[0:00] Recently, I’ve been messing around a lot of recently with the ESP32-S3 - it’s a great module to
[0:05] work with as there’s no need for a USB-UART chip.
[0:09] This means that we can get away with really simple PCBs - all you need is a Low Drop Out
[0:13] regulator and some decoupling capacitors.
[0:15] There are a lot of things to like with the S3 - it’s really flexible as it has a GPIO
[0:21] matrix for connecting pins to its peripherals.
[0:23] This means that, in general, you don’t need to worry too much about which pins you use
[0:27] for things like I2C or SPI.
[0:30] But, as always, there are some gotchas that can really trip you up.
[0:34] There are some really great guides out there for the original ESP32 - one of go to ones
[0:39] for me is the Random Nerd Tutorial page.
[0:41] But I haven’t really found a good one for the S3.
[0:44] So I thought I’d make one.
[0:47] I’ve put mine up on GitHub and I’m hoping we can have a bit of a community effort to
[0:51] gather everyone’s knowledge into one place.
[0:54] So, let’s run through what we know so far.
[0:56] I’ve gleaned a lot of this from the various data sheets, personal experience, and of course
[1:00] great input from all of you.
[1:03] There are some pins that you definitely need to be careful with.
[1:06] The most important of these are the strapping pins.
[1:09] GPIO0, 45 and 46 are the three most critical ones.
[1:14] Mess these up and you’ll probably have a dead board.
[1:17] GPIO0 is probably the one that you are most familiar with - it’s the pin that almost all
[1:22] dev boards will connect a push button to labeled “boot”.
[1:26] Holding this pin low when the board is powered on or reset will put the ESP32 in to firmware
[1:31] download mode.
[1:33] In previous ESP32 dev boards a clever arrangement of transistors could be used to replace this
[1:38] button dance.
[1:39] With the S3, in theory, pushing the buttons should not be needed.
[1:43] You can program the board directly through the USB pins.
[1:47] However, my experience, and anecdotal information from other people, indicates that this can’t
[1:52] be relied on.
[1:53] So you may sometimes find that you do need to force the ESP32-S3 into download mode.
[1:59] The value of GPIO46 can have an impact on the boot mode.
[2:04] This handy table gives you combinations of values - an important thing to note is that
[2:08] having GPIO46 high is not a valid if you want to go into firmware download mode.
[2:15] GPIO45 controls the voltage that is used for the internal flash - unless you really know
[2:20] what you are doing then messing with this pin can cause all sorts of interesting issues.
[2:25] GPIO3 is the last strapping pin, and arguably, less important, it only has an impact if you
[2:32] have burned some eFuses.
[2:34] If you have burned the appropriate fuses then GPIO3 can be used to switch the default JTAG
[2:40] debugging pins off.
[2:42] An important thing to remember about these pins is that apart from GPIO3 they all have
[2:46] either pull-up or pull-down resistors on them.
[2:49] GPIO0 has a pull up resistor and GPIO45 and GPIO46 both have pull down resistors.
[2:57] So that’s the strapping pins - my advice is to just not use GPIO45 or GPIO46 - leave them
[3:05] disconnected.
[3:06] But, there are some more pins that you need to be careful with.
[3:10] Two very obvious ones are the USB data pins, GPIO19 and GPIO20 - On most dev boards these
[3:18] won’t be broken out at all, but if you are designing your own PCB then it’s worth being
[3:23] aware of them.
[3:24] And if you use them in your code you will interfere with any USB communication.
[3:29] When I was playing with the Voltera and printing an S3 PCB at home I discovered some more pins
[3:33] to be careful of.
[3:35] GPIO pins 39 to 42 are used for JTAG debugging.
[3:40] Until you start using them, these pins can have unexpected values on them.
[3:43] There are also a couple of default UART pins - these will be used for UART0 unless you
[3:48] use them for something else.
[3:50] The final thing to watch out for, as I found to my cost, is only relevant if you are using
[3:55] an S3 module with octal PSRAM - these are the modules that have 8 megabytes of extra
[4:01] RAM.
[4:02] With these modules you cannot use GPIO35, 36 or 37 at the same time as PSRAM.
[4:09] If you do try and use them then your code will crash.
[4:13] I used these pins on my ESP32-TV and it was only after I got the boards manufactured by
[4:18] PCBWay and tried to enable PSRAM and then access the SD Card that I realised my error.
[4:24] Fortunately the TV firmware doesn’t require PSRAM so it wasn’t a complete disaster, but
[4:29] I am still going to redo the PCBs to avoid these pins.
[4:33] So we’ll have another board coming from PCBWay in the near future.
[4:37] And if you want a PCB for yourself, then check out the PCBWay link in the video description.
[4:43] That’s really it for pins that you need to be careful about.
[4:46] During boot time there are a surprising number of pins that have very short glitches on them
[4:50] - GPIO1 through to GPIO20 all have some kind of glitch, either a very brief low level or
[4:57] brief high level glitch of around 60 micro seconds.
[5:00] For most use cases, this probably won’t have any effect, but it could potentially cause
[5:05] any hardware connected to them to get into a funny state.
[5:09] I’ve documented all of this on the GitHub repo.
[5:12] One of the nice things with the S3 is that there are far fewer restrictions on the pins
[5:16] that you can use.
[5:18] There are some interesting things in the data sheet that need further investigation.
[5:22] I mentioned earlier that there is a GPIO matrix that means you can use any of the pins for
[5:27] the S3’s peripherals.
[5:29] I’ve made a lot of use of this on the ESP32-TV and everything has worked really well, however,
[5:34] there is also an IO mux.
[5:36] This is much less flexible than the GPIO matrix, but may offer improved high speed digital
[5:42] performance - though I have seen some messages on the forums that it may not make that much
[5:46] difference.
[5:47] Let me know in the comments if you’ve had any experience of this.
[5:50] And please take a look at the GitHub repo and open an issue or even better a pull request
[5:55] if you can see something that can be improved.