Question about ENC28J60 Ethernet shield

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi,

I want to DIY an ethernet shield by using ENC28J60 chip.
I found a schematic from nuelectronic:
http://www.nuelectronics.com/download/projects/Ethernet_v1_0.pdf

In this schematic, I don't understand why we need a 2-input AND gate 74HCT08D to process INT and MISO signal.

The 74HCT08D's function table:

Because the 74HCT08D is an AND gate chip, so if we connect the chip like the above schematic, the output signal must be as same as the input signal. So is this chip necessary for ENC28J06?

another question is If this chip is necessary, can I replace 74HCT08D with 74HC08D?

Thanks!

BR,
Wells.

2

They are used as level shifters between the ENC28J60 which runs at 3.3V, and the Arduino running at 5V

3

Thanks for your explaination!

But because the Arduino regards 2.4V and above as a logic high, I think there is no necessary to add level conversion for the MISO and INT signal. Is it right? :-[

4

Every single Arduino implementation for this chip I've seen has had the level shift hardware, so I used it in my design (thread just a few below your post here). That said, my design is not yet working, so who knows. :slight_smile:

If you DO get a homebrew implementation running, please post details as it may help me with my implementation.

5

There are more reasons that just whether the voltage level is high enough. The problem is without the buffer is that the Arduino's internal pull-up will take the ENC28J60's output to 5V. The absolute maximum rating for that pin is VDD +0.3V (VDD in this case is 3.3V). It may work, but it will put undue stress on the part.

Red is right that it will likely be read as high (although my datasheet says VIH (mininum input voltage for a high) is 0.6V. The ENC28J60 VOH (minimum output for a high is VDD -0.7V, in this case 2.6V.

Now to answer the other question, can a 74HC08 be used in place of a 74HCT08. Short answer is no.

74HC requires 3.15V for a high (74HCT is 2.0V). As stated above the minimum output of the ENC28J60 may only be 2.6V.

6

Thanks Williams,

I think I can understand the schematic now~
:smiley:

@ill_switch, if I make it successfully, I will post the detail infomation.
:slight_smile:

7

If you plan to use your diy ethernet shield with other SPI devices then you should replace the AND gates with a tri-state buffer. This will then properly isolate the pins.

For a detailed write up on this check out MISO and Arduino Ethernet Shields « John's Ramblings I did this myself in a slightly different way but never got round to documenting it like this.

I needed to run 2 SPI based devices together, the ethershield and a RF12 wireless module.

Hope this helps

Andy

8

Thanks AndyL, thanks for your suggestion.
after read your post, I studied the datasheet of ENC28J60 again.
I think this is the schematic which is mentioned by you.

It's so clearly that the MISO signal will not affect other SPI devices now.

:sunglasses: