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BT81X (815/6) Advanced Embedded Video Engine Datasheet

Version 1.0

Document No.: BRT_000220 Clearance No.: BRT#126

4.2.3

Clock Enable

At power-on the BT815/6 is in sleep mode. The internal relaxation oscillator is selected for the PLL clock

source. The system clock will be enabled when the following step is executed:



Host sends an “ACTIVE” command

If the application chooses to use the external clock source (12MHz crystal or clock), the following steps

shall be executed:



Host sends a “CLKEXT” command



Host sends an “ACTIVE” command

4.2.4

Clock Frequency

By default the system clock is running at 60MHz when the input clock is 12MHz. The host is allowed to

switch the system clock to other frequencies (72MHz, 48MHz, 36MHz, and 24MHz) by the host command

“CLKSEL”. The clock switching command shall be sent in SLEEP mode only.

After the chip is put into Active mode, software shall update the REG_FREQUENCY value to align with the

chosen system clock frequency by CLKSEL command, if the default 60MHz is changed.

When using the internal relaxation oscillator, its clock frequency is trimmed to be 12MHz at factory.

Software is allowed to change the frequency to a lower value by programming the register REG_TRIM.

Note that software shall not change the internal oscillator frequency to be higher than 12MHz.

4.3 Graphics Engine

4.3.1

Introduction

The graphics engine executes the display list once for every horizontal line. It executes the primitive

objects in the display list and constructs the display line buffer. The horizontal pixel content in the line

buffer is updated if the object is visible at the horizontal line.

Main features of the graphics engine are:



The primitive objects supported by the graphics processor are: lines, points, rectangles, bitmaps

(comprehensive set of formats), text display, plotting bar graph, edge strips, and line strips, etc.



Operations such as stencil test, alpha blending and masking are useful for creating a rich set of

effects such as shadows, transitions, reveals, fades and wipes.



Anti-aliasing of the primitive objects (except bitmaps) gives a smoothing effect to the viewer.



Bitmap transformations enable operations such as translate, scale and rotate.



Display pixels are plotted with 1/16th pixel precision.



Four levels of graphics states



Tag buffer detection

The graphics engine also supports customized built-in widgets and functionalities such as ASTC decode,

jpeg decode, screen saver, calibration etc. The graphics engine interprets commands from the MPU host

via a 4 Kbyte FIFO in the BT815/6 memory at RAM_CMD. The MPU/MCU writes commands into the FIFO,

and the graphics engine reads and executes the commands. The MPU/MCU updates the register

REG_CMD_WRITE to indicate that there are new commands in the FIFO, and the graphics engine updates

REG_CMD_READ after commands have been executed.

Main features supported are:



Drawing of widgets such as buttons, clock, keys, gauges, text displays, progress bars, sliders,

toggle switches, dials, gradients, etc.



JPEG and motion-JPEG decode



Inflate functionality (zlib inflate is supported)



Timed interrupt (generate an interrupt to the host processor after a specified number of

milliseconds)

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