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Forum Index : Microcontroller and PC projects : 3v3 inputs vs 1k resistors....
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9590 |
Hi folks. 
I have run out of 5v tolerant inputs on my latest design, and would love to use some of the spare standard 3v3 inputs, but the devices they connect to output 5v logic. I note that with the console, it is often said that you should use the 1k series resistor in case someone connects a 5v serial converter to the MM. With that in mind, is it acceptable practise to be able to use any 5v logic with 3v3 inputs, provided you have at least a 1k series resistor and don't directly connect the 5v logic to the 3v3 input? False logic?! 
If not acceptable practise, I can squeeze a potential-divider in there for each of those pins, to drop the voltage to them..... Smoke makes things work. When the smoke gets out, it stops! |
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| robert.rozee Guru  Joined: 31/12/2012 Location: New ZealandPosts: 2434 |
a resistor of a few k's (perhaps 4k7) would almost certainly be sufficient, though to be technically correct a divider would be worth the expense of the extra resistor when building a commercial product. i recall geoff did some tests a while back and concluded that there were not diodes to Vcc as one might expect. so with a single resistor, while current will be limited, the full voltage may still be present at the pin. the effect of this in the long-term, i have no idea. just spent 3 hours battling with a pickit3 and mplab to program up three 1455's. i feel the need for someone to write a program called pic16prog! cheers, rob :-) |
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boss Senior Member  Joined: 19/08/2011 Location: CanadaPosts: 268 |
Try this: bidirectional 5V <-->3.3V level converter 
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
Look at a BSS138 from Fairchild. Those work fine. I have made a 1455 together with a windows GUI program that can very quickly program another 1455. Though not ready for publish time yet. Programming 3 chips take about 30 seconds. Most time is to take one out of the zif socket and putting another one in. :) Microblocks. Build with logic. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9590 |
Thank you for the replies.
Looks like boss came up with the perfect solution, but I will also check out the BSS138.... Following boss's idea, I found this on eBay for the princely sum of US$1.... Smoke makes things work. When the smoke gets out, it stops! |
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