LPC4088 QuickStart Board - How to Expand - Display

Overview

The LPC4088 micrcontroller has an on-chip LCD controller. It is capable of driving displays with up to 1024x768 pixels. The large on-board SDRAM is used as framebuffer, storing the display content. There is a 61-pos expansion connector on the short edge of the LPC4088 QuickStart Board. The 0.3mm pitch of the connector makes it possible to carry all the signals in such a small space. The display expansion interface carry a full 24-bit parallel RGB-interface including the 24-data bits and timing signals like DCLK/HSYNC/VSYNC/DEN but also includes I2C and SPI interfaces along with a few GPIOs. These additional interfaces are for communicating with display controllers (some displays have such configurable controllers), reading possible configuration memories and communicating with touch screen controllers. An external LCD pixel clock can also be supplied via the display expansion connector.

Information

The more pixels to drive and the higher the pixel clock is the more the external memory bus of the LPC4088 is loaded. A large number of pixels also means that more execution/processing power is needed to update the display content.
It is not recommended to use a pixel clock above 33 MHz. That typically limits the display size to 800x600 pixels.

For performance reasons (on the LPC4088), a system with 16-bit color information per pixel is typically what is implemented. A 565-system is most commonly used. This means 5 bits for red, 6 for green and 5 for blue - 16 bits in total. 24-bit systems will double to load on the external memory bus since each pixel is stored as 32-bits (instead of 16-bits, or fewer).

EA Base Board and Display Boards

See the EA software library documentation for details about LCD driver that exist.

Embedded Artists supply a base board (soon to be released) that has the display signals available on a simple 2x25-pos, 100 mil pitch connector. This is to simplify your own display integration. Embedded Artists also supplies a few different display options as add-on boards that can be connected to this base board via the 2x25-pos, 100 mil pitch connector. These display boards are great to use for getting started quickly and test a specific display resolution in an application.

The 2x25-pos, 100 mil pitch connector carries 18 data bits per pixel by default (6 per RGB color). The LPC4088 LCD controller can produce 24 data bits per pixel and it is possible to output all these signals on the connector. The trade-off is that some signals (like the I2C interfaces) have to be removed. There are some jumpers to control this.

Note that the connector only supports connecting to a display via the RGB interface. Some lower resolution displays have the option to connect via a memory bus-like interface, typically 8 or 16-bit. Such display interfaces are not supported.

Example Schematic

One page covers display expansion in the example interface design schematic. A complete example is presented with a 4.3 inch 480x272 pixel LCD with touch screen interface. An LCD backlight generator is also included. It is simple to expand to other LCDs that have a parallel 16-24 bit RGB interface where timing comes from the HSYNC/VSYNC and/or DEN (Data Enable) signals.

Things to think about...

There are many steps to consider when designing a system with a custom LCD:

  • As a first step, the pixel color signals must be matched between the LPC4088 and display. The LPC4088 has many options for routing the pixel data to different pins and there are differences depending on the display mode selected. The base board supports the default settings when using TFT 4:4:4 / 5:6:5 / 8:8:8-modes.
    • Many displays have 24 or 18 bit color interfaces, i.e., 8 or 6 bits per color. This means that some color information is missing when using TFT 4:4:4 or TFT 5:6:5-mode. Always connect the available color bits from the LPC4088 to the MSB bits. A common solution is to connect the most significant color bit to the missing (LSB) bits. Alternatively, just ground the LSB bits.
    • The table below lists where the pixel color information can be found when working in TFT 4:4:4 / 5:6:5 or 8:8:8 mode. Remember that J8 pin numbering is reverse on the base board compared to the numbering on the LPC4088 QuickStart Board.
LCD-VD bitDisplay Expansion J8 pinTFT 4:4:4 modeTFT 5:6:5 modeTFT 8:8:8 mode
LCD_VD01RED0 (LSB)
LCD_VD12RED1
LCD_VD24RED2
LCD_VD36RED3
LCD_VD48RED0 (LSB)RED1RED4
LCD_VD510RED1RED2RED5
LCD_VD612RED2RED3RED6
LCD_VD714RED3 (MSB)RED4 (MSB)RED7 (MSB)
LCD_VD816GREEN0 (LSB)
LCD_VD917GREEN1
LCD_VD1018GREEN0 (LSB)GREEN2
LCD_VD1120GREEN1GREEN3
LCD_VD1222GREEN0 (LSB)GREEN2GREEN4
LCD_VD1324GREEN1GREEN3GREEN5
LCD_VD1426GREEN2GREEN4GREEN6
LCD_VD1528GREEN3 (MSB)GREEN5 (MSB)GREEN7 (MSB)
LCD_VD1629BLUE0 (LSB)
LCD_VD1730BLUE1
LCD_VD1832RED0 (LSB)BLUE2
LCD_VD1934BLUE0 (LSB)BLUE3
LCD_VD2036BLUE0 (LSB)BLUE1BLUE4
LCD_VD2138BLUE1BLUE2BLUE5
LCD_VD2240BLUE2BLUE3BLUE6
LCD_VD2342BLUE3 (MSB)BLUE4 (MSB)BLUE7 (MSB)
  • The HSYNC, VSYNC, DEN control signals and the DOTCLK pixel clock signals must be connected to the display. All displays typically require the DOTCLK signal but there are variations on the control signals.
    • Some displays require all three control signals (HSYNC, VSYNC, DEN).
    • Some displays require only HSYNC and VSYNC.
    • Some displays require only DEN control signals.
    • Some displays require that HSYNC is delayed compared to VSYNC, i.e., VSYNC must have a falling edge before HSYNC (assuming that HSYNC/VSYNC are active negative) . If this is needed, HSYNC can be delayed one DOTCLK cycle with two D-type flip-flops.
  • In most cases the LPC4088 can generate the appropriate DOTCLK frequency. The higher the frequency needed, the fewer available frequencies can be selected when dividing the core clock. If a specific frequency is needed, the LCDCLKIN signal can be used. It is an input to the LPC4088.
    • Most displays can accept a quite wide range of frequencies on the DOTCLK signal.
    • Embedded Artists has successfully used LCDCLKIN signals up to 35MHz. Above this it does not work. It is typically related to the core clock frequency.
      If not used, leave this signal unconnected (i.e., LCDCLK is generated by the LPC4088).
  • Some displays has a serial interface (typically SPI-like) for initialization of the controller chip inside the display. This is relatively common for smaller QVGA-sized displays but larger (resolution above QVGA – 320x240) displays typically do not require this initialization.
    • The display expansion connector has an SPI interface that supports both 3-wire transfer and 4-wire transfers. A 3-wire SPI interface typically means that 9 data bits are transferred in every time, 8 data bits and one bit indicating if it is a command or data byte. SPI-CLK, SPI-MOSI and SPI-SSEL are needed, i.e., 3 signals.
      For a 4-wire interface the transfers are 8 bits and a separate signal (the fourth wire) is used to signal if it is a command or data transfer. A GPIO signal is typically used for this.
      Few displays allow for read back of internal control registers via SPI. Only write transfers are typically supported.
  • Many displays have strict requirements of power sequencing during power up and down. Embedded Artists display boards has an I2C-GPIO expansion chip (PCA9532) and voltage control of +3.3V and +5V to allow power sequencing.
    • Embedded Artists recommend that the power sequences outlined in the display datasheets are followed for best long term performance of the display.
  • For simplicity, Embedded Artists has added a configuration i2c-e2prom on the display add-on boards that contains information about the connected display. This simplified development of portable software.
    • A standard 24LC64 chip is used with i2c-address 0x56 (1.0.1.0.1.1.0.rw). A0 on the chip is connected to GND and A1/A2 to VCC to get address 0x56.
  • Some displays also require a reset signal after VCC/VDD has stabilized. In that case an I2C-GPIO expansion chip can be used to create this signal. Alternatively a free GPIO signal in the LCD Expansion Connector can be used.
  • All TFT LCDs must have a control for backlight. A constant current source is needed. Some displays have one LED string while others have 2-4 strings making control slightly more complicated.
    • OLEDs do not require separate backlight since the pixels transmit light themselves.
  • As a final step a touch screen interface is typically needed.
    • Embedded Artists display boards often use the TSC2046 from TI. It has a simple SPI interface.
    • Embedded Artists also use the AR1021 from Atmel, in SPI more (the chip also supports an I2C interface).
    • Capacitive touch screen controllers typically have I2C or SPI interfaces.
  • Do not forget that some displays have configurations pins for selecting interface type functionality and sometimes display orientation.


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