micropython-nano-gui/drivers/sharp

A MICROPYTHON DRIVER FOR SHARP DISPLAYS

These monochrome SPI displays exist in three variants from Adafruit.

1. 2.7 inch 400x240 pixels
2. 1.3 inch 144x168
3. 1.3 inch 96x96 - Discontinued.

I have tested on the first of these. However the Adfruit driver supports all of these and I would expect this one to do so.

1. Display characteristics

These displays have extremely low current consumption: I measured ~90μA on the 2.7" board when in use. Refresh is fast, visually excellent and can run at up to 20Hz. This contrasts with ePaper (eInk) displays where refresh is slow (seconds) and visually intrusive; an alternative fast mode overcomes this, but at the expense of ghosting.

On the other hand the power consumption of ePaper can be zero (you can switch them off and the display is retained). If you power down a Sharp display the image is retained, but only for a few seconds. In a Pyboard context 90μA is low in comparison to stop mode and battery powered applications should be easily realised.

The 2.7" display has excellent resolution and can display fine lines and small fonts. However the display quality is not as good as ePaper. For good contrast best results are achieved if the viewing angle and the direction of the light source are positioned to achieve reflection.

1.1 The VCOM bit

The significance of this is somewhat glossed-over in the Adafruit docs, and a study of the datasheet is confusing in the absence of prior knowledge of LCD technology.

The signals applied to an LCD display should have no DC component. This is because DC can cause gradual electrolysis and deterioration of of the liquid crystal material. Display driver hardware typically has an oscillator driving exclusive-or gates such that antiphase signals are applied for ON pixels, and in-phase for OFF pixels. The oscillator typically drives a D-type flip-flop to ensure an accurate 1:1 mark space ratio and hence zero DC component.

These displays offer two ways of achieving this, in the device driver or using an external 1:1 mark space logic signal. The bit controlling this is known as VCOM and the external pins supporting it are EXTMODE and EXTCOMIN. EXTMODE determines whether a hardware input is used (Vcc) or software control is required (Gnd). It is pulled low.

The driver supports software control, in that VCOM is complemented each time the display is refreshed. The Adafruit driver also does this.

Sofware control implies that, in long running applications, the display should regularly be refreshed. The datasheet incicates that the maximum rate is 20Hz, but a 1Hz rate is sufficient.

If hardware control is to be used, EXTMODE should be linked to Vcc and a 1:1 logic signal applied to EXTCOMIN. A frequency range of 0.5-10Hz is specified, and the datasheet also specifies "EXTCOMIN frequency should be made lower than frame frequency".

In my opinion the easiest way to deal with this is usually to use software control, ensuring that the driver's show method is called at regular intervals of at least 1Hz.

2. Test scripts

1. sharptest.py Basic functionality test.
2. clocktest.py Digital and analog clock display.

sharptest should not be run for long periods as it does not regularly refresh the display. It tests writer.py and some framebuffer graphics primitives. clocktest tests nanogui.py.

To run the tests the fonts in the directory, writer.py and nanogui.py must be copied to the device or frozen as bytecode. Testing was done on a Pyboard D SF6W: frozen bytecode was not required. I suspect a Pyboard 1.x would require it to prevent memory errors.

3. Resources

Schematic for 2.7" unit

Datasheet 2.7"

Datasheet 1.3"