E-Ink Clock: Hardware
This page gives an overview of the hardware used. You can find the KiCAD files in the GitHub repository. The KiCAD schematic is filled with part numbers and can be exported using KiCAD's BOM functionality.
Attention: The PCB layout in the repository is out of date, since I had some issues in the first PCB. But it should be easy to change the PCB layout in such a way that it fits the schematic again.
The schematic is divided in four parts: the voltage generation circuits, the E-Ink display, the microcontroller and the DCF77 module.
The circuit needs six different voltages, of which five are generated here. The sixth voltage is the contrast voltage of the E-Ink display, which is generated in E-Ink part of the circuit.
- +3.3 V: This voltage is generated using a low-dropout linear regulator, namely a TPS71533 (U5; upper left). There is not much else to say about this, because this is quite standard. This voltage is used as supply voltage for all chips.
- +15 V & +22 V: These voltages are both needed by the E-Ink display and are generated using two independent TPS61041 switching boost converters (U6 & U8; middle row). The circuit was designed using the application note in the datasheet of the chip. Since I have to be able to completely turn off these voltages I added a load switch to turn them off.
-15 V & -20 V: These voltages are again needed by the E-Ink display and are generated using two independent LM2611 Ćuk converters (U7 & U9; bottom row). Building these blocks was, once again, done by strictly following the datasheet. Unfortunately, these two converters draw a high amount of current — I was able to measure about 1.3 A — in the startup moment. If I am going to revise this board, I might want to try to use charge pumps instead of Ćuk converters.
In a first version I tried to control the Ćuk converters using a MOSFET, but this did not work out. The first MOSFET I tried, not considering the high inrush current at this time, was obviously destroyed. The second MOSFET I tried was not able to supply the required startup current fast enough, leading to current limited short circuits. This was a very hard issue to debug, but luckily I found a post on mikrocontroller.net (German) that helped me to come to this conclusion.
All this can be achieved much simpler: In my first prototype I used the mother of all switching regulators, the MC33063A, to generate +22 V and -20 V and two linear regulators to obtain the +15 V, respective -15 V. While this is highly inefficient, it is easy and works well, even on a breadboard.
Not much to say here, the connections are exactly the same as Sprite_tm uses. You can also see that I use his construction to generate the contrast voltage, which happens to be -1.49 V in my case, but can be changed using a trimmer.
To connect the display with the PCB I used a one-sided Hirose FPC-connector with 39 pins and a pitch of 0.3 mm.
The LPC1227 was primarily chosen because of the built in real-time clock that is able to wakeup the microcontroller from its Deep Power-Down mode. There is nothing more to say about the microcontroller itself, because this is the standard wiring.
The SRAM module, a Microchip 23LC512, has a capacity of 512 kbit and is used as a 1 bpp image buffer (800 px · 600 px · 1 bpp = 480 kbit). Rasterizing to this buffer is much faster than rendering the image on the fly while updating the E-Ink display. This leads to huge power savings, since the E-Ink display update is — by far — the most power intensive phase.
This is a standard wiring for most DCF77 modules including a simple filter.
Just keep in mind that some
DCF77 modules do not work if
ON is directly connected to ground because they
need a high-to-low edge to start up.