--- title: "Pull ups, pull downs, schematic stuff" url: "https://maker.wiznet.io/forum/10592" markdown_url: "https://maker.wiznet.io/forum/10592/md" type: "Forum topic" category: "Ethernet Chips" author: "alexpescaru" created: "2014-10-12T21:01:41+09:00" last_activity: "2018-09-07T08:41:00+09:00" language: "en" views: 3176 replies: 10 source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)" --- # Pull ups, pull downs, schematic stuff ## Question Asked by alexpescaru on 2014-10-12 in Ethernet Chips. In your reference schematics, there are a number of 10K pull ups and pull downs. (PMODEx, RSVDx) I’ve seen in other implementations (Parallax for example) that those pins are directly connected to VCC and GND. Is it safe for the chip to work without them? I suppose that internally there is a microcontroller that, until it finishes its initialization process, those pins are not yet configured as inputs. Question is: until the configuration is done, is there any chance for they to behave like outputs and therefore they need to be connected through resistors? Thank you! ## Replies ### Reply 1 by kevinlee, 2014-10-12 Hi! alexpescaru, The Reference schematic you attached is older version. Please refer to the latest Reference schematic. In the latest circuit, resistors are not used for RSVD pins. Here is the link. [url][http://wizwiki.net/wiki/doku.php?id=products:w5500:refschematic\[/url\]](http://wizwiki.net/wiki/doku.php?id=products:w5500:refschematic%5B/url%5D) Thank you, Kevin ### Reply 2 by alexpescaru, 2014-10-13 Thanks all! And Kevin, you posted a link to the RJ45 section of the schematic. I was talking about the chip section. In the link you posted there is no RSVD… Or I didn’t looked in the right place? ### Reply 3 by kevinlee, 2014-10-13 Hi, I’m sorry that I couldn’t let you find the right schematic. There are two types of Reference Schematic for W5500 and related circuit. Please refer to these links. (PDF format schematic) [url][http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-externalmag_20140808.pdf\[/url\]](http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-externalmag_20140808.pdf%5B/url%5D) [url][http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-rj45withmag_20140808.pdf\[/url\]](http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-rj45withmag_20140808.pdf%5B/url%5D) Thank you, Kevin ### Reply 4 by alexpescaru, 2014-10-13 Thank you very much! Now I see. And two more questions, if I may… The capacitor for the 1.2V internal regulator, the 10nF one on pin 22. Is this value of 10nF critical or can be used one of 100nF (0.1uF), like the ones for decoupling AVCCs? Generally, for a voltage regulator output, a bigger capacitance is OK. In the schematic with external transformer… Even if the central taps are separated, on the RX side, you used 82 ohms resistors. I believe that you said that those 82 ohms are used only when the two central taps are interconnected. Why in this case, with separate central taps, you still using 82 ohms? Thank you! ### Reply 5 by jeongsk, 2014-10-14 Hi~ I will answer to the questions. You can use 100nF I used 82ohms and 6.8nF if I used the external transformer. What made you worried? Thank you!! ### Reply 6 by alexpescaru, 2014-10-14 From what I’ve understood, 82 ohms were used only when the central taps were interconnected. Like in the picture here: [wizwiki.net/wiki/doku.php?id=pro … fschematic](http://wizwiki.net/wiki/doku.php?id=products:w5500:refschematic) I was surprised to see 82 ohms when central taps were separate. Should I understand that I can also use 82 ohms (instead 49.9 ohms) when I am using a RJ45 with internal transformer with separate center taps? In the end, how critical are those resistors from value point of view? ### Reply 7 by alexpescaru, 2014-10-15 Could someone from Wiznet please clarify this? The two cases or reference schematic, one with internal transformer and one with external transformer. For internal transformer you used 49.9 ohms for RX side. For external transformer you used 82 ohms for RX side. Why is that? Because the characteristics of the transformers, both internal and externals, are the same: 1:1 ratio, 350uH, separate central taps for RX and TX. Again, how critical are the value of those resistors and how can they influence the W5500? Thank you! ### Reply 8 by jeongsk, 2014-10-16 Hi~ The Ref Schematic provided by us is under 2 kind circuit. External Trasfotmer :[url][http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-externalmag_20140808.pdf\[/url\]](http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-externalmag_20140808.pdf%5B/url%5D) Internal Tranformer :[url][http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-rj45withmag_20140808.pdf\[/url\]](http://wizwiki.net/wiki/lib/exe/fetch.php?media=products:w5500:w5500-ref-rj45withmag_20140808.pdf%5B/url%5D) It is due to R16,R17 that resistance of the internal transformer and external transformer is different. [![](https://maker.wiznet.io/forum_uploads/wiznet/original/1X/d73f3c217f6fa70d9fe1d96c2db095d8ca9ace5c.png) 895×546 63.1 KB](https://maker.wiznet.io/forum_uploads/wiznet/original/1X/d73f3c217f6fa70d9fe1d96c2db095d8ca9ace5c.png) The R16 and R17 is in the external circuit but there is no 150 ohm resistances in the internal circuit. It is that it is changed to 49.9 ohms in a in order to compensate for 150 ohms. the role of 49.9 ohms the High pass Filter with 6.8nF. If you change a value, the probability that W5,500 this doesn’t operate is high. And we don’t recommend the following circuit officially. It is that I comprise this circuit and European 1 customer explains the used example one. ### Reply 9 by alexpescaru, 2014-10-16 Thank you very much! Now it’s clear. ### Reply 10 by Matheus Eduardo, 2018-09-07 [@jeongsk](https://maker.wiznet.io/u/jeongsk) [@midnightcow](https://maker.wiznet.io/u/midnightcow) hat’s a little confusing, do we need to place those 150 Ohms always before the external magnetic? Could we simply wire the external magnetics the same way as the internal magnetics so we don’t need to change the R11-R12 to 82 Ohms? --- Source: https://maker.wiznet.io/forum/10592