"Remote Sense Z-Short" on Xilinx AC701 - Page 1
Forum section: electronics / general

author: Charles Swain
author_rank: Newbie
timestamp: April 02, 2020, 05:14:41 am
content: If anyone could please help me interpret this unfamiliar schematic symbol and whether this is some kind of layout instruction, then that would be fantastic.So I'm fairly new to FPGAs (no "real" experience outside SystemVerilog on a Cyclone III evaluation board) and decided to lean on Xilinx and use their products since they had a large amount of technical documentation available for them. So I'm fairly far along in the process, over 8 months in, and I've been parsing through the reference designs for the AC701 "Evaluation Board for the Artix-7 FPGA" (specifically an XC7A200T-FBG676, which is in the same family of Artix-7 FPGAs as my choice for my project). For the past month, all I've been doing is cross referencing everything in the first revision schematic of the board (XTP218) and a user guide (UG952) with everything in all the other Xilinx documents for the FPGA family, along with reading the datasheets for every single other part featured on the board. Of particular interest to me has been the power circuitry. Through some overseeing PMBus controllers managing over half a dozen regulators, power is provided to each rail for the FPGA, memory, many of the connectors, and what's called an "FMC" on the AC701. It has been an absolutely amazing learning experience that is putting me on track to actually provide a board with a powered up FPGA on it. Granted, the controlling circuitry is quite a lot more complicated and expensive than what would probably be a more "dumb" hard-wired power rail circuit and has great potential with the wrong commands to explode itself or the devices connected, and is extremely inefficient for a much more typical use application and has no guarantee of actually working with the particular use case, but has been in my eyes a worthwhile learning experience none the less that I'd like to continue if it weren't for a particular road block that I'm going to go into more detail about right now.I am currently stuck on what the meaning of what each instance of the part "0201_SHORT" is.Above is an example of what one looks like on the schematic, as it would appear on sheet 40 in the voltage sense circuit for VCCINT's regulator. Doing a practice board spin with this regulator circuit for powering my Artix-7 would be a nice exercise, even if I don't actually have the board made, but I need to understand what every instance of this part symbol in the regulator sheets means for the design and layout. In each PMBus controlled regulator's voltage and current sensing circuitry the 0201_SHORT symbol appears. There is even some text in the voltage sensing cases that I'd assume are addressed to the person doing the board spin: "PLACE REMOTE SENSE Z-SHORT CLOSE TO FPGA POWER PIN." All of these examples can be seen on sheets 40-44 and 46-50. Getting an answer to what this kind of symbol means would be great, since although the user guide, UCD90120A's datasheet, and each regulator's datasheet take the time to explain the presence of scaling circuitry and amplifiers on the voltage sense and current sense circuits, respectively, there doesn't seem to be a decisively clear and quick explanation for what a symbol like this would mean on a schematic as it is placed. I'm sure that if I kept searching and interpreting this it would become obvious after awhile, but considering there does not seem to any obvious examples of people encountering these things on other schematics and I haven't spoken to someone in awhile on this I'd like an outside opinion. My first unsure guess is that this means "create as strong a short as possible (as little trace length) between the lands for the corresponding power pins/balls on the FPGA and every pin on the *_SENSE_P nets," but the power controller is kind of big and my common sense it telling me not to put it near the FPGA. So then my attention turns to the feedback circuit that exists and how this connects to the feedback pins on regulators like the LMZ22010TZ, which appears as the regulator for VCCINT in the AC701. The datasheet for that part goes on length about how you should minimize trace lengths and keep part placement tight, especially in the feedback circuit, to avoid current loops and interference. The feedback pin is actually connected to the trimming, voltage sense, and power rail circuits via a resistor network with a switch for enforcing a low power mode. My second unsure guess is that the z-short symbol means "create as strong a short as possible (as little trace length) between either the FPGA power pins OR the start of the power rail itself at the regulator circuit AND any of the circuit corresponding to this feedback system presented (excluding or including the controller's monitor pin itself)." These are just pure guesses, however, and I need an actually accurate answer to proceed in good confidence explaining this feature of the drawings to myself and others.Well that is all I had to say on the matter, so far. For anyone interested in helping me who isn't already familiar with the AC701 or any of Xilinx's other very similar demonstration boards for their other product lines, the schematics can be found through accessing Xilinx's website and agreeing to their license for it. The following are links to the particular documents or places where the documents or schematics can be found:AC701 (including schematic):https://www.xilinx.com/products/boards-and-kits/ek-a7-ac701-g.htmlAC701 user guide:https://www.xilinx.com/support/documentation/boards_and_kits/ac701/ug952-ac701-a7-eval-bd.pdfUCD90120A datasheet:https://www.ti.com/lit/ds/symlink/ucd90120a.pdfLMZ22010 datasheet:https://www.ti.com/lit/ds/symlink/lmz22010.pdf

author: Someone
author_rank: Super Contributor
timestamp: April 02, 2020, 09:33:54 am
content: They are using it as a net-tie component so the remote sense routing isn't ripped up/flooded by the PCB layout tools.https://en.wikipedia.org/wiki/Four-terminal_sensinghttps://resources.altium.com/p/using-net-ties-to-meet-pcb-design-requirements