ESP8684 Hardware Design questions - Page 1
Forum section: electronics / projects

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: October 11, 2023, 01:08:38 pm
content: Hello,I am recently designing a system that needs to have potential access to WiFi based on a STM32. For the WiFi bridge, I thought as a good candidate to be ESP8684, therefor I have a couple of questions.I already have a system clock of 26MHz, which seems to be the correct frequency the ESP needs to operate, with WiFi support, this comes from an Oscillator.Reading various ESP design manuals it seems like I should drive the XTAL_P pin from the Oscillator. Now some manuals from other ESP devices mention the use of a DC Blocking Capacitor,some other manuals mention the use of an inductor ( probably for the higher harmonics ). Searching on the Internet, others seem to have straight driven the oscillator signal into the ESP.In some other ESP devices manuals, it is mentioned that the amplitute should be kept within 1.2V ( makes sense for a 3v3 powered device with pre-biased input and using a DC block cap ).Does anyone have experience with something like this?I would also like to connect the ESP with my main microcontroller over SPI. It seems that there is a ESP SPI-slave "driver" program, at least for other ESP devices. I would be connecting tothe main microcontroller with the ESP anyhow with UART to provide for a programming Bridge, but I would like the SPI for faster data transfers and more robust communications, comparedto UART.Has anyone tried doing that with the ESP8684?It also seems that I have to be using in that scenario SPI2. If I understood that right, the SPI2 pins can be assigned to whatever pins I like, so I shall route the ESP as I like ( with bestpractices in mind of course ). Is my assumption correct here?And finally, watching a Phil´s Lab video ( highly recommended channel BTW ), he mentioned that the output impendance of the RF port of ESP32-C3 ( if not mistaken ) is 35 + 10j. SinceEspressif is using a balun / single ended line at the output, my guess is that the same would be for ESP8684. My problem is that both in the datasheet of ESP32-C3 mentions 35R +0j,should be an initial assumption point for measuring the port impedance. It also seems to be heavily depended on package, the QFN6x6 seems to have a port impedance of 30 + 10j and theQFN5x5 respectively a 35+10j port impedance.Cool... the ESP8684 comes in a QFN 4x4 package. So... lacking a network analyser on my desk ( I still have my liver in place ) what should I assume that the port impedance is?30 + 0j, 35 +0j, 30 + 10j, 35 + 10j?Now... I am going to say something many people would not like and I´d say... it doesn´t matter much. Using a PI filter what changes is the source side capacitance mostly and given the PCBparasitics, not by a lot I might also add. But... but... it´s good to know the exact. And it´s nice to know the exact because the ESP8266, which is a QFN 5x5 package ) has a port impedance of 39 + 6j.So it´s not like the imaginary part of the impedance can not change and ESP8684 is an upgrade / replacement to ESP8266, but still can not extrapolate from that.Excluding the SPI question, which might be a bit more involved, I would have hopped that the other two questions would have been answered by datasheets. So either google search has becometotal horses!st ( which it has, but that´s another thing ) or I am that bad at searching. Though for a microcontroller costing 1 - 1.2€ with embedded flash, it´s a hard pass not to just ask around a bit.With best regards,Lefteris

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: October 11, 2023, 02:53:20 pm
content: So... because of parasitics and very small variations in the values of the components,I will be going with a pi filter with 3p, 2,2nH, 2,2p looking from the port down to the antenna.Varying the values anywhere from 30 + 0j to 39 + 15j, with a quality factor of only 2, it should be good enough.If anyone comes up with more exact values for the RF port matching, please feel free.- Lefteris