uVGA library

by Eric PREVOTEAU

1 Quick start

Import zip file using Arduino IDE library manager or place this folder in your libraries directory and start or restart the Arduino IDE

1.1 Wiring

• Hsync pin (default: 22) -> 82R resistor -> VGA 13 depend on HSync FTM channel

• Vsync pin (default: 29) -> 82R resistor -> VGA 14 can be changed to any pin except pin of port D

  on teensy 3.2, default Vsync pin is pin 10.

• Teensy pin 5 (port D, bit 7) -> 470R resistor -> VGA pin 1 (red)

• Teensy pin 21 (port D, bit 6) -> 1k resistor -> VGA pin 1 (red)

• Teensy pin 20 (port D, bit 5) -> 2k2 resistor -> VGA pin 1 (red)

• Teensy pin 6 (port D, bit 4) -> 470R resistor -> VGA pin 2 (green)

• Teensy pin 8 (port D, bit 3) -> 1k resistor -> VGA pin 2 (green)

• Teensy pin 7 (port D, bit 2) -> 2k2 resistor -> VGA pin 2(green)

• Teensy pin 14 (port D, bit 1) -> 390R resistor ->VGA Pin 3 (blue)

• Teensy pin 2 (port D, bit 0) -> 820R resistor ->VGA Pin 3 (blue)

• Teensy pin GND -> VGA pins 5,6,7,8,10

for more accurate colors, replace 2k2 by 2k and 470R by 510R

1.2 Basic usage

#include <uVGA.h>

uVGA uvga;

#define UVGA_DEFAULT_REZ
#include <uVGA_valid_settings.h>

void setup()
{
	uvga.begin(&modeline);
}

This will use the default resolution set for your CPU frequency.

You can choose a different modeline by changing the define. Accepted values are in uVGA_valid_settings.h

Then call any drawing or text functions.

For text, use uvga.print(...) and uvga.println(...) just like Serial.print(...) and Serial.println(...)

1.3 Basic usage with preallocated frame buffer

#include <uVGA.h>

uVGA uvga;

#define UVGA_DEFAULT_REZ
#include <uVGA_valid_settings.h>

UVGA_STATIC_FRAME_BUFFER(uvga_fb);

void setup()
{
	uvga.set_static_framebuffer(uvga_fb);
	uvga.begin(&modeline);
}

UVGA_STATIC_FRAME_BUFFER macro creates a frame buffer named uvga_fb, stored in DMAMEM area and uses UVGA_HREZ, UVGA_VREZ, UVGA_RPTL #define created in uVGA_valid_settings.h

2 Colours

color format is RGB332 (RRRGGGBB)

3 API

class instantiation

• uVGA(int dma_number = 0, int sram_u_dma_number = 0, int sram_u_dma_fix_number = 0, int hsync_ftm_num = 0, int hsync_ftm_channel_num = 0, int x1_ftm_channel_num = 6,int vsync_pin = 29, int graphic_dma = DMA_NUM_CHANNELS - 1)

Initialize class internal parameters.

Class requirements:

• 4 DMA channels (0-15), can be any.
• 1 FTM with 4 channels (on teensy 3.6, only FTM0 and FTM3 are possible. on teensy 3.2, FTM0 only). All others channels will be free.

If the first 3 parameters (DMA channels) are set to 0, DMA channels will be allocated using DMAChannel library.

On the chosen FTM, library will use channels:

• hsync_ftm_channel_num
• hsync_ftm_channel_num+1
• x1_ftm_channel_num
• x1_ftm_channel_num+1

hsync_ftm_channel_num and x1_ftm_channel_num MUST be even (0, 2, 4, 6)

The pair (hsync_ftm_num, hsync_ftm_channel_num) defines the pin generating Hsync signal. Look at teensyduino's teensy3/core_pins.h file, CORE_FTM*_CH*_PIN

On teensy 3.6, valid pairs (not on port D) are:

• (0,0) => pin 22
• (0,2) => pin 9. This FTM can use pin 13 but you will have to configure it yourself and problem may occur due to LED on this pin.
• (3,4) => pin 35
• (3,6) => pin 37. This FTM can use pin 57 but you will have to configure it yourself.

On teensy 3.2, valid pairs (not on port D) are:

• (0,0) => pin 22
• (0,2) => pin 9. This FTM can use pin 22 but you will have to configure it yourself.

1 or 3 DMA will be used to generate video signal, the 4th one is (will be) used to accelerate some drawing functions. Currently, only 2 functions support DMA acceleration (HLine & VLine) however, unlike the first 3 DMA channels which never runs at the same time, this one can run at any time and drawing a long line greatly disturb the other DMA channels thus DMA acceleration is currently disabled

If any parameter is invalid, the library will fallback to its default value.

• void uvga.set_static_framebuffer(uint8_t *static_frame_buffer)

Force library to use a dedicated frame buffer instead of letting it allocates the frame buffer itself.

If used, this function MUST be called before uvga.begin call.

Frame buffer can be easily created using a macro one line:

UVGA_STATIC_FRAME_BUFFER(your_frame_buffer_name_here);

• void uvga.trigger_dma_channel(uvga_trigger_location_t location, short int dma_channel_num)

Start a DMA channel automatically when a specific location is reached on screen.

If used, this function MUST be called before uvga.begin call.

It is possible to trigger multiple events but only one DMA channel can be trigger by one event.

Possible locations are:

• UVGA_TRIGGER_LOCATION_END_OF_DISPLAY_LINE

when HSync occurs, whenever line has pixels or not.

• UVGA_TRIGGER_LOCATION_END_OF_VGA_IMAGE

immediately after the last pixel of last image line.

• UVGA_TRIGGER_LOCATION_START_OF_VGA_IMAGE

before the first pixel of first image line. Warning the coordinates of the trigger is NOT x = -1, y = 0 (the "pixel" on the left of the first pixel of the image) but x = 0, y = -1 (the "pixel" above the first pixel of the image) which is technically the last time the VSync TCD is called.

• UVGA_TRIGGER_LOCATION_START_OF_DISPLAY_LINE

immediately after the beam moved to the left. Occurs whenever line has pixel or not.

Usage restriction:

• UVGA_TRIGGER_LOCATION_END_OF_DISPLAY_LINE

It depends on Hsync polarity. If polarity is positive, it occurs at position modeline.hsync_start. If polarity is negative, it occurs at modeline.hsync_end. This trigger generates a DMAMux event linked to FTM = hsync_ftm_num, channel = hsync_ftm_channel_num

• UVGA_TRIGGER_LOCATION_END_OF_VGA_IMAGE

supported in most modeline configurations. Exception when:

• UVGA_DMA_AUTO + frame buffer does not fit totally in SRAM_L + repeat_line > 2. In this mode, DMA channel linking from 1st and 2nd DMA channel are already used. The channel linking of the 3rd DMA channel is used in this case. The trigger occurs a bit later than in all other configuration but it should works properly
• Not available if frame buffer fit totally in SRAM_L + repeat_line = 1 + vertical resolution > 511.
• Not available if UVGA_DMA_AUTO + frame buffer does not fit totally in SRAM_L + repeat_line = 1 + first line in SRAM_U is > 511.

The 2 modes where this trigger is not available should not be a problem as they only occurs if horizonal resolution is ridiculously small (<128 and ~ <200 pixels/line)

• UVGA_TRIGGER_LOCATION_START_OF_VGA_IMAGE

supported in all modeline configurations

• UVGA_TRIGGER_LOCATION_START_OF_DISPLAY_LINE

supported in all modeline configurations. This trigger generates a DMAMux event linked to FTM = hsync_ftm_num, channel = x1_ftm_channel_num + 1

Manually modifiying DMAMux configuration to route the DMAMux event to multiple DMA channels will result in unpredictable behavior (see Kinetis Reference Manual, 23.4.1 Channel Configuration register DMAMUX_CHCFGn). The same result can be obtained by using several DMA channels linked together.

• void uvga.disable_clocks_autostart()

In standard case, uvga.begin immediatly start image generation. However, in some case, it may be required to delay this start to perform additionnal tasks.

If used, this function MUST be called BEFORE uvga.begin call. Later, uvga.clocks_start() must be called to start pixel DMA.

• void uvga.clocks_start()

Start image generation. If uvga.disable_clocks_autostart() was not called, there is no need to call this function else this function MUST be called AFTER uvga.begin()

• uvga_error_t uvga.begin(uVGAmodeline *modeline)

Initialize the display

Returns: 0 on success, uvga_error_t code on failure

Not all resolutions work on all monitors.

see below for uVGAmodeline description

• void uvga.end();

Stop the display. NOT TESTED

• void uvga.clear(int col=0);

Fills screen with color col, or black if col is not specified.

• void uvga.get_frame_buffer_size(int *width, int *height);

Retrieve the width and the height of the frame buffer. Width and height of the frame buffer are computed from modeline settings.

• int uvga.getPixel(int x, int y)

Return the color of the pixel at (x,y)

• void uvga.drawPixel(int x, int y, int col);

Draw pixel at (x,y) in colour col

• void uvga.drawLine(int x0, int y0, int x1, int y1, int col);

Draw line from (x0,y0) to (x1,y1) in colour col

• void uvga.drawTri(int x0,int y0,int x1,int y1,int x2,int y2,int col);
• void uvga.fillTri(int x0,int y0,int x1,int y1,int x2,int y2,int col);

Draw or fill triangle (x0,y0),(x1,y1),(x2,y2) in colour col

• void uvga.drawRect(int x0, int y0, int x1, int y1, int col);
• void uvga.fillRect(int x0, int y0, int x1, int y1, int col);

Draw or fill rectangle with corners (x0,y0),(x1,y1) in colour col

• void uvga.drawCircle(int x, int y, int r, int col);
• void uvga.fillCircle(int x, int y, int r, int col);

Draw or fill circle center (x,y) radius r in colour col

• void uvga.drawEllipse(int x0, int y0, int x1, int y1, int col);
• void uvga.fillEllipse(int x0, int y0, int x1, int y1, int col);

Draw or fill ellipse bounded by rectangle (x0,y0),(x1,y1) in colour col

• void uvga.drawText(const char *text, int x, int y, int fgcol, int bgcol= -1, uvga_text_direction dir = UVGA_DIR_RIGHT);

Draw text at any pixel position.

(x,y) is the top-left corner of the text before rotation.

fgcol is the colour of the text. bgcol is the colour of the text background or -1 for a transparent background.

dir is the direction of the text.

• UVGA_DIR_RIGHT is left to right,
• UVGA_DIR_TOP is bottom to top,
• UVGA_DIR_LEFT is right to left,
• UVGA_DIR_BOTTOM is top to bottom,

• void uvga.scroll(int x, int y, int w, int h, int dx, int dy,int col=0);

Scroll an area of the screen, top left corner (x,y), width w, height h by (dx,dy) pixels. If dx>0 scrolling is right, dx<0 is left. dy>0 is down, dy<0 is up. Empty area is filled with color col (only when horizontal (dy=0) or vertical scroll (dx=0))

• void uvga.copy(int s_x, int s_y, int d_x, int d_y, int w, int h);

Copy image area from position (s_x, s_y) to position (d_x, d_y).

Area is w pixels width and h pixels height.

The function supports overlapping area and off-screen. If source area is fully off-screen, nothing occurs. If source area is partially off-screen, w and h will be automatically adjusted to fit fully in screen.

• void uvga.drawBitmap(int16_t x_pos, int16_t y_pos, uint8_t *bitmap, int16_t bitmap_width, int16_t bitmap_height);

Draw an out of screen bitmap (size: bitmap_width * bitmap_height pixels) on screen at position (x_pos, y_pos).

The bitmap must have the same color mode as the modeline. The function will automatically clip the bitmap if it should be copied partially out of frame buffer.

• void uvga.moveCursor(int column, int line);

Move the print position to (column, line)

• void uvga.setPrintWindow(int x, int y, int width, int height);

Restrict the printing window to an area of width x height characters, at a position (x,y) (in pixel).

• void uvga.unsetPrintWindow()

Restore the print window to be the whole screen.

• void uvga.clearPrintWindow();

Clear the print window to the current text background colour.

• void uvga.scrollPrintWindow();

Scroll the print window up one line and moves the print position to the bottom.

• void uvga.setForegroundColor(uint8_t fg_color);

Set the text colour to fg_color (RGB332).

• void uvga.setBackgroundColor(int bg_color);

Set the text background colour to bg_color (RGB332) or -1 for transparent background

• virtual size_t uvga.write(const uint8_t *buffer, size_t size);
• virtual size_t uvga.write(uint8_t c);

These functions are similar to Serial.write, except output gets printed to the screen. These functions enable print and println to work correctly.

• void uvga.waitBeam()
• void uvga.waitSync()

These functions wait for the beam position to be off-screen. waitBeam will return immediately if the beam is already off-screen, waitSync will always wait for the next frame. These can be used to reduce flicker.

4 modeline

Used resolution is described using a modeline stored in an uVGAmodeline structure. Most of the data can be obtain from a standard modeline for Xorg server.

Standard modeline format is:

ModeLine "640x480" 25.20 640 656 752 800 480 490 492 525 -HSync -VSync

which means

• Pixel clock in MHz: 25.20
• horizontal resolution: 640
• HSync start: 656
• HSync end: 752
• Total pixels in line (including sync): 800
• vertical resolution: 480
• VSync start: 490
• VSync end: 492
• Total line per image (including sync): 525
• Polarity of horizontal signal: -HSync
• Polarity of vertical signal: -VSync

The modeline structure contains the following fields:

• int pixel_clock:

  it is the pixel clock. Its value cannot be higher than F_BUS frequency which depends on CPU frequency. The pixel clock is in Hz, not in MHZ.

• int hres:

  number of pixels per line. Unlike all other modelines values and because VGA is an analog signal, the number of horizontal pixels can be freely defined. For example. @180MHz and UVGA_HSTRETCH_WIDE, in 800x600@56, the maximum number of pixel is 445. @144Mhz, in 640x480@56Hz, you can fit 724 pixels per line :)

• int hsync_start:

  position of the start of the horizontal sync signal.

• int hsync_end:

  position of the end of the horizontal sync signal.

• int htotal:

  number of pixels per line including sync signal.

• int vres:

  number of lines per frame.

• int vsync_start:

  position of the start of the vertical sync signal.

• int vsync_end:

  position of the end of the vertical sync signal.

• int vtotal:

  number of lines per frame including sync signal.

• uvga_signal_polarity_t h_polarity:

• uvga_signal_polarity_t v_polarity:

  polarity of horizontal and vertical signal. Possible values are:

  • UVGA_POSITIVE_POLARITY
  • UVGA_NEGATIVE_POLARITY

• uvga_color_mode_t img_color_mode:

  video mode to use. The only possible value is UVGA_RGB332.

• int repeat_line:

  number of times to display each line of the frame buffer. The height of the frame buffer is vrez / repeat_line.

  • 1 means 640x480 (hres x vres) has a frame buffer of 640x480 pixels
  • with 2, frame buffer is 640x240 pixels
  • with 3, frame buffer is 640x160 pixels
  • with 7, frame buffer is 640x69 pixels

  frame buffer height is always rounded to immediate next integer

Some additionnal settings allow fine tuning of the video mode

• int horizontal_position_shift:

  Delay before displaying the start of line. Default value is 1. Increase to move image to the right if first pixels are not visible.

• uvga_pixel_hstretch pixel_h_stretch:

  increase width of pixels on monitor. This works by throttling DMA and inserting wait state. Default is no wait state. The other settings insert 4 and 8 wait states. Possible values are:

  • UVGA_HSTRETCH_NORMAL = 0 wait state
  • UVGA_HSTRETCH_WIDE = 4 wait states
  • UVGA_HSTRETCH_ULTRA_WIDE = 8 wait states

• uvga_dma_settings dma_settings:

  The library can display images using either 1 or 3 DMA channels. With 3 DMA channels, the image is more stable but requires roughly hres + vres * 4 more bytes. However, if the frame buffer is small enough to fit in SRAM_L (lower SRAM), there is no need to use 3 DMA channels to obtain stable image. Possible values are

  • UVGA_DMA_AUTO : let library decide if multiple DMA channels are required
  • UVGA_DMA_SINGLE : force library to use only one DMA channel

5 Miscellanous informations

• To enable debug mode in the library, edit uVGA.cpp and uncomment the line

#define DEBUG

This will dump a huge amount of data regarding FTM and TCD.

• If you create your own modeline structure from a standard modeline and the monitor fails to sync properly, it is probably because the library send image pixels during HSync signal. Try to lower hres value and if it is not enough, you can also increase horizontal_position_shift value a little.

• Frame buffer size is not width * height bytes because all lines address and size must be aligned on a multiple of 16.

A line uses (hres + 1 + 0xF) & ~0xF bytes. +1 comes from the black pixel added at the end of each line. UVGA_FB_ROW_STRIDE macro computes this automatically

6 How it works

A lot of magic... 1 FlexTimer (2 channels in complementary mode + 2 channels in PWM mode) and 1 (or 3) DMA channel

The FlexTimer uses 2 combined channels in complementary mode (asymetric PWM) to create HSync signal at the correct time. On the same FlexTimer, a 3rd channel (named X1 here) is used to start the DMA at the correct time on each line.

The DMA generates both image and VSync signal and once started never stops. The DMA uses a set of TCD (transfer control descriptor) linked together using DMA scatter/gather mode. The last TCD is linked to the first one thus the DMA never has nothing to do. Each TCD describes a line to process and is started by X1 FTM channel (the 3rd channel). The 4th channel (X1 FTM channel + 1) is not used by the library itself but generates a "new display line" event on DMAMux.

On a normal line, DMA copies as fast as possible all pixels of the line + 1 (1 black pixel is added at the end of each line to power off RGB pins). After the image, 3 TCD are used to set VSYNC signal properly, 1 before the vsync to wait it (may not exist depending on modeline), 1 at the beginning of vsync to set it and 1 after the vsync to clear it. Vsync TCD uses nearly no ressource because they copy 4 bytes only on each line.

It is not possible to fine tune DMA copy speed. Because it has the highest priority, it starts at the correct time (nearly every time :) )

I tried to adjust speed of DMA using FTM and PIT but it slowed down far too much and is not accurate due to round error (PIT@60MHz to obtain a 45MHz signal gives a FTM modulo of 1 with an error of 15Mhz). Moreover, additionnal wait cycles are added to start and stop DMA.

The 2 ways I found to slow down DMA is:

• the hard way: using DMA bandwith control to add wait states. Unfortunately, there is only 3 settings available: no wait state, 4 cycles wait state, 8 cycles wait state
• the extreme way: modifying CPU frequency

Due to the fact VGA is an analog signal, the width of each pixel is no really "defined". Using a 800x600 resolution, I successfully packed more than 1000 pixels on each line.

To improve video stability, pixel DMA channel has the highest priority among other DMA channels. Moreover, the crossbar switch is configured to give DMA a maximal priority to SRAM backdoor and GPIO. To gain one more cycle, SRAM backdoor port and GPIO port are parked to DMA when they are not in used. This gives a huge boost in performance. Finally, SRAM backdoor is configured to give absolute priority to DMA on SRAM_L and favor DMA on SRAM_U.

Everything is not perfect. Due to the fact pixel duration is approximated, the monitor may or may not totally understand what it receives and with some colors, pixels may be blurry (VGA like :) )

A last problem comes from SRAM. SRAM is splitted in 2, SRAM_L and SRAM_U. SRAM_L is accessed using CODE bus and has 0 wait state. SRAM_U is accessed using system bus and has at least 1 wait state. Unfortunately, the biggest part of the SRAM is SRAM_U.

To fix this problem, a 2nd DMA channel is used. For all lines located in SRAM_U, a copy will be performed to bring them back in SRAM_L before displaying them. This channel will be started using TCD dma channel link of the previously displayed line. In case repeat line factor is bigger than 1, to reduce bandwidth usage, the copy will happen only on new line, not on its duplicates.

However, this 2nd channel triggers a new problem. It is not possible for a TCD to modify destination address between TCD minor loop. This problem does not exist in the 1st channel because the destination address is always the same address. To bypass this problem, after TCD minor loop of the 2nd channel is processed, it triggers a start on the 3rd channel. TCD of this 3rd channel will simply reset the value of destination address (yes, the DMA reprograms one of its register itself :) ).

All these copies waste a bit of RAM bandwidth but the 2nd DMA channel copies are performed using burst mode and all these DMA TCD and DMA channel are automatically processed by the DMA engine without any CPU help.

Note: if all frame buffer lines are in SRAM_L, only the first DMA will be used

Finally, these system works perfectly... as long as nothing disturb it.

Hsync position is always correct because FTM channels cannot be bothered by anything.

Vsync position is roughly correct. Even if DMA starts a bit late, due to signal duration, it does not seem to disturb monitor (at least my old LCD 17").

The main problem is pixel generation. If the DMA is delayed, minor line oscillation can be visible. Due to the high priority, DMA seems to always obtain access to SRAM before CPU. The only thing which seems to delay DMA channel start is... the DMA itself. If another DMA channel performing a "long" transfer, despite having a lower priority, it delays pixel DMA channel. Performing various data copies using CPU is OK. A simple Serial.print is not OK.