More people watch TV (Part 2)

I have three problems that I need to tackle; USB, 3x HDMI ports, HDMI-CEC. So again, lets head to the service manual to avoid embarrassment.

 

Connectors

There are footprints for 2 USB connectors. Even though I don’t want to plug a memory stick in, it did pique my interest as to the differences. The top-most connector (CN125) is labelled DIGITAL_USB, as opposed to the plain USB of the bottom connector (CN112). Neither of the connectors have a direct connection to the 5V_VCC rail, but instead go through an IC labelled STMP2161. On my spare PCB this part is missing (as I expected), and I couldn’t find a datasheet or anywhere to get one from. Thankfully, the part has both pins and nets labelled, so it isn’t too difficult to at least identify the function of the part, allowing a suitable replacement to be found. The device features ENABLE and FAULT pins, of which the FAULT pin is connected to a net labelled USB_OCD. This device is probably a current limited load switch (OCD being short for Over Current Detected).

 

USB SchematicTests have shown the Raspberry Pi has a maximum current consumption of between 500-600mA. This is just a guess, but I doubt that the STMP2161could handle 600mA, so if anything I am lucky that I have to source my own current limited load switch. Failing that I could bypass the switch circuitry and use a resettable fuse instead.

Back to the differences between the two connectors. the DIGITAL_USB goes to a CT216T-R – the Cheertek Integrated DVB-T Receiver. Given this device has digital video and audio decoding capabilities, it is likely that the DIGITAL_USB connection would be used for playing video from a memory stick.

The plain USB connector goes to the MST6WB7GQ-3 – the main MSTAR microcontroller. This is the brains that bring everything together. I couldn’t find any other information about the MSTAR processor, or its USB capabilities. The service manual makes reference to an “Analog USB” port which I didn’t even think was possible.

Enabling the power for the DIGITAL_USB connector is easy, but I wonder how much work would be involved in getting the actual USB data side to work. I am going on the presumption that all MSTAR microprocessors go out the same, and it is just a configuration page in external EEPROM that enables the features. This idea explains a number of things found in the Service Menu (found by pressing 4725 whilst in the TV’s menu). Firstly, there are options for “Digital USB Hotplug”, “CEC Enable”, HDMI3, HDMI4 but they are all greyed out. Secondly, there is the ability to modify NVM (non-volatile memory) data. I think that modifying the correct byte in NVM will enable the greyed out features.

Before I modify the NVM, it makes sense to make a backup. But first I need to find the NVM. Hunting for NVM on the schematic leads to a net labelled SCL_NVM. This is an I2C clock line for the NVM device, in this case a Microchip’s 24C32. Admittedly, I’m getting a bit ahead of ourselves, but it is obvious how the building blocks all come together and what started as a horrifically complicated design is getting simpler.

Back to the USB, and there is another unidentified device – a AZ099-04S. I am guessing this is probably something to help with ESD – maybe an array of bi-directional¬†transient voltage suppressor (TVS) diode clamps. The last interesting part of the schematic is “Impedance 90R olacak”. USB is a differentially driven bus with a characteristic impedance of 90 ohms, and olacak is Turkish for “it will happen”. As to why there are Turkish instructions of the schematic, Vestel is a Turkish firm.

Jobs before the next post are dump the contents of any EEPROM on the board. This includes the main NVM EEPROM (U103) that hopefully enables features, and the VGA EEPROM (U112) responsible for identifying the TV as a PC monitor and presumably including EDID (Extended Display Indification Data). Additionally, the HDMI switch contains an EDID table that should be visible over I2C. There are two I2C flash chips; one on the MPEG decoder (U114), and one on the main microcontroller (U132), that can also be read.

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