2017-08-23

What ESP8266 modules should look like

Context

I've been using ESP8266 modules over the last 2 years for various IoT projects. These modules come with a number of very annoying characteristics which make their adoption problematic to beginners :

  1. they don't boot by default, it is mandatory to solder a number of wires and pull-up resistors just in order to get them to boot.
  2. their pitch is not 2.54 mm, so it's not possible to plug them on a breadboard to simply connect the required resistors and wires
  3. there are two pins (RST and GPIO0) to change between operation and programming, so when you soldered to achieve point 2 above, you have to desolder and solder differently just to flash, keeping a flying wire for the reset which must not stay connected (obviously)
Some vendors understood this and started to propose very nice development boards like the NodeMCU or Wemos D1. The NodeMCU clearly is too big to serve as a production model, but it exposes all I/O, integrates a USB UART, a "flash" and a "reset" button, and connects the UART's RTS and DTR signals to the RST and GPIO0 wires so that the programming software can automatically toggle these lines to program. The Wemos D1 provides all these features except the "flash" button, and is half as small. In fact it's only twice as large as the ESP12 module. This one may be used as-is for some projects.

But both boards present a big problem : the USB UART cannot be disconnected and it draws a lot of power. So if you want to use these boards for production, you can only use them for mains-powered devices, not battery powered ones. Some people explain how to cut wires on these boards to reduce the power consumption but it's a real pain to do. Thus often you're back to using the raw ESP12 device as-is and solder the wires yourself.

But then comes another problem : most USB UART devices adopt the now ubiquitous "FTDI" pinout, which exposes GND, VCC, RXD, TXD, DTR and CTS. The problem is that while DTR is an output and may be connected to GPIO0, CTS is an input and you don't have another output to use to select between running and programming, so it's still required to plug/unplug wires during programming. Some more advanced circuits implement an automatic RST+GPIO combo signal based on DTR only. But for me all of them have proven very unreliable, even after various modification attempts. First, GPIO0 sometimes emits a strong ~20 MHz signal preventing the DTR pin from going low and triggering the RST ; this does not happen while RST is held down however. Second, most often as soon as you open a terminal , DTR is triggered and resets the device again, which is not fun. Fortunately, during this time, RTS is low so it's possible to consider the combination of RTS and DTR instead of each individual signal. So as if it was not enough, DTR and RTS must not be directly connected to GPIO0 and RST, it's required to implement an exclusion between the two so that GPIO0 is triggered only when DTR is low and RTS is high, and RST is triggered only when RTS is low and DTR is high.

So I spend quite some time scratching my head trying to find the appropriate solution. The important points are :
  1. the board must be able to boot in running mode with only the power connected. This means that the pull-ups and pull-downs must be connected
  2. the USB-UART controller must not be physically present on the board, as it sucks power, takes space and costs money. Instead only the 6 pins required to connect a serial adapter must be present
  3. the board must use 2.54mm pitch, be very narrow, not larger than the ESP12 itself so that there are still pins left around it on breadboards
  4. the serial connector must respect the FTDI-compatible pinout so that any adapter will fit at least to allow regular communication with the device, and emergency flashing by moving RST by hand if needed.
  5. the RST pin must not be connected to the serial adapter but be available for RTC-based wakeup if required. Thus the module must use the CH_PD pin instead (also called "EN" or "CHIP_EN")
  6. the serial adapter must be modified to route RTS instead of CTS on pin 2
  7. the serial adapter must provide the logic to combine RTS and DTR as described above. If small enough it can be placed on the final board, otherwise it's better to have it as an intermediary board.
By searching for existing designs I found one part of the solution : Wemos not only does nice ESP boards, they also make an FTDI compatible USB UART on which you can decide to route RTS instead of CTS to pin 2. That confirmed to me that point 6 above can be addressed and become a prerequisite. I ordered a few and decided that in the mean time I'd modify my FTDI adapters.

For point 3, I cut some experimentation board to the same dimensions as the ESP12, plus one row for an optional serial connector. It happens that the 8 pin rows on each side are the same length as the module once expanded to 2.54mm, and that exactly 6 pins fit in the module's width!
I decided to place 2.54mm male connectors in the middle, spaced by 7.62mm so that they can fit on the middle row of a breadboard and even in a DIL16 socket on any board.

This left a central area where a few wires were routed and where there's enough room to install the pull-ups and pull-downs, addressing point 1 (I moved them elsewhere on this prototype as it was a pain to solder them after the connectors were in place).

By placing the serial connector close to the antenna there's no risk of touching the SPI pins at the bottom. Also the antenna is generally supposed to be located in an accessible place so it makes sense to install this connector at the same place. It also turns out that it was convenient to route RXD and TXD.

For point 5, I noticed during tests that a weak pull-up would still be better on RST otherwise it catches RF noise around (ie if you touch it with your finger). But that's a minor detail and doesn't prevent it from working.

So let's proceed with these various steps in order of dependencies

Making a usable ESP board

I didn't want to start making my own PCBs, it was a late afternoon's project. I decided to go with experimentation board, that I cut to the appropriate dimensions to hold the ESP-12E module and a 6-pin connector for  the serial port. Then I've cut unused traces as well as a central area to have two sets of connections. For those interested in trying it, you need to keep 11 rows of 6 holes. It's mandatory that it's single-sided because the ESP module will be placed on top of it and we don't want to risk accidental contacts :


Then I prepared the ESP-12E module. I've soldered very thin wires in each hole. The wires need to be long, at least 4-5 cm, or it will be a pain to place them into the PCB holes later. For this I've cut multi-strand wires and used their thin individual wires :


Now the difficult part starts. Before proceeding, it's important to pull the wires to ensure they're firmly attached. If some wire pops on the other side of the module, it needs to be cut. While pulling the wires, try to arrange them so that each side is approximately parallel with wires approximately 2.54mm apart. These ones will be placed into the PCB. Inserting the first side is not very difficult but requires patience. What is difficult is to insert the second one without removing any wire from the first one :-)

Given that the pitch between the two series of solders is not the same, some wires could cross. Of course we don't want this. So we want to insert the wires into the second row of holes on the PCB, like this :


It should then ressemble approximately this after you pull hard enough on the wires to ensure that none is blocked by another one or touching a neighbour :



Then quickly solder them, cut the remaining wires very short, and grind the solders so that the're as flat as possible :


Then the fun can begin. Cut two 8-pin right-angled male connectors that you solder 7.62mm apart, above the 2nd and 5th columns precisely, and solder them on the outside :



The last part for this board is to install the 6-pin connector for the serial adapter, and to connect the wires and components. I've used only 10K resistors, except in series with GPIO0 where I've added a 470R to protect the serial adapter against the output signal that's sometimes present on this pin during reset. I've also installed a 10 µF decoupling capacitor between VCC and GND because there was enough room for it and it was easy. The wiring diagram looks like this (it's easier than a schematics given that these are almost only wires), followed by the final assembly :

 

 

It's worth mentionning that if you don't see the pull-up resistor for the RESET pin on the photo, it's because I omitted it (as not strictly required), but I'll change this as the reset pin is now too sensitive to my finger, and I tend to reset the device when I touch it.

Modifying a USB-TTL serial adapter to provide both DTR and RTS

I'm using different flavours of FTDI adapters, and all of them have CTS on pin 2 :


I need to have RTS here. So I expected to be able to cut the PCB trace and solder a wire but I can't follow this trace which probably is under the IC so I don't want to damage my board.

Instead I decided to proceed differently by using a 6-pin male-female connector to extend the existing connector, not connecting pin 2 to the adapter but instead cutting the other pins, bending pin #2 over the PCB and connecting it to RTS which is on the chip's pin #3 according to the datasheet. That's all! Now you have a modified FTDI adapter with RTS on pin 2! This modification is very simple to operate. However don't make the same mistake I did, you need to glue the connector once it works, otherwise it will come out of the adapter once you unplug the adapter and pull off your wire. BTW since I couldn't find a 6-pin connector, I had to cut  a 8-pin one.

In the mean time I found that Wemos proposes such an adapter on which pin #2 can be configured to be RTS or CTS, so I will probably not develop further on this adapter's mod.





Converting the DTR/RTS signals to GPIO0/EN

It's not practical to directly connect the signals to the board, it's required to implement exclusion between the signals so that when DTR is low but RTS high, the chip is forced to flash mode, and when DTR is high and RTS low, the chip is reset. Otherwise the chip will work in a single mode, or all software will have to be modified to consider your protocol.

The truth table looks like this :


So it's as simple as doing :
  • GPIO0 = DTR | !RTS
  • EN = RTS | !DTR
In practice it's often made with NPN transistors and resistors on their base, but I do have a few low-voltage dual-mosfets in SOP8 packages which are very convenient because they don't require to cross PCB traces nor to add resistors. Some of them are IRF7313, but IRF7301, IFR7303 and FDS6990 will work fine as well. I also have a few other ones which have the exact opposite pinout, but I don't remember their identifier, so I stopped at the first one I found that I knew. The IRF7313's pinout is this one :


 

 The schematics we want is trivial (GPIO0 resistor included here but it's better placed on the ESP board) :



So the wiring can even be made using flying wires :


But I decided to be reasonable and to use another piece of PCB to make this one, so that connectors are firmly attached. The purpose will be to use the same pinout as the modified FTDI adapter on one side, and as the new ESP PCB on the other side. The result comes below :




I noticed that I wrote the pinout on the least convenient side, so it's better to do it the other way around to match what is displayed on the FTDI
module and the ESP module. And it worked like a charm on first test!


Now what is needed in the end ?

After all these modifications, and after having found the Wemos serial adapter which supports RTS, I concluded that the only part which does not exist is the ESP module for end users. So in fact given that the dual mosfet above is very small and consumes zero power, it should fit on the small board I installed the ESP-12 on. This board would then be programmable and compatible with deep-sleep. Also, even smaller MOSFETs exist, I've seen some dual in SOT363 package.

So by having just a board with 2.54mm pitch, the same pinout as the FTDI board for the serial connection, the MOSFET mounted and the few pull-ups, it would be possible to have the equivalent of the current ESP-12E module, but which could easily be programmed, either for development, or just for production. I think it should be sold with the connectors unsoldered. This way everyone can use it as a simple replacement for the current ESP-12 with a different pitch, yet program it and have it run by simply sending the power. Those who want to turn it into a development model can solder the two 8-pin barrels underneath, and it becomes compatible with breadboards and DIL16 adapters.

Those who simply want the ability to reprogram it in field just have to solder the 6-pin barrel connector and they'll be able to plug their FTDI-like adaptor in situ to reprogram it.

The best thing that could happen would be that some of the vendors like AI-Thinker create a new ESP module with such characteristics. It would solve all those issues at once. Right now when you look up "ESP8266" on Google image, you find tons of connection diagrams explaining how to flash them, which proves it's all but easy, or the NodeMCU models which provide the solution for development only. Let's think about it a bit further and make it usable for everyone! I'm not an electronician, just an occasional hobbyist and I could make it. But for people like me, it takes an amazing amount of time. Having the correct boards from the factory would be so much great!

I've ordered some ESP8285 which include the flash, maybe its possible to rebuild a board from scratch featuring an appropriate pinout. The same could be said for ESP32 which doesn't seem to have improved anything in this area unfortunately :-/


1 comment:

  1. As discusses above ESP8266 modules are used for various IOT projects. Having proper IOT Training will help you to overcome number of annoying characteristics which make their adoption problematic to beginners.

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