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Electronics => Electronics => Topic started by: Admin on October 12, 2007, 07:08:40 AM

Title: resistors on tx/rx lines
Post by: Admin on October 12, 2007, 07:08:40 AM

this is mostly a stupid check for me . . .

I want to protect my tx/rx lines with resistors, so is this correct?

txA --- 1.18k resistor ---- rxB
rxA --- 1.18k resistor ---- txB
gndA ------------------------- gndB

anyone know how much current the UART for an ATmega needs? I couldn't find it on the datasheet, although Im sure its hidden somewhere in its 400+ pages of glory . . .

(and in case you were wondering, the 1.18k resistor was cheaper than the 1k resistor for 0402 smd)

Title: Re: resistors on tx/rx lines
Post by: jsmoker on October 12, 2007, 09:48:19 AM

Hmm, I'm no expert, but I found this:

http://ieeexplore.ieee.org/iel5/4234/20101/00929606.pdf

Title: Re: resistors on tx/rx lines
Post by: hgordon on October 12, 2007, 01:15:44 PM

I always use 2.7k resistors for i/o protection - that basically limits current to a bit more than 1mA on 3.3V circuits and a bit less than 2mA on 5V circuits, which puts things in the range that most microcontroller interfaces can handle.

Title: Re: resistors on tx/rx lines
Post by: Admin on October 12, 2007, 02:02:19 PM

Hmmmm Im assuming that the Rx/Tx lines on the connecting device (unknown at the moment) will also have protection resistors . . . any idea what the minimum allowable current is?

Title: Re: resistors on tx/rx lines
Post by: hgordon on October 12, 2007, 02:24:26 PM

It looks like they have pretty good protection on their chips.  An ATmega8 data sheet shows 40mA current handling per i/o pin, and pull-up resistors are spec'd at 30-80kohm, which implies that minimum currents are in the ballpark of 0.1mA.

Title: Re: resistors on tx/rx lines
Post by: Admin on October 12, 2007, 02:51:07 PM

Where did you find this in the datasheet (which section)?

Title: Re: resistors on tx/rx lines
Post by: hgordon on October 12, 2007, 03:08:17 PM

http://www.atmel.com/dyn/resources/prod_documents/doc2486.pdf - page 242 - Electrical Characteristics