What are this instructions for?

Discussion in 'hardware' started by AlexC, Aug 17, 2012.

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  1. AlexC

    AlexC Registered Member

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    I've just read this in a forum:
    http://www.techsupportforum.com/forums/f108/solved-toshiba-satellite-screen-wont-turn-on-641125.html

    What will press and hold the power button for 30-45 seconds with battery and power removed do at hardware level? Just curious.

    Thanks
     
    Last edited: Aug 17, 2012
  2. Dark Shadow

    Dark Shadow Registered Member

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    I believe it drains the power from all the componets and reset hardware clearing memory I think.
     
  3. Bill_Bright

    Bill_Bright Registered Member

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    It does absolutely nothing. I am always amazed to see these ridiculous suggestions - some have been floating around for more than 10 years.

    On a PC, the power button on your case does just one thing, it puts a short on (connects) two pins on the motherboard. This completes a "momentary circuit".

    A momentary circuit is one where once the button is pressed and the short detected by the power supply (which takes 1 clock cycle - pretty darn quick)from that point on, the switch is ignored, until released and pressed again.

    In other words, by pressing then holding the button for additional 5 seconds, 15 seconds or 45 seconds accomplishes one thing - it makes your finger sore.

    Unplugging the PSU from the wall (or, if equipped, setting the PSU's master power switch in back to off), resetting the BIOS by removing the CMOS battery for a few seconds, the reset jumper or reset button (found on some newer motherboards) dumps any residual data left in any circuit.

    Residual voltages are nothing new in electronics. Cathode Ray tubes (CRTs) can hold 1000s on the anode for days, unless discharged, as can some storage capacitors. Engineers have known that well since Electronics Basic 101. So for many years designers have designed motherboards to quickly discharge. In the old days, it was to keep from being killed, or at least thrown across the floor. Today it is to prevent damage to sensitive devices.

    You will often find comments that holding the button drains the storage capacitors in the PSU. Hogwash! All PSUs have bleeder resistors or bleeder circuits that designed specifically to discharge these capacitors circuits almost immediately after power is removed.

    In fact, the ATX12V Form Factor PSU Design Guide states computer PSUs need maintain power during loss of power for just 17ms. Milliseconds! (See paragraph 3.2.11 Voltage Hold-up Time).

    On notebooks, as was the case in that thread, the button does the same thing. HOWEVER, some notebooks let you reset the charging/battery circuits, but I have never seen any published instructions for any computer that says to hold down the power button. Including in my own Toshiba notebook manual. Still, in any case, that is something that should be verified in that specific notebook's user manual, but if anyone has seen this published (by a computer maker, not just a forum responder) please provide a link.

    I suspect what really happened with that notebook is the issue was corrected by removing total power from the notebook - removing the battery and power block/charger for a few seconds - Not by holding down the power button.

    If residual voltages are still present after a couple seconds of removing power, there is a fault in the power supply and it needs to be replaced. CMOS modules were selected for use on motherboards because one of their primary electrical characteristics is they [almost] instantly dump any stored data once the "holding voltage" on it's power input is removed. That occurs the PSU is unplugged and the circuit is "short-circuited" to "ground" with the jumper or switch, or when the battery is removed.

    If motherboard designers wanted the CMOS information to "linger", they would not have selected CMOS modules to hold the data! They would use some sort of flash - EEPROM chip.

    So my point is, resets are almost instant in high speed digital electronics.

    As a long time electronics technician with a shop, I know for a fact holding the power button is not a standard for, or a feature on PC motherboards. If it was, it would be specified in the ATX Form Factor Standard that specifies how ATX motherboards, ATX cases and ATX PSUs connect and interact - physically and electronically. Sadly, there is no such standard for notebooks (which is why they are so proprietary - and so much harder to work on, upgrade, customize, or build for yourself).

    It takes forever for the industry to agree on standards. It took 2 years after the 802.11n (wireless-n) protocol was presented for the makers to agree on it, then it sat in 11n-DRAFT mode for another 5. USB and SATA have had similar issues getting across-the-board adaption by everyone.

    So if this were a notebook industry-wide standard - adapted by Dell, Toshiba, Lenova, HP, Acer, ASUS, BioStar, MSI, FoxConn and the almost countless other notebook/notebook motherboard makers (AND the various BIOS makers too!), it surely would be officially published somewhere. But it is not.
     
  4. Dark Shadow

    Dark Shadow Registered Member

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    I guess pressing and holding down the power button is a myth then,Like unicorns and flying Dragons.Thanks Bill for the explanation.:thumb:
     
  5. Bill_Bright

    Bill_Bright Registered Member

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    Well, not sure myth is right because residual voltages are real and the need to discharge them is too. But we are not talking deadly or even high voltages. 12 at most and typically 3, or ~1.5VDC - because NO ONE is digging around inside the PSU, right? There are no "user-serviceable" parts inside a PSU, but anything that plugs into the wall can kill!

    Also, when the IBM PC and clones first came out, they complied with the AT Form Factor standard for AT motherboards, AT Cases and AT PSUs.

    One significant difference between the AT PSU and today's ATX PSU is the AT PSU used a wiring "harness" that ran out to the case's front panel. And when you pressed the power button, you were actually pressing the power button on the PSU.

    With today's (and the last 10 years) PCs and the ATX Form Factor standard, the PSU plugs into the motherboard, not the case. You remotely start/stop the PSU by shorting the ATX required +5Vsb (standby) voltage on the two motherboard pins, signaling the PSU to boot, or signaling the BIOS to start the power off, or delayed power off sequence (depending on BIOS setting).

    There was no standby voltage with AT.

    So in the olden days, and on other electronics, holding the power button actually did hold down the power button to the PSU, and may have helped ensure a full discharge.

    But again, with ATX, the front panel button is simply a remote button, not directly connected to the PSU.

    So I think this "myth" might be rooted in real requirements from ancient (electronically speaking) times and antique hardware. But it does not apply today with today's high-speed, low voltage, digital electronics.
     
  6. Dark Shadow

    Dark Shadow Registered Member

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    @ Bill Bright,Some of that technical information is over my head, but I do appreciate the knowledge you share.Thanks:thumb:
     
  7. Bill_Bright

    Bill_Bright Registered Member

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    That's okay. But still it is important to clarify things rather than assuming something you read is true - or true for you - especially when it comes to rumors and myths.
     
  8. NGRhodes

    NGRhodes Registered Member

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  9. AlexC

    AlexC Registered Member

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    Thanks Dark Shadow, Bill_Bright and Nick Rhodes for the information :thumb:
     
  10. Bill_Bright

    Bill_Bright Registered Member

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    Yeah, sorry. I was not very clear. You are correct that I was talking about PCs and this thread is about a notebook - sorry for taking it a bit OT.

    However, I was still correct because I was talking about the old wives's tail or "trick" of holding down the power button to "drain residual voltages". Even on notebooks, that does not happen. I did not make that clear when I said it does absolutely nothing. Sorry again.

    When you hold down the power button on a notebook, you are not draining circuits, you are counting down a timer - or allowing the timer to continue. And when the count hits zero, the hard-reset initiates.
     
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