Atmel specific advice needed: 5V Atmega 3.3V input level - Page 1
Forum section: electronics / beginners

author: bryce1
author_rank: Regular Contributor
timestamp: August 07, 2014, 09:40:18 am
content: Hi,I like the enc28j60 for ethernet stuff. It runs at 3.3V but hast 5V tolerant inputs. I use it in a couple of projects and I found, that Atmegas running it 5V work reliably with 3V input levels.But now, I am designing a commercial product and need advice from more experienced engineers:Would you design in level conversion or do you think I can get away with what (I think) I know will work?Thanks!

author: sleemanj
author_rank: Super Contributor
timestamp: August 07, 2014, 10:07:12 am
content: What you are asking, is for the ATMega running at 5v, what is the threshold voltage, and is 3.3v sufficiently above it to guarantee registering a high.This should help you answer your question:http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=109947&start=0

author: idpromnut
author_rank: Unknown
timestamp: August 07, 2014, 11:08:56 am
content: Alternatively you could run the atmega at 3.3v (pretty sure most of them will run that low, depending on your clock rate).

author: bryce1
author_rank: Regular Contributor
timestamp: August 07, 2014, 11:16:10 am
content: no option as I need more than 10Mhz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 11:26:48 am
content: Find this in the datasheetGeneral Purpose I/O PinsStatic / DC CharacteristicsV_IH min, V_IL maxTim

author: bryce1
author_rank: Regular Contributor
timestamp: August 07, 2014, 11:33:16 am
content: Pin Threshold and Hysteresis tells me it should register >2.6V as high when the chip runs on 5V.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2014, 04:48:40 pm
content: Where's that?My copy of ATmega164P/324P/644P datasheet shows (p.325) 0.3*Vcc low and 0.6*Vcc high. Or < 1.5V and > 3.0V.The voltage change should still be sufficient, so you could adjust the offset by using a voltage divider to VCC to shift the level up a little bit.Tim

author: zapta
author_rank: Super Contributor
timestamp: August 07, 2014, 05:06:02 pm
content: Quote from: bryce1 on August 07, 2014, 11:16:10 amCheck the speed vs voltage graph in the datasheet, with the atmega328p, 3.3V should give you up to 13.3Mhz, 3.8V will give up to 16.1Mhz.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2014, 05:54:23 pm
content: I have considered using the enc28j60 before but never gotten around to trying it.The enc28j60 datasheet is explicit that its inputs are 5 volt tolerant (although it only specifies input current up to Vdd) so I would not worry about level shifting in that direction. The Atmel 5 volt parts have an input threshold voltage between 0.8 and 2.0 volts so the 3.3 volt CMOS signal levels from the enc28j60 should work fine.If I was paranoid about this, I would conduct a test using the minimum and maximum operating voltages for each part over temperature. Running the enc28j60 at 3.10 volts and the ATmega at 5.5 volts should be worst case. I would also check the enc28j60 input current when driven with 5.5 volts to see if there is any unexpected behavior.

author: Svuppe
author_rank: Regular Contributor
timestamp: August 08, 2014, 09:04:20 am
content: Another option could be to switch to the XMega series AVR. They are 3.3 volt devices and run at speeds up to 32 MHz.

author: bryce1
author_rank: Regular Contributor
timestamp: August 08, 2014, 12:55:33 pm
content: Quote from: T3sl4co1l on August 07, 2014, 04:48:40 pmp. 349Quote from: David Hess on August 07, 2014, 05:54:23 pmnote that I this is about the enc28j60 OUTPUTs. although its inputs are 5V tolerant, it still outputs only 3.3V.Quote from: zapta on August 07, 2014, 05:06:02 pmthat is cutting it close, I like to have some headroom.Quote from: Svuppe on August 08, 2014, 09:04:20 amdesign requires atmega

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2014, 01:36:03 pm
content: Quote from: bryce1 on August 08, 2014, 12:55:33 pmUm...

author: bryce1
author_rank: Regular Contributor
timestamp: August 11, 2014, 06:07:26 am
content: Sorry.Figure 28-103.ATmega644P: I/O Pin Input Hysteresis vs. V CCp468.