How to isolate interface pins? - Page 1
Forum section: electronics / projects

author: Kohath
author_rank: Newbie
timestamp: November 11, 2019, 07:15:43 am
content: I'm trying to use the ZED-F9P (pretty cool u-blox GPS module) and one of the things they mention is that:QuoteVCC and all data lines are 3V3, so I don't need to translate voltages, and I can use a buffer for most of them (e.g.https://au.rs-online.com/web/p/buffers/9210275/), but the I2C line is bidirectional.How can I isolate a bidirectional line?I don't want to put on a big relay, and am unsure about the IC to search for. Looking at voltage translators and buffers, I'm just getting more confused.

author: Kohath
author_rank: Newbie
timestamp: November 11, 2019, 07:22:42 am
content: Maybe I should just buy some of the autobidirectional voltage translators like TI says (https://training.ti.com/tlm-bot-choosingautobidirectional?cu=1134826), and the C099-F9P (evaluation board for the ZED-F9P) schematic uses SN74CB3Q16244DL, and they have changed that to some YF08E's on the actual boards we got. I'd like to get some from RS Components, but really just want to understand more about what to look for.

author: Berni
author_rank: Super Contributor
timestamp: November 11, 2019, 07:35:50 am
content: This is not a special problem for the uBlox F9P it applies to almost all chips.Its that ESD protection diodes tend to be placed between every pin and the supply rails. This makes sure that the IO pin never goes far outside of GND or Vcc as that could damage the chip. The problem is that if you power off the chip then Vcc and GND are at 0V and so driving a logic 1 into a pin will cause these diodes to conduct current into the Vcc rail. Because of this its actually possible to power most ICs by connecting GND and Vcc to any pin you like and it will work. But if too much current flows these diodes can blow up.Yeah for I2C this is a bit unfortunate since it has pullups. You won't damage anything by powering down the uBlox module, but the unpowered module will likely sink current into the SDA and SCL lines and pull both of them low causing the whole bus to stop working, so you can't communicate to any other device at all until you power the module back up to make it let go of the bus. Using one of those bidirectional I2C level shifters won't solve the problem because the level shifter will think that the uBlox is trying to transmit a logic low on the bus and allow it to do so.The real solution is to use a analog switch or MUX (Like for example a 74HC4066) to disconnect the SDA and SCL lines and control the switch from the same signal that switches the power to the module on and off.By the way any cool project you have with these RTK capable GPS modules? They have some pretty amazing performance.EDIT: Oh and for other lines like UART the fix is a lot simpler, simply reconfigure the MCUs IO pins for the UART to stay logic low or configure as input pins. If the pin is at 0V then it wont push any current trough the ESD diodes and you are safe.

author: Kohath
author_rank: Newbie
timestamp: November 12, 2019, 01:13:16 am
content: Ahh − Nice one Berni! So the ESD diodes provide a path from IO pin to VCC and they aren't rated for that continuous current. I want to have all of those lines open, and I haven't switched the ZED-F9P VCC, but perhaps I should... In any case, I will get some HEF4066BT,653 and put all my lines through them.About the project, nothing really innovative - just working on a GPS product for the agriculture sector.Talking about TVS, it's funny because I planned a PCB with unidirectional TVS on the RS232 lines, and was wondering why I was getting garbled output. And recently I bought and soldered a 200kHz oscilliscope from AliE (e.g.https://www.aliexpress.com/item/32878694776.html), and the penny dropped...

author: Berni
author_rank: Super Contributor
timestamp: November 12, 2019, 03:29:53 pm
content: Well if you never switch off the Vcc to the Ublox F9P then none of this matters anyway as long as you run everything from the same 3.3V rail. The only pin to watch out for on these is the Reset line since handling that in the wrong way can cause it to loose the GPS almanach in memory despite being provided backup power.And if you work with electronics properly i suggest getting a real scope for 400 bucks. Without a proper scope you are often working blind.