit , setting clock pin high close minimum chip, so, without adding delay, arrange data pin either already high or switch low (which quick) before clock line goes high. bit of hack tho'

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