This wiki page explains the device structure. It should contain enough information to enable users to create their own device.

I will start working on this wiki page and it might be incomplete for a (long?) time. Please excause...

The U8glib device is a C structure:

struct _u8g_dev_t
{
  u8g_dev_fnptr dev_fn;         /* device procedure */
  void *dev_mem;                /* device memory */
  u8g_com_fnptr com_fn;         /* communication procedure */
};
typedef struct _u8g_dev_t u8g_dev_t;

There are three parts in a device structure:

1. dev_fn (required): A callback procedure, which handles messages from the graphics procedures.
2. dev_mem (optional): A pointer to some local memory.
3. com_fn (optional): A communication procedure

Basic idea is, that (parts of) the frame buffer is stored via dev_mem. Display specific procedures (display init, pixel set and clear) are handled by dev_fn and communication specific details (TWI, SPI, etc) could be handled via com_fn.

It is always possible that dev_fn directly writes to the display device. In this case set com_fn to u8g_com_null_fn.

Here is a template for a very simple device:

uint8_t my_dev_fn(u8g_t *u8g, u8g_dev_t *dev, uint8_t msg, void *arg)
{
  /* handle messages here */
  return 1; /* successful handled */
}

u8g_dev_t u8g_dev_my_simple_display = { my_dev_fn, NULL, u8g_com_null_fn };

All the communication between the graphics procedures and the device callback procedure (dev_fn) is done via messages. A message is a number, which is passed in the argument msg of the dev_fn:

uint8_t my_dev_fn(u8g_t *u8g, u8g_dev_t *dev, uint8_t msg, void *arg)

In some cases, there are additional arguments and parameters together with msg. In this case arg is a none-NULL pointer to these additional arguments.

A typical device callback will look like this:

uint8_t my_dev_fn(u8g_t *u8g, u8g_dev_t *dev, uint8_t msg, void *arg)
{
  switch(msg)
  {
    case U8G_DEV_MSG_INIT:
      /* setup/init display */
      /* ... */
      break;

    case U8G_DEV_MSG_PAGE_NEXT:
      /* write a page to the display */
      /* ... */
      break;

    /* maybe handle more messages */
  }
  /* make a call to a suitable page buffer procedure */
  return u8g_dev_pb8v1_base_fn(u8g, dev, msg, arg);
}

This is a small reference of all messages:

• U8G_DEV_MSG_INIT: Setup and init display, no args.
• U8G_DEV_MSG_STOP: Shutdown display, no args (not used at the moment).
• U8G_DEV_MSG_CONTRAST: Set contrast or bightness of the display, arg is a pointer to uint8_t which contains the contrast value (0..255).
• U8G_DEV_MSG_SLEEP_ON: Display should enter sleep mode, no args.
• U8G_DEV_MSG_SLEEP_OFF: Display should recover from sleep mode, no args.
• U8G_DEV_MSG_PAGE_FIRST: Picture loop will start now. Page buffer procedures will reset the page handling to the first page and clear the buffer.
• U8G_DEV_MSG_PAGE_NEXT: A page has been finished, dev_fn should transfer page to the device, page buffer procedures will setup next page. 0 should be returned if there are no further pages required. There are no args for this message.
• U8G_DEV_MSG_GET_PAGE_BOX: Return dimensions of the page. arg is a pointer to u8g_dev_arg_bbx_t. This messages is handled by page buffer procedures.
• U8G_DEV_MSG_SET_PIXEL: Set a pixel. arg is a pointer to u8g_dev_arg_pixel_t. This messages is handled by page buffer procedures.
• U8G_DEV_MSG_SET_8PIXEL: Set a sequence of 8 pixel. arg is a pointer to u8g_dev_arg_pixel_t. This messages is handled by page buffer procedures.
• U8G_DEV_MSG_SET_COLOR_INDEX (until v1.12): Not used
• U8G_DEV_MSG_SET_COLOR_ENTRY (since v1.13): Set color palette entry. arg is a pointer to u8g_dev_arg_irgb_t.
• U8G_DEV_MSG_SET_XY_CB: Not used
• U8G_DEV_MSG_GET_WIDTH: Return the width of the display. arg is a pointer to u8g_uint_t. The width must be stored at the address pointed to by arg.
• U8G_DEV_MSG_GET_HEIGHT: Return the height of the display. arg is a pointer to u8g_uint_t. The height must be stored at the address pointed to by arg.
• U8G_DEV_MSG_GET_MODE: Return the display mode as return value of dev_fn, no other args. Possible return values are U8G_MODE_BW and U8G_MODE_GRAY2BIT

(device for stdout)