Rant:  Top View vs Bottom View on Datasheets. - Page 1
Forum section: electronics / general

author: JoeN
author_rank: Frequent Contributor
timestamp: March 28, 2017, 05:08:15 am
content: Why would anyone do a bottom view on a datasheet as the primary source of the pinout diagram? It doesn't seem common, but Linear still has it on some of their components (LTZ1000, LT1029, etc.) Maybe it is for the TO packages only? I don't understand why you would want it under any circumstances, it is just confusing to me. Does anyone like it like this? Here is an example where they mix and match views:On the LM399, they have gone with a top view:LTZ1000, which is a similar can, bottom view.No rhyme or reason...

author: Berni
author_rank: Super Contributor
timestamp: March 28, 2017, 05:30:33 am
content: Yeah it makes no sense. The bottom view only confuses people.I have a worse one tho. Its a Gyro/Accelerometer/Compas IMU chip from Bosh:https://ae-bst.resource.bosch.com/media/_tech/media/datasheets/BST-BMX055-DS000-02.pdfCheck page 155. For some reason they number all the pins in the clockwise direction!I had already drawn its annoying LGA footprint in Altium and then i also wanted to mark down the axies directions inside the footprint so i know where to point it on my PCB. But as i read the diagram on Page 161 nothing seams to come together, how is pin one on that side? Have they accidentally mirrored the picture of the chip in that diagram? Hmm it looks like that on the front page too... Whats going on here? Wait... the pins go the other way around? YOU CHEEKY BASTARDS!Have i not tried to mark down the accelerometer axies on my footprint i probably would have not realized this and went on to produce a PCB with a mirrored footprint for it.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 28, 2017, 05:32:01 am
content: Look at the bright side. It is labeled. And makes some sense for TO-3 and similar since you can't see or easily access the pins from the top. You will probably be coming from the side or bottom when probing, unlike the various flat paks, DIPs and so on where you will be coming from the top. BGA would be the one to break that pattern. And at least a few do, by showing both top and bottom views - labeled.

author: JoeN
author_rank: Frequent Contributor
timestamp: March 28, 2017, 05:44:28 am
content: Quote from: Berni on March 28, 2017, 05:30:33 amYeah, that is a pretty weird screwup. The rule for quad packages is simple, you put the dot in a corner, the pin next to it is 1, and then you go counterclockwise. Hard to screw that up.I also don't like that the first pinout is on page 155. Pinouts are usually much closer to page 1 for every manufacturer I have ever seen, even for really complex devices.

author: Berni
author_rank: Super Contributor
timestamp: March 28, 2017, 06:13:09 am
content: Quote from: JoeN on March 28, 2017, 05:44:28 amYeah that's also one of my gripes about some manufacturers datasheets that don't stick to the standard datasheet formula of where things are,That reminds me of another documentation failure. There is a pressure sensor that marks the pinout on the mechanical drawing. Its a bit odd but hey it works fine, but the problem arises that there are two possible pinouts marked simply as A or B and despite going trough the entire datasheet multiple times i could not find if i am supposed to use the A or B one. What made things worse is that the A and B one had power supply pins in different locations. In the end i got fed up and soldered some wires to the sensor and applied power according to the A pinout using a low current limit, PSU went in to over current so i changed it around to the B pinout and it was drawing normal current and it had a output signal so i used that pinout.

author: DimitriP
author_rank: Super Contributor
timestamp: March 28, 2017, 07:33:06 am
content: It's makes sense. The drawing shows labeled the visible pins.Quote from: CatalinaWOW on March 28, 2017, 05:32:01 amWhat he said ^^^^

author: capt bullshot
author_rank: Super Contributor
timestamp: March 28, 2017, 07:44:56 am
content: I'd consider bottom view a relict from the past when you were making layout using sticky tape etc. As you've done this (at least for simple single sided boards) mirrored, the bottom view actually was somewhat helpful. Using todays CAD tools it's just a pita.

author: bktemp
author_rank: Super Contributor
timestamp: March 28, 2017, 07:57:13 am
content: I find many mechanical drawings in datasheets even more annoying:Often only the dimensions between individual points are given. So you need to add dimensions A+B+C and subtract E and D to get the distance between G and H.And you have to do this for almost every pin/line/edge/circle you want to extract from the drawing. And often you have to combine dimensions from top+bottom views.I did some footprints for SD and micro SD card slot. Because of the non equal pin spacing and the complex shape it took hours to get all the dimensions right. I even had to ask others to help me understand the drawing and find some missing dimensions (in one example I had to guess one dimension, because it was missing completely).I know a correct mechanical drawing must not overspecify dimensions otherwise the tolerances will not add up, but it would be really great in many cases to have an easy to read drawing with typical dimensions all referenced to a single point (maybe one edge or the centre) for generating the footprint.I haven't seen any really good mechanical drawing in a datasheet for an electronical component. Many datasheets are ok, but there are also a lot of datasheets with either missing or completely obfuscated dimensions.

author: JoeN
author_rank: Frequent Contributor
timestamp: March 28, 2017, 07:59:03 am
content: Quote from: capt bullshot on March 28, 2017, 07:44:56 amYeah, single sided copper on the bottom boards, I guess that makes sense. You don't see those very often anymore, unless you are putting together Velleman kits (which I did recently with my niece). Even the cheap Chinese eBay kits are now double-sided FR4.But, to completely analyze this to exhaustion, that logic would be equally applicable to all through hole parts like DIPs. I've looked up a few Linear datasheets on DIP parts and all of those have the top view, as well as some of the can parts like LM399.Oh well...

author: Brumby
author_rank: Unknown
timestamp: March 28, 2017, 08:28:06 am
content: Can't say this issue has really bothered me. So long as I know which way is up.Quote from: CatalinaWOW on March 28, 2017, 05:32:01 amYep - that says it.

author: TerraHertz
author_rank: Super Contributor
timestamp: March 28, 2017, 09:54:42 am
content: With any multi-leaded through hole component (eg TO-92) I have to layout on a PCB, I do a little 3D sketch to make sure I have the pins ordered right. Usually put this on the schematic too.So, I don't care at all which way the datasheet shows it. Just so long as it's correctly labeled top or bottom view.

author: Prabhat
author_rank: Newbie
timestamp: April 14, 2023, 09:37:41 am
content: Oh. This is a common problem. Thank god. I am feeling like shit today because I just destroyed a 15k Euros PCB board because of pinout top/bottom views.I have done many passive antenna PCBs before but this was my first active antenna board (12 layers, RF at the top and bottom, analogue, digital and baseband at middle layers, 20 GHz antenna array PCB with electronics for beamforming). Unfortunately, the whole PCB is a piece of junk now because I assumed that the beamforming chip pinout was top-view but it is actually a bottom view and my chip layout in the PCB is flipped (It is a BGA chip with 9x11 400um bumps). The worse part is that the datasheet does not say if the pinout is a top or bottom view. When the prototype did not work during testing, we guessed that the pinout might be wrong and when we asked the chip manufacturer they say "It is a bottom view". And puff.... 15k Euros gone. The last 12 months of my work gone.

author: tggzzz
author_rank: Super Contributor
timestamp: April 14, 2023, 04:33:04 pm
content: Quote from: Prabhat on April 14, 2023, 09:37:41 amShame it wasn't a through-hole component. If it had been you could have tried the board simply by mounting it on the other side of the board.That does work; I saw it used 35 years ago when the manufacturer (AMD IIRC) issued a preliminary datasheet for a prototype component with the pinout as viewed by the semiconductor mob

author: MarkS
author_rank: Unknown
timestamp: April 14, 2023, 06:41:49 pm
content: This has bitten me before. I learned the hard way to pay attention to that. It's very annoying.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 14, 2023, 07:58:06 pm
content: What is top and bottom anyway? Do the australians not live upside down?

author: tggzzz
author_rank: Super Contributor
timestamp: April 14, 2023, 10:15:36 pm
content: Quote from: SiliconWizard on April 14, 2023, 07:58:06 pmThey don't think so. But then they do think that during the course of the day the sun goes from right to left.

author: TimFox
author_rank: Super Contributor
timestamp: April 14, 2023, 10:45:40 pm
content: Bottom view, with pins labeled in a clockwise direction, was the standard for vacuum-tube base diagrams, since tube sockets were always wired from the bottom.Old conventions can live on.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 15, 2023, 01:52:35 am
content: Quote from: TimFox on April 14, 2023, 10:45:40 pmExactly, & it made perfect sense, as analog clocks all were numbered that way.If you were probing tube sockets from the top, you just (easily) kept in mind that the rotation was a mirror image of the normal diagram.With the first DIL ICs, as people trying to troubleshoot them were as likely to be probing the pins from the top or bottom of the PCB, it again made sense to maintain the clockwise diagram, as those brought up on tubes would automatically make the adjustment.It was found, though, that much probing was done from the top, so it became more common to show the top view, with the numbers naturally rotating anticlockwise.Discrete transistors were normally probed from the bottom, so it became natural to draw them, or anything else in a similar package from the bottom view.If you want something confusing---back in the day, optocouplers were commonly packaged in a 6-pin package (DIP 8 with two missing pins).As they were seemingly all pin for pin compatible, & many had very similar characteristics, it became relatively common to swap types with little chance of a problem.Siemens, however, decided to go their own way with a totally different pinout to optos which were otherwise similar to "jellybean" ones.A "trap for young (& old) players!

author: vk6zgo
author_rank: Super Contributor
timestamp: April 15, 2023, 01:56:18 am
content: Quote from: tggzzz on April 14, 2023, 10:15:36 pmIt does that if you look towards the North, no matter where you are.

author: tggzzz
author_rank: Super Contributor
timestamp: April 15, 2023, 04:14:27 pm
content: Quote from: vk6zgo on April 15, 2023, 01:56:18 amNot if you are standing on your head.If you can'tseethe sun moving, it doesn't count