---
title: "Using W5100 with SPI bus"
url: "https://maker.wiznet.io/forum/10463"
markdown_url: "https://maker.wiznet.io/forum/10463/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "atlantis"
created: "2014-08-21T17:44:43+09:00"
last_activity: "2020-09-07T09:16:40+09:00"
language: "en"
views: 2267
replies: 6
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# Using W5100 with SPI bus

## Question

Asked by atlantis on 2014-08-21 in Ethernet Chips.

Currently I am designing my own device with W5100 chip and Atmega644 micronontroller.
I would like to utilize SPI bus for communication between both chips. I’ve already read [datasheet](https://www.sparkfun.com/datasheets/DevTools/Arduino/W5100_Datasheet_v1_1_6.pdf), I’ve seen [reference schematic](http://www.wiznet.co.kr/UpLoad_Files/ReferenceFiles/W5100_Ref_sch_MAG_R2.1.pdf) as well, but few issues are still unclear for me.

1. What should I do with /CS, /WR and /RD pins, when I am using SPI instead of parallel bus? Should I left them unconnected, or rather pull them up to VCC with single resistor, just like in case of [Arduino Ethernet](http://arduino.cc/en/uploads/Main/arduino-ethernet-R3-schematic.pdf) board?

2. What should i do with SEN pin? Should I simply pull it up to VCC? I am asking, because there is some strange solution with inverting buffer on the Arduino Ethernet board schematic.

3. Should I left DATA pins unconnected, or connect them to GND, just like on the schematic of Arduino Ethernet?

4. Can I connect GNDA pins to the common ground plate, without using ferrite bead?

5. Is it possible to use single SPI bus for W5100 communication ans ISP Atmega programming? I mean, does chip sets high-Z state on MISO, when SPI is not used (SCS line in not active state)?

## Replies

### Reply 1 by hjjeon0608, 2014-08-29

Hi,

1. You should control CS when you access the W5100. SPI needs it also.
   You let RD and WR unconnected, but I recommend you pull them up.

2. Yes.

3. Both unconnected and connect them to GND are possible, but I recommend you pull them down.

### Reply 2 by hjjeon0608, 2014-08-29

1. Yes. But I recommend use bead.

2. If CS is high(1), MISO is high-Z.

### Reply 3 by atlantis, 2014-08-30

I am not so sure about /CS pin. Is it possible, You simply mistaken it with /SCS pin?
In [W5100 datasheet](https://www.sparkfun.com/datasheets/DevTools/Arduino/W5100_Datasheet_v1_1_6.pdf) (page 9) /CS pin is described in context of /WR and /RD pins. On the [Arduino Ethernet Shield schematic](http://arduino.cc/en/uploads/Main/arduino-ethernet-shield-06-schematic.pdf) this pin is simply pulled up to VCC. /SCS is used with SPI instead.

### Reply 4 by hjjeon0608, 2014-08-31

I’m sorry.
I mistake.

I think CS as SCS.
The description you said is right.

Thanks.

### Reply 5 by btprg, 2020-09-03

Hallo! Just a related question: in SPI mode why recommends the reference schematic to pull SEN to 3.3V with a 4.7k resistor? (Also for unused ~WR, ~RD etc.) Why cannot I connect SEN and other inputs directly to 3.3V? Thank you!

### Reply 6 by btprg, 2020-09-07 (in reply to reply 5)

up! zzz

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Source: https://maker.wiznet.io/forum/10463