Truly LCD front panel: the backlight (2)

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Here is the follow-up of the post where I described how I took out the front panel of a router (yes, a router) and found a way to interface it with Arduino or other development board. It should be noted that the front panel electronics use 3.3V levels, therefore the popular 5V Arduino boards cannot drive the front panel. Using level shifters would complicate things and increase the possibility of something going wrong, so I ended up using a 3.3V STM32 blue pill development board. This is programmed from Arduino IDE, so the code I write is compatible with Arduino development boards.

While I was sampling various pins of the front panel connector with a logic analyzer, I noticed a strange protocol on pin 18. I was able to trace the PCB track from pin 18 near an area that seemed like a DC-DC converter. It directly drove an integrated circuit marked T43. Searching for it revealed some LDO linear voltage regulators, but this was not the case. Pin 18 carried a digital protocol that would be of no use for an ordinary voltage regulator. But without information I could only write code that would mimic the protocol I sampled. Things changed once the GPL source code has been made public. The signal on pin 18 had a meaning. It was necessary to turn on/off and dim the backlight. Upon powering the front panel on the breadboard, the backlight stayed off. You can turn it on by setting pin 18 high but if you want to adjust its level you must send two bytes using a custom serial protocol. Before getting to the code let’s see an overview of the pins and connections on the breadboard.

Front panel connector adapter on breadboard
Front panel connector adapter on breadboard

Router's LCD and keypad interfaced to Arduino (1)

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A router with display is not something you see everyday. That’s why when I saw two such routers that were discarded I bought them. At that time I had absolutely no idea if I could install an open source firmware on them (such as OpenWrt). I didn’t even know what type of display do they use. Currently, there is no way of installing a 3rd party firmware on those devices. But the front panel of the router can be interfaced to a microcontroller (only a specific hardware version).

The devices I’m talking about are SerComm SHG1500 routers, used for ADSL internet. They are based on Broadcom BCM6361 SoC and, although this platform is supported by OpenWrt, a specific build for this device or a way to upgrade firmware isn’t known. So I gave up this idea. But upon opening the case, the front panel with LCD display looked like a module that could be used for my projects. The LCD is color TFT, 2.8” size. Next to it there is a capacitive keypad with 5 keys. Front panel plugs into main board using a 2x15 pins, 1.27 mm pitch connector. It seemed good enough to start gathering information.

SerComm SHG1500 front panel with LCD and capacitive keypad
SerComm SHG1500 front panel with LCD and capacitive keypad