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A MICROPYTHON DRIVER FOR SHARP DISPLAYS

These monochrome SPI displays exist in three variants from Adafruit.

I have tested on the first of these. However the Adfruit driver supports all of these and I would expect this one also 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. In other respects 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.

1.2 Refresh rate

The datasheet specifies a minimum refresh rate of 1Hz.

2. Test scripts

sharptest.py Basic functionality test.
clocktest.py Digital and analog clock display.
clock_batt.py As above but designed for low power operation. Pyboard specific.

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. The clack_batt.py demo needs upower.py from micropython-micropower.

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. Device driver constructor

Positional args:

spi An SPI bus instance. The constructor initialises this to the baudrate and bit order required by the hardware.
pincs A Pin instance. The caller should initialise this as an output with value 0 (unusually the hardware CS line is active high).
height=240 Dimensions in pixels. Defaults are for 2.7" display.
width=400
vcom=False Accept the default unless using pyb.standby. See 3.2.

3.1 Device driver methods

show No args. Transfers the framebuffer contents to the device, updating the display.
update Toggles the VCOM bit without transferring the framebuffer. This is a power saving method for cases where the application calls show at a rate of < 1Hz. In such cases update should be called at a 1Hz rate.

3.2 The vcom arg

It purpose is to support micropower applications which use pyb.standby. Wakeup from standby is similar to a reboot in that program execution starts from scratch. In the case where the board wakes up, writes to the display, and returns to standby, the VCOM bit would never change. In this case the application should store a bool in peristent storage, toggling it on each restart, and pass that to the constructor.

Persistent storage exists in the RTC registers and backup RAM. See micopython-micropower for details of how to acces these resources.

4. Application design

In all cases the frame buffer is located on the target hardware. In the case of the 2.7 inch display this is 400*240//8 = 12000 bytes in size. This should be instantiated as soon as possible in the application to ensure that sufficient contiguous RAM is available.

4.1 Micropower applications

These comments largely assume a Pyboard host. The application should import upower from micropython-micropower. This turns the USB interface off if not in use to conserve power. It also provides an lpdelay function to implement a delay using pyb.stop() to conserve power.

In tests the clock_batt demo consumed 700μA between updates. A full refresh every 30s consumed about 48mA for 128ms. These figures correspond to a mean current consumption of 904μA implying about 46 days operation per AH of battery capacity. LiPo cells of 2AH capacity are widely available offering a theoretical runtime of 92 days between charges.

Lower currents might be achieved using standby but I have major doubts. This is because it is necessary to toggle the VCOM bit at a minimum of 1Hz. Waking from standby uses significan amounts of power as the modules are compiled. Even if frozen bytecode is used, there is still significant power usage importing modules and instantiating classes; this usage is not incurred in the loop in the demo.