VCOM Kick‐back voltage reading
This special feature is only available for epdiy PCB's with TPS65185 on board (v6 & v7)
Datasheet of the TPS65185 PMIC and DC to DC booster
The question came out in Issue 393 and the is a pending to be reviewed Pull-Request 395:
Would it be possible to detect a smashed epaper display by reading this value and compare it against a certain threshold value?
Here we will document how we arrived to the conclusion that it could be used for broken display detection albeit not with 99.9% of accuracy.
For starters I will reproduce exactly what is stated on page 21 of the official Texas Instruments datasheet:
SECTION 8.3.7.1 Kick-Back Voltage Measurement
TPS65185x can perform a voltage measurement on the VCOM pin to determine the kick-back voltage of the panel. This allows in-system calibration of VCOM. To perform a kick-back voltage measurement, follow these steps:
- Pull the WAKEUP pin and the PWRUP pin high to enable all output rails.
- Set the HiZ bit in the VCOM2 register. This puts the VCOM pin in a high-impedance state.
- Drive the panel with the Null waveform. Refer to E-Ink specification for detail.
- Set the ACQ bit in the VCOM2 register to 1. This starts the measurement routine.
- When the measurement is complete, the ACQC (Acquisition Complete) bit in the INT1 register is set and the nINT pin is pulled low. v The measurement result is stored in the VCOM[8:0] bits of the VCOM1 and VCOM2 register. The measurement result is not automatically programmed into nonvolatile memory. Changing the power-up default is described in the following paragraph.
Now some remarks: We do not have the original NULL waveform but we got the hint from Wenting that in fact, is a waveform full of 0 (Zeros) that mean "no action" in every pixel. Grayscale or MODE_DU waveforms so usually have 2's (LIGHTEN) or 1's (DARKEN) that are triggering the Gate negative or positive MOSFETs per pixels, that let pass the -15 (Darken) or the +15 (Lighten) to each pixel so we can show the image. It does that in many pushes, sometimes up to 30, in professional Eink controllers.
Codewise I will document this further adding a PR that will enable sending this I2C commands to the TPS65185.
My thesis is that normal VCOM values for different Display I tested ranged from 270 mV to 550 mV (Is really not the real VCOM from fabricant, is just a reference voltage reading). When you break the Glass substrate with a hammer or crack that joins the various layers, you are really making a short circuit, connecting VCOM with it's nearest layer. Now this is pure theory just backed up by our experience documenting this feature (Check video here)
My theory of how is the internal glass structure:
As a resume, even without using this special Waveform, the VCOM readings show:
- A normal reading of 250 to 600 mV on normal room temperature
- A very low reading of 10 to 40 mV if you leave the naked display for 5 minutes in a freezer temperature of -18 ° Celsius (This is a temperature where any normal display will fail to work anyways since the fluids stop to be liquid)
- A zero reading if the FPC cable is disconnected or completely broken because of a hit
- A very high reading, actually the top most Voltage of VCOM that the PMIC can achieve, of 5100 mV if the display is smashed and the layers are broken touching each other as in the right side of the graphic above.