🚀 go-pugleaf

In article <1992Jul30.165112.10990@ole.cdac.com>, ssave@ole.cdac.com (Shailendra Save) writes:
> In article <1992Jul30.070338.6778@uwm.edu> pegasus@csd4.csd.uwm.edu (David R Hucke) writes:
>>
>>Hi everybody!
>>
>>Well I finally got my 8051 compatible micro-controller in the mail
>>and got it all wired with a crystal and MAX232 chip and reset hardware.
>>I had an LED hooked up to port 1 pin 1 with a resistor and connected to
>>ground like this:
>>So my first program was this:
>>setb P1.1  ;; Set Port 1 Pin 1 to 1  This turned on the LED as expected...
>>Now my second program was this:
>>clr P1.1  ;; Set Port 1 Pin 1 to 0 This should have turned off the LED.. 
>>It did not turn off the LED... In fact nothing I do turns the LED off...
>>Anybody have any idea what is going wrong?
>>When I go into Serial Load mode (RST goes low) the LED still on also.
>>Any help would be appreciated...
>>pegasus@csd4.csd.uwm.edu                David R. Hucke
>>                                        UW-Milwaukee Film Major
> 
> 
>    When you clear a port pin, it is pulled to a tristate 1. It does not
              ^^^^^                               ^^^^^^^^ no its not
> put a 0 onto the pin.  I would think you want to write a "0" onto the
        ^ yes it does
> port to turn the led off.  Same with RST.

the output on most CMOS uPs has a nominal active low (about 3*LSTTL)
and in the 8051 clear/0 = low
When a high/set/1 is written to a port there is a short duration strong
pullup to overcome line capacitance (usually 1 or 2 clock cycles) which is then
turned off and a weak (uA) pullup remains. This strong pullup can result in
glitches if the port is operated in a "wired-OR" mode

> 
> 
> -S
> ssave@ole.cdac.com
-- 
Bruce Spedding
Materials Science and Performance
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