cannot get a clock line using Scanvideo_dpi

[ChrisABailey]

I am trying to use the scanvideo_dpi code to drive a small LCD. The LCD needs a pixel clock, VSync and HSync. I am having trouble getting the timing to work in several areas. I am new to PIO programming and I'm having trouble following all the code but basically I have done the following. I started with the TestPattern code in Pico-Playground but I have created my own video timing:

#define vga_mode psp_mode_480x272_60

const scanvideo_timing_t psp_timing_480x272_60_default =
        {
                
                .clock_freq = 8000000,

                .h_active = 480,
                .v_active = 272,
                .h_front_porch = 8,
                .h_pulse = 4,
                .h_total = 531,
                .h_sync_polarity = 0,

                .v_front_porch = 8,
                .v_pulse = 4,
                .v_total = 292,
                .v_sync_polarity = 0,

                .enable_clock = 1,
                .clock_polarity = 0,

                .enable_den = 0
        };

const scanvideo_mode_t psp_mode_480x272_60 =
        {
                .default_timing = &psp_timing_480x272_60_default,
                .pio_program = &video_24mhz_composable,
                .width = 480,
                .height = 272,
                .xscale = 1,
                .yscale = 1,
        };

I update the system clock to ensure it is a power of 2 times the pixel clock:
set_sys_clock_khz(128000,true);

finally I added the following to CMakeLists.txt

    target_compile_definitions(test_pattern 
        PRIVATE
        PICO_SCANVIDEO_ENABLE_CLOCK_PIN=1
        )

The code runs and I have a logic analyzer that shows basically good data on the R/G/B pins
but I do not think the sync pulses match what I would expect:

1. I cannot get a clock on either GP18 or GP19. The code looks like it should be on Pin 18 but also a bit of code that looks wrong (see below)
2. The R0 pin appears to "glitch" after every pixel so instead of being a steady low for 15 pixels and steady high for 15 pixels it it has a short pulse at the end of each pixel going the opposite of whatever it should be:

3. VSync also does not match what I would expect (but maybe it would work with analog VGA?

the code that I think might be an issue is in scanvideo.c:

    uint pin_mask = 3u << PICO_SCANVIDEO_SYNC_PIN_BASE;
    bi_decl_if_func_used(bi_2pins_with_names(PICO_SCANVIDEO_SYNC_PIN_BASE, "HSync",
                                               PICO_SCANVIDEO_SYNC_PIN_BASE + 1, "VSync"));

#if PICO_SCANVIDEO_ENABLE_DEN_PIN
    bi_decl_if_func_used(bi_1pin_with_name(PICO_SCANVIDEO_SYNC_PIN_BASE + 2, "Display Enable"));
    pin_mask |= 4u << PICO_SCANVIDEO_SYNC_PIN_BASE;
#endif
#if PICO_SCANVIDEO_ENABLE_CLOCK_PIN
    bi_decl_if_func_used(bi_1pin_with_name(PICO_SCANVIDEO_SYNC_PIN_BASE + 3, "Pixel Clock"));
    pin_mask |= 8u << PICO_SCANVIDEO_SYNC_PIN_BASE;
#endif

I think the pin mask for the pixel clock should be:
pin_mask |= 4u << PICO_SCANVIDEO_SYNC_PIN_BASE;
if the out pin is 1 past the VSync but this may be my lack of knowledge on the details of the PIO Side set commands. I have tried making this change but it did not fix my issue with the clock.

Has anyone used this code to drive an LCD directly from the parallel output pins?

[ChrisABailey]

So I discover one change that fixes several of my problems:

If you enable PICO_SCANVIDEO_ENABLE_CLOCK_PIN you must also enable PICO_SCANVIDEO_ENABLE_DEN_PIN

Once I did both the Clock appears correctly on GP19 and DE on GP18. This also fixed the glitch on low order Red Bit.

I'm checking the other timing now (I have not wired it up to my LCD to verify every thing). The basic waveform for VSync/HSync/DE still seem to not stay low Like I would expect.

Here is the data sheet which looks like most LCD data sheets I have seen:

and here is the new scan:

[lurch]

The code in the pico-playground and pico-extras repos isn't officially supported, and I suspect that the engineer(s) who originally wrote the scanvideo code are too busy with other projects to debug code that hasn't been touched in over a year. So you might get a quicker response if you ask for help at https://forums.raspberrypi.com/viewforum.php?f=143 (if you haven't done so already).