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David Prentice
02/02/12 07:21
Read: 189 times
Wormshill
England


 		 #185775 - Design the hardware first.
Responding to: ???'s previous message
The traditional 8051 has a relatively long start-up time compared to other microcontroller families. So an LED or motor that turns on with the power and is not turned off until the software is running is an 'embarrassment'. All the same even a 10ms glitch is unwise on any controller, just not so noticeable.

The solution is simple. An external pull-up or pull-down resistor.

You have to ensure your default state when the controller is in RESET.
During RESET, the pins are all inputs on a PIC, 8051, ...

If you designed active-low, you use a pull-up. Active-high needs a pull-down.

Even though a PIC port defaults to 0, it remains an input until you set its DDR. With default active-low, the PIC writes 1 to the port before it sets the DDR for o/p. With default active-high, you write 0 to the port before you make it an o/p. The external 'pull' resistor ensures off until the software is running.

The traditional 8051 does not have a DDR. It is therefore wiser to use active-low. All the same an external pull-down can ensure 'off'. You then need a PNP transistor to provide source current.

If you have made a design mistake, change to a variant with push-pull output. Then you are in the same position as your PIC colleague.

David.

List of 34 messages in thread
TopicAuthorDate
Power-On Port Status Problem      Adam Blue      01/30/12 08:42      
   Wrong idea      Per Westermark      01/30/12 08:48      
   you will not      Erik Malund      01/30/12 08:49      
   you are not the only one      Erik Malund      01/30/12 09:18      
   ATMEL be ashamed      Adam Blue      01/30/12 15:05      
      No, YOU should be ashamed...      Kai Klaas      01/30/12 16:15      
      Nonsense!        Andy Neil      01/30/12 16:46      
      PIC Does so      Neil Kurzman      01/30/12 23:08      
         Oh really??      Michael Karas      01/31/12 07:46      
            "Glitch" in the sense of kind of "setback in a plane"?      Kai Klaas      01/31/12 10:19      
   Try These Ideas        Michael Karas      01/30/12 17:21      
   "Glitch"      Adam Blue      02/01/12 13:12      
      what is not “ATMEL true”.      Erik Malund      02/01/12 13:37      
      I think you are making excuses      Erik Malund      02/01/12 13:41      
      Oh, boohoo!        Rob Klein      02/01/12 14:04      
      What a bunch of fatty sausage...      Michael Karas      02/01/12 14:59      
      "very easy problem"      Andy Neil      02/01/12 15:02      
   Power-On Glitch      Adam Blue      02/01/12 15:40      
      Nonsense - again      Andy Neil      02/01/12 16:28      
      Please explain what the "power-on glitch" is ...      Richard Erlacher      02/01/12 19:26      
      Into The Frying Pan        Michael Karas      02/01/12 20:52      
      Power-on problem...      Kai Klaas      02/02/12 05:55      
      You are still blaming the tools      Per Westermark      02/03/12 05:55      
   Design the hardware first.      David Prentice      02/02/12 07:21      
      do not Design the hardware first.      Erik Malund      02/02/12 07:30      
   Power-On Glitch      Adam Blue      02/02/12 15:07      
      that you had the worst possible reset circuit      Erik Malund      02/02/12 15:59      
      RC-reset...      Kai Klaas      02/02/12 17:20      
      Sorry, double post, don't know why...      Kai Klaas      02/02/12 17:21      
         No, you're not the only one      Richard Erlacher      02/02/12 21:12      
      Would I Hire?      Michael Karas      02/02/12 22:44      
         incidentially      Erik Malund      02/03/12 07:09      
   Power On satus Problem      Satish S Munot      02/29/12 10:16      
      echo, echo, echo      Erik Malund      02/29/12 10:30      

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