NXP¶

NXP has integrated LVGL into the MCUXpresso SDK packages for several of their general purpose and crossover microcontrollers, allowing easy evaluation and migration into your product design. Download an SDK for a supported board today and get started with your next GUI application.

Creating new project with LVGL¶

Downloading the MCU SDK example project is recommended as a starting point. It comes fully configured with LVGL (and with PXP/VGLite support if the modules are present), no additional integration work is required.

HW acceleration for NXP iMX RT platforms¶

Depending on the RT platform used, the acceleration can be done by NXP PXP (PiXel Pipeline) and/or the Verisilicon GPU through an API named VGLite. There is a single NXP draw context that covers both GPUs allowing to have enabled either one or even both at the same time. While enableing both 2D accelerators, the VGLite can be used to accelerate widget drawing while the PXP accelerated blit and fill operations.

Supported draw callbacks are available in "src/draw/nxp/lv_gpu_nxp.c":

    nxp_draw_ctx->base_draw.draw_arc = lv_draw_nxp_arc;
    nxp_draw_ctx->base_draw.draw_rect = lv_draw_nxp_rect;
    nxp_draw_ctx->base_draw.draw_img_decoded = lv_draw_nxp_img_decoded;
    nxp_draw_ctx->blend = lv_draw_nxp_blend;

If enabled both GPUs, the PXP is the preffered one to be used for drawing operation. A fallback mechanism is implemented so that if the feature is not supported by PXP (or if PXP fails), the VGLite will take over to handle the task. At the end, the CPU will assure that every widget drawing is fully covered (if not already done by GPU).

PXP accelerator¶

Several drawing features in LVGL can be offloaded to the PXP engine. The VGLite (if supported) and CPU are available for other operations while the PXP is running. An RTOS is required to block the LVGL drawing thread and switch to another task or suspend the CPU for power savings.

Features supported:¶

RGB565 and ARGB8888 color formats
Area fill + optional transparency
BLIT (BLock Image Transfer) + optional transparency
Color keying + optional transparency
Recoloring (color tint) + optional transparency
Image Rotation (90, 180, 270 degree) + optional transparency
Recoloring (color tint) + Image Rotation (90, 180, 270 degree) + optional transparency
Screen Rotation (90, 180, 270 degree)
RTOS integration layer
Default FreeRTOS and bare metal code provided
Combination of recolor and/or rotation + color key/alpha blend/transparency is supported but PXP needs two steps. First step is to recolor/rotate the image to a temporarly buffer (please check LV_MEM_SIZE value for allocation limit) and another step is required to handle color keying, alpha chanel or to apply transparency.

Known limitations:¶

Rotation is not supported for images unaligned to blocks of 16x16 pixels. PXP is set to process 16x16 blocks to optimize the system for memory bandwidth and image processing time. The output engine essentially truncates any output pixels after the desired number of pixels has been written. When rotating a source image and the output is not divisible by the block size, the incorrect pixels could be truncated and the final output image can look shifted.

Basic configuration:¶

Select NXP PXP engine in lv_conf.h: Set LV_USE_GPU_NXP_PXP to 1
Enable default implementation for interrupt handling, PXP start function and automatic initialization: Set LV_USE_GPU_NXP_PXP_AUTO_INIT to 1
If SDK_OS_FREE_RTOS symbol is defined, FreeRTOS implementation will be used, otherwise bare metal code will be included

Basic initialization:¶

If LV_USE_GPU_NXP_PXP_AUTO_INIT is enabled, no user code is required; PXP is initialized automatically in lv_init()
For manual PXP initialization, default configuration structure for callbacks can be used. Initialize PXP before calling lv_init()

      #if LV_USE_GPU_NXP_PXP
        #include "src/draw/nxp/pxp/lv_gpu_nxp_pxp.h"
      #endif
      . . .
      #if LV_USE_GPU_NXP_PXP
        PXP_COND_STOP(!lv_gpu_nxp_pxp_init(), "PXP init failed.");
      #endif

Project setup:¶

Add PXP related files to project:
- src/draw/nxp/pxp/lv_gpu_nxp_pxp.c, src/draw/nxp/pxp/lv_gpu_nxp_pxp.h: init, uninit, run/wait PXP device, log/trace
- src/draw/nxp/pxp/lv_draw_pxp_blend.c, src/draw/nxp/pxp/lv_draw_pxp_blend.h: fill and blit (w/o transformation)
- src/draw/nxp/pxp/lv_gpu_nxp_osa.c, src/draw/nxp/pxp/lv_gpu_osa.h: default implementation of OS-specific functions (bare metal and FreeRTOS only)
  - optional, required only if LV_USE_GPU_NXP_PXP_AUTO_INIT is set to 1
PXP related code depends on two drivers provided by MCU SDK. These drivers need to be added to project:
- fsl_pxp.c, fsl_pxp.h: PXP driver
- fsl_cache.c, fsl_cache.h: CPU cache handling functions

Logging:¶

By default, LV_GPU_NXP_PXP_LOG_ERRORS is enabled so that any PXP error will be seen on LVGL output
For tracing logs about the PXP limitations or size thresholds, the user can enable LV_GPU_NXP_PXP_LOG_TRACES

Advanced configuration:¶

Implementation depends on multiple OS-specific functions. The struct lv_nxp_pxp_cfg_t with callback pointers is used as a parameter for the lv_gpu_nxp_pxp_init() function. Default implementation for FreeRTOS and baremetal is provided in lv_gpu_nxp_osa.c
- pxp_interrupt_init(): Initialize PXP interrupt (HW setup, OS setup)
- pxp_interrupt_deinit(): Deinitialize PXP interrupt (HW setup, OS setup)
- pxp_run(): Start PXP job. Use OS-specific mechanism to block drawing thread. PXP must finish drawing before leaving this function.
There are configurable area thresholds which are used to decide whether the area will be processed by CPU or by PXP. Areas smaller than a defined value will be processed by CPU and those bigger than the threshold will be processed by PXP. These thresholds may be defined as preprocessor variables. Default values are defined in lv_draw_pxp_blend.h
- LV_GPU_NXP_PXP_BLIT_SIZE_LIMIT: size threshold for image BLIT, BLIT with color keying, BLIT with recolor and BLIT with rotation (OPA >= LV_OPA_MAX)
- LV_GPU_NXP_PXP_BLIT_OPA_SIZE_LIMIT: size threshold for image BLIT, BLIT with color keying, BLIT with recolor and BLIT with rotation and transparency (OPA < LV_OPA_MAX)
- LV_GPU_NXP_PXP_FILL_SIZE_LIMIT: size threshold for fill operation (OPA >= LV_OPA_MAX)
- LV_GPU_NXP_PXP_FILL_OPA_SIZE_LIMIT: size threshold for fill operation with transparency (OPA < LV_OPA_MAX)

VGLite accelerator¶

Extra drawing features in LVGL can be handled by the VGLite engine. The PXP (if supported) and CPU are available for other operations while the VGLite is running. An RTOS is required to block the LVGL drawing thread and switch to another task or suspend the CPU for power savings.