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???
08/01/05 23:37
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 		 #98644 - HC vs HCT vs TTL
Responding to: ???'s previous message
Just a-wondering.

What is the micro's VCC?

What is the HC165's VCC?

Here's a lesson learned. I did a USB device and the micro was powered by 3.3V downregulated from the USB 5V. The micro has LVTTL port outs and I was bitbanging an 'HC595 shift register. Originally the 'HC595 was powered from the 3.3V rail but I decided to minimize draw on that regulator so I powered it from the raw USB 5V. That USB 5V can be as high as 5.25V or more. The micro outputs switched between essentially ground and the 3.3V rail, but since the 'HC595 input thresholds vary with its VCC, when you have a high-ish 5V, that threshold can be greater than 3.3V. In other words, never a valid high!

Changing to an 'HCT595, with TTL thresholds (2.4V high) fixed the problem.

-a

List of 28 messages in thread
TopicAuthorDate
74hc165 read problem            01/01/70 00:00      
   pull ps?            01/01/70 00:00      
      external pull up            01/01/70 00:00      
   74LS165 schematic and assembly program            01/01/70 00:00      
      re 74LS165 schematic and assembly progra            01/01/70 00:00      
   connections and timings            01/01/70 00:00      
      Port0 can show very high rise times            01/01/70 00:00      
      thanks oleg and kai            01/01/70 00:00      
      re connections and timings            01/01/70 00:00      
         What pull-ups?            01/01/70 00:00      
            Also....            01/01/70 00:00      
               re also            01/01/70 00:00      
                  Very strange...            01/01/70 00:00      
                     re Very strange...            01/01/70 00:00      
                        Something wrong with 74HC165?            01/01/70 00:00      
                           If you have an oscilloscope..            01/01/70 00:00      
                              re If you have an oscilloscope..            01/01/70 00:00      
            re What pull-ups?            01/01/70 00:00      
               HC vs HCT vs TTL            01/01/70 00:00      
   165 access problem            01/01/70 00:00      
   using opto to pulldown pin of 74hc165            01/01/70 00:00      
      which optocouplers            01/01/70 00:00      
      time for schematics            01/01/70 00:00      
   here is solution            01/01/70 00:00      
   it is working            01/01/70 00:00      
      because of something in your hardware            01/01/70 00:00      
         nop not due to hardware            01/01/70 00:00      
            still could be            01/01/70 00:00      

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