New-Tech Europe | May 2017

Fig. 3: PIC32 microcontroller LCC graphics flowchart

the EBI module is a more efficient peripheral when communicating to external SRAM. Software The internal SRAM method uses the write strobe of the PMP for the pixel clock. Jumper rows one and two on the LCC graphics board must be set for this configuration. In this setup, all colour is 8BPP and no external SRAM is used. SRAM from inside the MCU is continuously writing its pixel values to the PMP. For 8BPP colour, a 332 RGB colour format is used, that is three colour values for red, three for green and two for blue. This is a common colour format, because red is an easier colour for the human eye to detect than blue. The external SRAM method uses the read strobe of the PMP for the pixel clock. Jumper rows two and three on

the LCC graphics board must be set for this configuration. In this setup, all colour is 16BPP and the external SRAM contains the graphics frame that is continuously being read. For 16BPP colour, a 565 RGB format is used with five colour values for red, six for green and five for blue. In both methods, when connecting to an LCD panel with more than 16 colour lines, the unused colour lines are tied to the most significant bits of the last colour bit being used. This ensures that a full colour scale from white to black can be achieved. Conclusion This article has shown how a low-cost controllerless graphics system can be implemented with microcontroller peripherals to create a virtual graphics controller using only a small amount of CPU time.

the frame is being updated. This is necessary because there is only one PMP peripheral and it is being shared by the virtual graphics controller or DMA transfer. This method does affect the refresh rate of the screen. The amount of pixel updates needs to be monitored to prevent too large a refresh rate change, otherwise the change will be perceptible by the human eye. This is done by using a pixel count variable within the virtual graphics controller that is updated on every pixel write and cleared during every DMA interrupt. With the introduction of the EBI peripheral, the suspension time needed for a frame update is dramatically reduced. When the EBI is used for writing, less data need to be stored and restored since the PMP registers are no longer being shared for reading and writing. Also,

New-Tech Magazine Europe l 41