BB View 7" LCD on Debian Buster 10.3: Abandonware revisited

 

BB View LCD by element14. Note: the FPC cable when correctly inserted may not show printing topside

The BB View LCD came out about mid-2013, and was one of the earliest LCD panels for the beaglebone. It came with a "cape", ie a beaglebone interface board. I bought mine in December 2014 and could not get it to work. No doubt it worked for many other people: it came with kernel patches and source code for Angstrom, the TI SDK as well as Debian. It is still listed for sale in element14 and Mouser, but recently has become "unavailable".

Currently, there is a worldwide semiconductor shortage, and component leadtimes are now a few months. The temptation to press the BB View 7" LCD back into service became irresistible. I think my problem was I used the kernel patches with a later, 2014 version of Debian while the patches were mid-2013. The beaglebone black could recognize the cape and read its eeprom, but the LCD stayed blank, and the backlights kept blinking in perfect unison with the beaglebone CPU LED.

While no walk in the park, I thought it should possible to get it working. After all I have source code and it should only be a matter of time. And given the pandemic, I have four weeks of lockdown time, to be precise. And hopefully I should learn a little about kernel display drivers too.

First you need a beefy 5V power supply. I used a 5V@3A one. While the average current draw of the BB View LCD is not that high at 240mA, there are regular nasty spikes. I got the occasional CPU crash along with sdcard errors.

Beaglebone Black Serial Debug Header: Pin 1 is on the right

You also need to connect to the serial debug port. Any USB serial TTL adapter will do. The pinout is:

Beaglebone    Pin Description      USB Serial TTL Adapter
1                      Gnd                         Gnd
4                      TX                           RX
5                      RX                           TX

The Beaglebone Black manual specified an FTDI USB adapter, but I used a cheap CH340, which worked well enough. The baud rate is 115200, 8-bits and 1-stop with no hardware flow control.

FTDI Serial Debug cable. Click on picture for datasheet

The terminal program I use is minicom, set to 115200 baud, 8-bits, 1-stop and without hardware flow control. If you have the proper FTDI cable, you should use hardware flow control.

This may be a no-brainer to some, but an LCD's size is measured at its diagonal. The BB View LCD comes in 2 software-incompatible sizes, 4.3" and 7". For the unwary, a 7" LCD measures close to 4.3" on one side.

Also, the FPC cable is inserted with the metalled side facing down on both ends. And may not look like the picture on top. Each matching connector has a little lever which you pull up. Do not force the FPC cable.

The Debian image I used was the latest as of this writing, which is bone-debian-10.3-iot-armhf-2020-04-06-4gb.img.xz. 

To transfer the image to sdcard, I did:

# xzcat bone-debian-10.3-iot-armhf-2020-04-06-4gb.img.xz | dd of=/dev/sdb

Do take care to ensure /dev/sdb is the sdcard else you might end up wiping the drive on your laptop/desktop.

The changes to the sdcard /boot/uEnv.txt boot config file are:

dtb_overlay=/lib/firmware/BB-VIEW-LCD7-01-00A0.dtbo

disable_uboot_overlay_video=1

disable_uboot_overlay_audio=1

You can download the uEnv.txt file here. Notice that it specifies a binary device tree overlay file, BB-VIEW-LCD7-01-00A0.dtbo. The latest and greatest bone-debian-10.3-iot-armhf-2020-04-06-4gb.img.xz happened to have them pre-compiled. I have not seen it in earlier debian images, so this is indeed a stroke of luck. I have uploaded a copy here for your convenience. 

By the way it also has BB-BONE-LCD7-01-00A0.dtbo which also works except for an annoyingly dim backlight setting and the inability to adjust it.

These dtbo files are usually something that you get by recompiling the BB View Debian source files. However, the manufacturer supplied BB View Debian source file BB-VIEW-LCD7-01-00A0.dts did not work for me when compiled to dtbo files. On close examination it is missing pin definitions for 8 LCD panel data pins, lcd_data16 - lcd_data23.

Typically the beaglebone black boots by default from the eMMC, so you need to hold down the 'Boot' button on the beagleboard before you power on. The BB View cape gets in the way, but it helpfully provides a second 'Boot' button on the cape. 

Given the power demands of the BB View, I found it more reliable not to boot from the power switch. Rather there is a 'Power' button on the beaglebone black which you can use. 

The LCD displayed the login screen for tty1, and you can log in by plugging a USB keyboard into into the USB Host (ie USB Type B) connector.

Working BB View 7" LCD with Debian Buster 10.3

To display picture files from the command prompt I used fbi with a little help from Drew Fustini:

root@beaglebone:~# apt-get update

root@beaglebone:~# apt-get install fbi

# wget https://kernel.org/theme/images/logos/tux.png

# fbi -d /dev/fb0 -T 1 -a tux.png

For video I used mplayer:

# apt-get install mplayer

This worked for me as there is no audio when using the BB View LCD:

# export SDL_NOMOUSE=1

# mplayer -nolirc -vo sdl:driver=fbcon  -ao null  test.mpg

My test mpg file happened to have a resolution of 480x360. The BB View 7" LCD is 800x600 but mplayer could not quite get the scaling correct. Whan scaled manually this works:

# mplayer -nolirc -vo sdl:driver=fbcon  -ao null -x 800 -y 533 test.mpg

mplayer 

Debian 10.3 did not seem to recognise the touchscreen, but a little peek at the decompiled dtbo file mentioned a tscadc kernel modue:

        fragment@5 {

                target = <&tscadc>;

                __overlay__ {

                        status = "okay";

                        tsc {

                                ti,wires = <4>;

                                ti,x-plate-resistance = <200>;

                                ti,coordinate-readouts = <5>;

                                ti,wire-config = <0x00 0x11 0x22 0x33>;

                                ti,charge-delay = <0x400>;

                        };

                        adc {

                                ti,adc-channels = <4 5 6 7>;

                                ti,chan-step-opendelay = <0x098 0x3ffff 0x098 0x0>;

                                ti,chan-step-sampledelay = <0xff 0x0 0xf 0x0>;

                                ti,chan-step-avg = <16 2 4 8>;

                        };

                };

        };

A search of the dmesg log produced:

[Thu Jun 24 10:21:40 2021] input: ti-tsc as /devices/platform/ocp/44e0d000.tscadc/TI-am335x-tsc.0.auto/input/input0

Which led me to

# ls -l  /sys/devices/platform/ocp/44e0d000.tscadc/TI-am335x-tsc.0.auto/input/input0/event0

A quick check of the shows that tapping the LCD does indeed produce a response:

# hexdump -C -v /dev/input/event0

...

00000780  81 9f d5 60 e6 e6 06 00  03 00 01 00 2c 08 00 00  |...`........,...|

00000790  81 9f d5 60 e6 e6 06 00  03 00 18 00 92 03 00 00  |...`............|

000007a0  81 9f d5 60 e6 e6 06 00  00 00 00 00 00 00 00 00  |...`............|

000007b0  81 9f d5 60 1f 03 09 00  01 00 4a 01 00 00 00 00  |...`......J.....|

000007c0  81 9f d5 60 1f 03 09 00  03 00 18 00 00 00 00 00  |...`............|

000007d0  81 9f d5 60 1f 03 09 00  00 00 00 00 00 00 00 00  |...`............|

Another bit of electronics rescued from the scrap heap cannot be that bad.

Happy Trails.

PS

Why not use the element14 files and go with the BB View manual? Believe me, I tried. For 14 days. I started off with the oldest Debian images I could find, co-incidentally the same Debian 7.5 of 2014-05-14 that I used back then. And got the same result. And since there were 3 different patches, one each for Angstrom and TI SDK, I tried them as well but none worked. The Angstrom version was older 2013-06-20 and worked slightly better: the LCD backlight did not blink in concert with he CPU light and was quite controllable on its own. But in all cases the LCD stayed resolutely blank. 

There is a wealth of information on the subject, not to mention source code. But there is no schematic, so reading it would be harder. Two weeks later, I could rebuild the latest TI SDK, or even mainline Linux kernel using Linaro, but neither version worked with the BB View LCD. The problem as mentioned earlier was the device tree source code, which I suspect was incomplete for the 7" LCD. The 4.3" version probably worked, but I did not test this. 

It was starting to become clear why there was such heavy development over so many years: there is precious little standardization in ARM systems on how to interface to external boards. Each ARM board basically is unique, and back in 2013 this was hardcoded into the kernel source.

At some point each variation was defined in a 'device tree' that was compiled separately from kernel. In practice this simply means a separate sub-directory with its own Makefile all under the same master ARM build. 

Eventually, with a little encouragement from Linus Torvalds, the device tree code was shifted to the ARM bootloader, 'Das U-Boot'. This is where the Beaglebone Debug Serial Port comes in handy. Many of the BB View LCD problems are reported only as system console messages, ie to the Debug Serial Port, and may not even show up in the kernel log. 

Thus, unlike the manufacturer, Debian admirably maintained support for the BB View. As usual, the documentation became hopelessly out of date, as befits abandonware. 

After 2 weeks, Lady Luck took pity on me. I always have a late version of Beaglebone Debian, in this case Debian Buster 10.3 of 2020-04-06. The micro-sdcards are too tiny to label, and I swapped it into the BB View LCD system by mistake. It complained it missed the file BB-VIEW-LCD7-01-00A0.dtbo which just happened to be in the eMMC partition (as /dev/mmcblk1p1). One small change in the boot config file and it worked! Took all of 5 minutes.

Which leaves me 2 more weeks of lockdown time to fill. Maybe it's time to watch 'Das Boot' again ...

Das Boot: Wolfgang Petersen's 1981 classic