$2 esp8266 4MB ESP-12-E module

One of the recent goodies to arrive in the mail was a new esp8266 module.  When I ordered it, the price was 208c, but as of the time of this writing it has gone up.  Although it is advertised as an ESP-12-E module, the module I received is labeled ESP-12-Q, as can be seen in the photo above.  Other vendors are selling the same modules for 201c now.

One of the reason I ordered the module is because I intend to test the ADC function, and the ADC pin is not broken out on my ESP-01 module.  The other reason is that I read a couple postings from people that had received ESP8266 modules with a larger 4MB flash instead of the 512KB flash typical of the first ESP8266 modules released.

Since the modules have pads at 2.0mm spacing, I made an adapter board with some protoboard.  I just hot glued the module to the protoboard, and soldered short wires from the pads to the 0.1" spaced pin headers.  It is a bit more tedious work than using one of the pre-made adapter boards on the market, but I didn't have to wait for an adapter board, and my solution leaves more available space for connections on a breadboard.  Aside from the wire connecting EN with VCC, the rest of the connections are wired to the respective protoboard header pins.

After connecting power and Tx/Rx to a USB-TTL module, and pulling down GPIO15 to ground with a 1K resistor (to select boot from serial flash), I opened a terminal program to 9600bps, pulsed the reset line with a brief 1K short to ground, and got the following output:
NodeMcu 0.9.6 (Doit.am Version) build 20150701  powered by Lua 5.1.4

ESP8266 Started
Start soft AP
>

I also did a wifi scan with my phone and found an SSID named DoItWifi.  Since eLua was pre-flashed to the module, I could easily check the flash size with the node.info() function:
> =node.info()
0       9       6       16398478        1458400 4096    2       40000000

The 4096 indicates 4096KB (4MB) of flash.  The 512KB of flash on the ESP-01 module had been enough for the kind of progams I was writing using esp8266/Arduino, but now with a 4MB module I have plenty of memory to start experimenting with the ESP RTOS.

I measured the power consumption of the module at idle and with a wifi client connecting.  Idle consumption was consistently around 60mA, and with a wifi client connecting power consumption was around 90mA with a couple peaks to 140mA.

With this module I also decided to do some tests on the boot modes.  While the boot modes are reasonably well documented, there is little documentation on the default state of GPIO0, 2, & 15 at boot time.  This posting on the espressif bbs states they all have weak pullups, and my testing confirms this.  I held reset low with a 1K resistor to ground, and measured the voltage on the pins.  TXD0, RXD0, GPIO0, 2, & 15 all read 3.26V, while GPIO4 & 5 were 0V.  Connecting a 10K resistor to GPIO0, 2 & 15 caused the voltage to drop to 0.8V for each of them.  Calculating for the voltage divider ratio results in an internal pullup resistor value of around 33K Ohms.

I also tested the pins after booting from the on-board flash.  GPIO15 was low, GPIO0 was still pulled high, however GPIO2 was in output high state (a 10K resistor to ground resulted in a voltage drop of less than 100mV).  I also did some tests without GPIO15 pulled low, which, according to the documentation, should put it in the boot from SD card mode.  Every time I reset the module without GPIO15 pulled low, the module started outputting a 26Mhz signal on GPIO0.  Some googling indicates that this is what some people are calling the "zombie" mode.  Perhaps all that is required to avoid the zombie mode is to ensure GPIO15 is pulled low.

The final thing I tested was the deep sleep wakeup pin, GPIO16.  By connecting this to RST, it is possible to reset the module out of deep sleep.  I had previously tried connecting GPIO16 to RST on a ESP-03 module, but then had problems resetting the module.  The module was faulty in other ways, so I wasn't certain of the problem.  With the ESP-12-Q module, GPIO16 is driven high during reset and after reset.  Perusing the ESP8266 GPIO control files in the SDK indicates that GPIO16 does not have an internal pullup available, explaining why it is in output high mode.  This means that in order to be able to manually reset the module, a diode or a resistor of about 1K should be used between GPIO16 and RST.  Otherwise connecting RST to ground with a 1K resistor will not result in enough of a voltage drop to reset the module, and a dead short from RST to ground could result in enough current through GPIO16 to damage the chip.

With the 4MB ESP modules available for $2, I think they will now be my preferred choice for projects requiring an MCU.  The modules use too much power to work from a CR2032 coin cell, so for such things I'd likely use an AVR.