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Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>
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Michel-FK
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docs/developer_guide/hardware_reference/screen.md
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docs/developer_guide/hardware_reference/screen.md
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The second most important part in the **FunKey S** is certainly its
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screen: in an overall form factor of roughly 45mm x 45mm x 15mm (1.75"
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x 1.75" x 0.6"), it has to be comfortable enough to provide a good
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gaming experience.
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In theory, 1.75" would allows to shoehorn a 2.4" (diagonal) square
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screen, but in practice, 2.4" screens are seldom square and more
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rectangular in shape.
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Unless you are a large manufacturer and selling millions of devices,
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you are limited to using the existing screen sizes that are available
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on the market, which most of the time were designed for a
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long-forgotten specific devices (think of PDAs, MP3 players,
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clam-shell phones, pods, etc.) and standard aspect ratio are either
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1:1, 5:3 or 16:9. Thus, for a given pixel technology, this results in
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rather standard screen sizes.
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So the next available size down are 2" and 1.8", but these screens
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tend to be quite thick and based on an older technology, so their
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typical resolution is rather limited @ 128x160 pixels: too small for
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gamers.
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Still going down in size, you can find 1.5" to 1.55" screens with an
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interesting resolution of 240x240 or even 320 x 320 ("Retina") pixels,
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but most of them use a fast MIPI DSi interface. These particular
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screens were popular as they were used in the 6th-generation "square"
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iPods, but unfortunately, they require a dedicated controller on the
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host side, which is only available in higher-end SoC.
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Another important characteristic of the required screen is that it
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needs to have a narrow flex cable in order to be rolled into the hinge
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(like a _fly-paper_) in order to spread the mechanical flex constraint
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over several loop rather than at always the same section, resulting in
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less wear and an expected longer lifespan.
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Fortunately, we found this 1.54" LCD screen on AliBaba:
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What makes this screen special is its standard SPI interface, which
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only requires a few wires and thus a narrow flex cable like the MIPI
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DSi interface, so it is easy to roll into a hinge. But unlike the MIPI
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DSi one, the SPI interface is readily available on most of the SoC,
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including the Allwinner V3s.
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This 1.54" display features a 240x240 16/18-bit full color pixel
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resolution and is an **IPS** display, so the color looks great up to
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80 degrees off-axis in any direction.
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However, in order to achieve a 50 fps @ 240 x 240 pixel resolution in
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RGB565 (2 bytes / pixel), this requires a ~44 MHz SPI clock rate,
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which is rather high.
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Once again, we were fortunate as both the V3s CPU and the screen
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built-in controller (a Sitronix [ST7789V][1]) both support this high
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clock speed (after checking with the manufacturer and despite the
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controller datasheet that specifies only a serial clock cycle (Write)
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of 66 ns or 15 MHz!).
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We were even luckier as its backlight consists in 3 white LEDs in
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parallel and not in series, such that no additional step-up DC-DC
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converter is required, as a standard 3.3V / 60 mA (typical) power
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supply is sufficient. Of course, we won't be able to drive this
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current directly from a CPU GPIO and the backlight will require an
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additional transistor to interface to the LCD backlight.
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Its flex cable requires a mating Hirose 0.4 mm pitch
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[DF37NB-24DS-0.4V][2] dual row SMT connector, out of which only one
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single row is actually used.
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## Customization
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Unfortunately, the flex cable for the stock LCD screen we found does
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not match our particular **FunKey S** mechanical design.
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For the prototypes, we designed custom flex extension cables for a ~
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$100 cost, but we had to design [our own custom flex][3] and have this
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standard screen assembly attached to it for mass production, with a
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one-time tooling fee of ~ $800.
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## Schematic
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The schematic is quite simple:
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{.lightbox}
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The main component is of course the Hirose screen connector **J3**,
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with the following signals:
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- LEDA: the backlight LED common Anode connection (+)
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- GND
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- +3V3 power supply
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- /SPI_CS: SPI Chip Select
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- SPI_MOSI: SPI Master Out / Slave In
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- SPI_CLK: SPI Clock
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- TE: Tearing Effect sync signal from the screen
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- RS: LCD-specific Register/Memory Select (or Data/Control Select)
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- LCD_RESET: LCD Reset
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All data signals feature an ESD TVS protection diode **D19-D24** and
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**D37**, and except for the power supplies and LEDA + LCD_RESET and TE
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signals, all signals are directly connected to the V3s CPU's SPI
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interface, so there is not much to say about these.
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The LCD_RESET signal is controlled by a V3s GPIO (PB2 on pin 41).
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The TE signal from the screen controller is fed to an
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interrupt-enabled input pin PB1 (pin 40) in order to synchronize the
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sending of data to the screen with its inactive period in order to
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avoid the hardware tearing effect.
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## Backlight PWM
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The backlight control requires a few more components: a MOSFET-P
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transistor **Q1** and 2 resistors **R5** and **R7** to provide its
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polarization, more on this below.
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As the backlight LEDs cathode (-) pin are directly tied to GND within
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the screen, we need to drive these LEDs "from the high-side",
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i.e. between the +3V3 power supply and the LEDA pin, so a MOSFET-P
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transistor is necessary:
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{.lightbox}
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As we want the backlight to be on by default, we need to drive it to
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GND by default: this is the role of **R7**. The role of **R5** is then
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to make sure that -Vgs is driven below its threshold voltage and turns
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off the transistor when the CPU drives a GPIO high.
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As an ultimate sophistication, we can drive the backlight from the CPU
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using one of its built-in PWM controllers PWM0 with a varying
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duty-cycle, thus controlling the LCD backlight brightness
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accurately. For this purpose, the other end of the **R5** resistor is
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connected to the PB4 output (pin 43) on the V3s, wich provides this
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function.
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[1]: https://www.numworks.com/shared/binary/datasheets/st7789v-lcd-controller-73f8bc3e.pdf
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[2]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/1811071030_HRS-Hirose-DF37NC-24DS-0-4V-51_C324720.pdf
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[3]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/DISPLAY%20-%20SF-TC154B-8737A-N%20-%20CUSTOM%20FLEX%20-%200.3.PDF
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--8<--
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includes/glossary.md
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--8<--
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