Connecting Rx pins together on a bunch of Pic Micros - Page 1
Forum section: electronics / general

author: mrcrud5
author_rank: Contributor
timestamp: May 01, 2018, 05:44:42 pm
content: Hi guys,I have a project I am working on in which I'd like to have 8 pic microcontroller EUSART RX pins connected together all receiving data from one source. This may be a silly question, but I just want to make sure there is nothing wrong with doing this. The reason I ask is because this is something I've never done before and I'd like to be sure there is nothing I'm missing before I go forward with this design. I cant seem to find anything saying not to do this in the data sheet. At the moment I plan on connecting the pins directly to each other. Should I maybe be using some series resistors or diodes?Anyways thanks for the help!

author: Howardlong
author_rank: Super Contributor
timestamp: May 01, 2018, 06:15:45 pm
content: No problem at all assuming you’re not running super high speeds and the devices are physically an extraordinarily long way away from each other. I’m also assuming they’re all on the same board and power domain.

author: mrcrud5
author_rank: Contributor
timestamp: May 01, 2018, 06:32:38 pm
content: Quote from: Howardlong on May 01, 2018, 06:15:45 pmThanks for the reply! Yep, they are really close together and speed isn't too high. Each board is identical is will be meant to stack on top of each other.

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2018, 08:25:46 pm
content: Any input pin presents some sort of load on the circuit driving it, so in theory you could load down your source enough to cause problems, but in practice that would likely require more ICs than you would reasonably try to connect together. If in doubt, use a scope to take a look at the signal to make sure it's not too ugly.

author: Howardlong
author_rank: Super Contributor
timestamp: May 01, 2018, 09:06:28 pm
content: Back in the days of bipolar TTL, fanout was a real problem, particularly going from a lower power to full fat TTL. This was due to static loading, each TTL input took a significant amount of current.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: May 03, 2018, 10:24:28 am
content: Can you guarantee that at power up, all these rx pins are inputs?If (more than) one is output for only a fraction, that could cause trouble...

author: iMo
author_rank: Super Contributor
timestamp: May 03, 2018, 10:30:31 am
content: I always put resistors into Rx/Tx lines. Resistor's value depends on the baudrate, 1k works fine with 115k2. That may help with ringing and in your case with collisions as mentioned above.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 03, 2018, 10:43:09 am
content: Basically all MCUs guarantee that the pin output drivers are disabled during and after reset. If otherwise, it should be clearly documented in the datasheet. I'm quite sure it's not an issue on a PIC UART pin. Just be careful not to configure them as outputs by accident.Using a series resistor (such as 47R-100R) at the driving source as series termination would help with EMI and ringing, but not probably required.1k is beyond reason for series termination or short protection within a PCB where excessive voltage sources do not appear. 100R at each pin would do the job.

author: Howardlong
author_rank: Super Contributor
timestamp: May 03, 2018, 03:40:58 pm
content: FWiW, on lines with CMOS single ended fast rise and fall times, my automatic rule of thumb is a 33 ohm damping resistor close to the outputs to reduce ringing and overshoot, and please the EMI folks.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 03, 2018, 05:08:27 pm
content: I have yet to see any microcontroller that wouldn't guarantee its GPIOs (unless they are output only) to be in Hi-Z state upon reset. If they weren't, most designs out there would fry, and having to protect them from this issue would be extremely costly overall.Modern µCs IO's can drive several mA of current (source and sink) and their input capacitance is usually in the order of 10-20 pF, so unless you're aiming at the 100 MHz+ frequency range, you're not likely to have any issue faning a single output out to a dozen inputs.As suggested above, series resistors (depending on the data rate) are a good idea to lower EM emissions.