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Screenkey Design - Frequently Asked Questions

  1. What ScreenKey development kits are available?

    2. Please see "Getting Started" section under "Technology" for more information. There are two development kits available:

    DemoComII is a Microcontroller Prototyping/Evaluation Kit with two ScreenKeys onboard. This kit is useful for design engineers who wish to get to understand ScreenKey interfacing before embarking on their own hardware and software development. 'C' source code is supplied. This can be compiled with a free gnu public licence compiler called SDCC.

    OEM-5400 is a Windows Prototyping/Development kit that enables users to begin prototyping and preparing proof-of-concept ScreenKey user interfaces straight "out of the box". A panel of 12 ScreenKeys is provided as part of this kit. Various Windows software tools are provided that enables users to quickly design images and text to appear on the ScreenKeys and to enable these to interact with existing applications or to program support into proprietary application using high-level programming languages.

  2. Can I adjust the LCD contrast?

    3. The greatest influence on contrast is the supply voltage. The recommended supply voltage is 4.9v - 5.0v. However, it is possible to drive ScreenKeys with a voltage as low as 4.7v. Changes in supply voltage can result in considerable visual contrast changes, with higher voltages increasing contrast.

    The refresh frequency register (described in the datasheet) can also be used to produce slight contrast adjustments. However, this register is not specifically for adjusting contrast. Its purpose is to generate the correct waveforms for driving (refreshing) the LCD display. This register should be set according to the driving clock frequency as per the appropriate datasheet. One effect that this register has is that the contrast is affected by using lower values.

  3. Must the clock signal always be present?

    4. The clock signal supplied to the ScreenKey has two purposes. Firstly, it is used by the ScreenKey internal circuitry to synchronously process the serial data. The second purpose of this clock signal is to produce the waveforms to refresh the LCD display. It is important that this clock should be permanently applied so long as power is applied to the ScreenKey. If the clock is stopped for a long period it is possible that the LCD liquid may be damaged. Short periods without the clock (e.g. during reset) do not need special handling.

  4. What clock speed should I use?

    5. The clock speed to be used depends entirely on your driving circuitry, CPU availability, etc. Please refer to the Application Notes for some examples of how to interface to ScreenKeys. ScreenKey datasheets permit any clock frequency between 50KHz and 4MHz. Using a faster clock will enable the LCD data to be changed more rapidly. You should try to use the fastest clock possible depending on your circumstances.

    If your clock uses a dedicated crystal, then either 2MHz or 4MHz should be considered. If using a software generated clock, then you must consider your processing speed and ability to feed data and clock together. A hardware generated clock is recommended.

  5. What is the current consumption?

    6. Actual current consumption is dependent on the LEDs that are lit and at what brightness.

    The LC (or RG) Series ScreenKey has two base colors, red and green, made up of 4 red and 4 green LEDs:

    • 2 x (red on) x (10mA half brightness / 20mA full brightness)
    • + 2 x (green on) x (10mA half brightness / 20mA full brightness)
    • + 7mA

    Example:

    • Red only at full brightness = 2 x 20mA + 7mA = 47mA
    • Green only at half brightness = 2 x 10mA + 7mA = 27mA
    • Orange at full brightness = 2 x 20mA + 2 x 20mA + 7mA = 87mA

     

    The RGB Series ScreenKey has three base colors, red, green and blue, made up of 4 red, 4 green and 4 blue LEDs:

    • 1 x (red on) x (10mA half brightness / 20mA full brightness)
    • + 1 x (green on) x (10mA half brightness / 20mA full brightness)
    • + 1 x (blue on) x (10mA half brightness / 20mA full brightness)
    • + 7mA

    Example:

    • Red only at full brightness = 1 x 20mA + 7mA = 27mA
    • Green only at half brightness = 1 x 10mA + 7mA = 17mA
    • White at full brightness = 1 x 20mA + 1 x 20mA + 1 x 20mA + 7mA = 67mA

  6. Do I need to supply an external graphic controller?

    7. No, every ScreenKey has an onboard graphics controller. Each ScreenKey has onboard accessible RAM that is bit-mapped to a particular pixel on screen. This memory begins at address 0x80 in the key. Images are displayed on the ScreenKey by writing bit patterns to this memory. There is no need to refresh this memory as long as power is applied.

  7. Does the display have to be refreshed?

    8. No, every ScreenKey has an onboard graphics controller. Each ScreenKey has onboard accessible RAM that is bit-mapped to a particular pixel on screen. This memory begins at address 0x80 in the key. Images are displayed on the ScreenKey by writing bit patterns to this memory. There is no need to refresh this memory as long as power is applied. If power is removed, then the pixel pattern must be rewritten to the bitmapped RAM in the ScreenKey.

  8. Do the backlight LEDs require a separate Vcc to the LCD?

    9. No, a common Vcc is used to drive the internal circuitry, the LCD and the LEDs. A proprietary ASIC inside the ScreenKey generates a constant current source as the actual driver for the LEDs.

  9. Do ScreenKeys support I2C?

    10. No. ScreenKeys implement a synchronous packetised protocol using a user supplied clock and data line. Each byte transmitted to the ScreenKey is comprised of 12 bits:

    • Start bit (low)
    • 8 Data bits (LSB first)
    • Parity bit
    • 2 Stop bits (high)

    An instruction packet is comprised of:

    • Start byte (0x00, even parity)
    • Command byte (odd parity)
    • Data bytes (odd parity)
    • End byte (0xAA, even parity)

  10. Is it possible to drive the data and clock lines with a 3.3v driver?

    11. Inside each ScreenKey is an ASIC chip and the spec for the input lines are as follows:

    Re :Screenkey with 5 volts DC VCC

  • Max Low voltage for Data and Clockline inputs is 1.5v (0.3xVCC)
  • Min high voltage for Data and Clockline inputs is 3.5v (0.7xVCC)

    So a 3.3v level for a high signal would be outside the above spec.

    Note: If the data line or clock line is driven from a 3.3v driver, a possible solution may be to fit pull-up resistors (1k or 4k7) from the data and clock lines to the 5v DC VCC at the ScreenKey, provided the 3.3v device output can except pull-ups to +5v DC.  You would have to check that this arrangement gives voltages that obey the above ScreenKey spec. 

     

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