Re: [PATCH v2] auxdisplay: hd44780: move cursor home after clear display command

David Reaver
2023-07-26T15:16:48Z

Hugo Villeneuve <hugo@hugovil.com> writes:

> From: Hugo Villeneuve <hvilleneuve@dimonoff.com>
>
> The DISPLAY_CLEAR command on the NewHaven NHD-0220DZW-AG5 display
> does NOT change the DDRAM address to 00h (home position) like the
> standard Hitachi HD44780 controller. As a consequence, the starting
> position of the initial string LCD_INIT_TEXT is not guaranteed to be
> at 0,0 depending on where the cursor was before the DISPLAY_CLEAR
> command.
>
> Extract of DISPLAY_CLEAR command from datasheets of:
>
> Hitachi HD44780:
> ... It then sets DDRAM address 0 into the address counter...
>
> NewHaven NHD-0220DZW-AG5 datasheet:
> ... This instruction does not change the DDRAM Address
>
> Move the cursor home after sending DISPLAY_CLEAR command to support
> non-standard LCDs.
>
> Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
> ---
> drivers/auxdisplay/hd44780_common.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/auxdisplay/hd44780_common.c b/drivers/auxdisplay/hd44780_common.c
> index 3934c2eebf33..7cbf375b0fa5 100644
> --- a/drivers/auxdisplay/hd44780_common.c
> +++ b/drivers/auxdisplay/hd44780_common.c
> @@ -82,7 +82,15 @@ int hd44780_common_clear_display(struct charlcd *lcd)
> hdc->write_cmd(hdc, LCD_CMD_DISPLAY_CLEAR);
> /* datasheet says to wait 1,64 milliseconds */
> long_sleep(2);
> - return 0;
> +
> + /*
> + * The Hitachi HD44780 controller (and compatible ones) reset the DDRAM
> + * address when executing the DISPLAY_CLEAR command, thus the
> + * following call is not required. However, other controllers do not
> + * (e.g. NewHaven NHD-0220DZW-AG5), thus move the cursor to home
> + * unconditionally to support both.
> + */
> + return hd44780_common_home(lcd);
> }
> EXPORT_SYMBOL_GPL(hd44780_common_clear_display);
>
>
> base-commit: d192f5382581d972c4ae1b4d72e0b59b34cadeb9

I tested this on a 16x2 HD44780 on my BeagleBone Black with:

$ printf '\f' > /dev/lcd
$ printf 'Hello\nWorld!\n' > /dev/lcd
$ printf '\x1b[LR' > /dev/lcd
$ printf '\x1b[LR' > /dev/lcd
$ printf '\x1b[LR' > /dev/lcd
$ printf '\f' > /dev/lcd
$ printf 'Goodbye\nWorld!\n' > /dev/lcd

As expected, "Goodbye World!" was correctly placed left-aligned on the
display, split over both lines.

Tested-by: David Reaver <me@davidreaver.com>