Jellybean part for disconnecting 2 IO - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: July 17, 2018, 09:41:04 pm
content: I'm looking for a cheap part i can use as a digital logic DPST switch.Basically space is at a premium and i need to add a programming+uart pogo-based header to the project.I want to use the same two physical connections for USART and for the two JTAG programming lines.(micro is nRF52832)From what i can see, these could 'in theory' be shorted, as the USART will go tristate at reset and programming/debug modes are only activated at reset. However this might be a bit risky. Who knows what undocumented commands the programming pins might respond to.So i think i'll add a switch system to stop uart comms feeding into the programming pins during operation.Does anyone know of a super cheap, like <10c, part that is basically a digital DPST switch?(Note one of the programming lines is bidirectional)Thanks

author: tautech
author_rank: Super Contributor
timestamp: July 17, 2018, 09:49:48 pm
content: 4052 or 4056 too big ?

author: DaJMasta
author_rank: Super Contributor
timestamp: July 17, 2018, 10:10:51 pm
content: There are a lot of 1:1 switches that could be doubled up (or multiple in a package), then some 2:1 or 1:2 configurations, but if you have one of your two selections that can go tristate, you could also just use one SPST switch that's tristate to cover the other selection.Are they single directional in usage? Maybe some clever placement of Schottky diodes would suffice.

author: Mr.B
author_rank: Unknown
timestamp: July 17, 2018, 11:05:15 pm
content: I have used these before for a similar application:ON Semiconductor / Fairchild FSA2147K8Xhttps://nz.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/FSA2147K8X?qs=sGAEpiMZZMtxrAS98ir%252bs%2fWcmqYyr0irsUbXiDz78qg%3dHowever, pricing is about 10 times more than your requirement unfortunately. (5 times more in 1000 qty)

author: Psi
author_rank: Super Contributor
timestamp: July 18, 2018, 01:11:23 am
content: The clock pin is one direction, data pin is both.Yeah, it really needs to be a tiny package. ~3x3mm or somethingThis looks workable,https://www.digikey.com/product-detail/en/nexperia-usa-inc/74LVC2G66GN115/1727-7447-2-ND/7669985but would be nice if there was something more jellybean and from a common brand.

author: DaJMasta
author_rank: Super Contributor
timestamp: July 18, 2018, 05:53:11 am
content: I think the 2G66 would be the standard version of that? Seems like there's at least TI/NXP/ON Semi variants of it, though maybe not in a standard package...

author: daveshah
author_rank: Unknown
timestamp: July 18, 2018, 06:29:26 pm
content: Personally faced with the same problem I solved it in software and used SEGGER RTT over SWD rather than the UART for debugging an nRF with a pin constrained pogo pin setup. Not sure if that would work for you.

author: JS
author_rank: Frequent Contributor
timestamp: July 18, 2018, 07:13:39 pm
content: What rails do you have around? If you have an extra rail you could use a single mosfet, just be careful where the parasitic diode is pointing when it's off, or a JFET but the gate voltage must be more under control on those, much smaller range to work with.JS