SPRUJ28E November 2021 – September 2024 AM68 , AM68A , TDA4AL-Q1 , TDA4VE-Q1 , TDA4VL-Q1
The DPI interface will normally be driven from a graphics driver that will be configured to match a frame geometry and refresh rate for a panel display. The DPI driver will use the expected refresh rate and geometry for the active and blanking parts of a frame to determine the pixel rate clock. The DSITX controller will then translate this to match the incoming byte information to send across the DSI DPHY link.
A system will always expect the bandwidth of the incoming pixels × BPP to match the transmission of bytes from the DSI DPHY based on the number of active lanes. The relationship of DPI clock to TX byte clock must ideally hold with the following formula:
Fpixel_clk = Ftx_byte_clk × active_lanes × 8 ÷ bits_per_pixel;
The DSI will supply packets to the DPHY interface using the internal sequencing and counters clocked from the tx_byte clock. The normal system operation will mean that the DSI must always have the correct number of tx_byte clock cycles to match the number of pixel clock cycles. This means that all the events used for the packet generation will be directly impacted by the delay that exists in the DPHY High Speed request to ready.
The DSITX controller relies upon the DPHY PLL being programmed and locked before starting the DPI video. The DSITX controller can then be configured and enabled (clock lane and data lanes) before starting the video transmission.
The DSITX controller will begin data transmission with a High Speed request after it receives the falling edge of the VSYNC on the DPI interface. The controller will then begin to build the bytes for the VSYNC packets (VSS, HSA, Blanking packets) during the delay period before the ready is returned from the DPHY. This delay will add an interval effect on all the packets that follow.
The DPI FIFO size must be sufficient to buffer all the incoming pixel data during the active line stage while the DSITX controller continues to send the previous lines packet information due to this interval effect.
The DPI and DSI system may not be able to guarantee the exact frequency on the pixel clock input or tx_byte clock from the DPHY. The DSI configuration can be adjusted to support a small deviation in frequencies using the size of the HFP packet so that it changes the alignment of packets on the active lanes to absorb the different pixel clock cycles during the line. More details are given in section0.