The Tech Blog

Back to Basics – Watchdog Timers(WDTs)

Posted under Back to Basics, Educational, Technology on December 25th, 2022 by Amaldev

WDTs are probably one of the most skipped sections in an embedded system because a first-time firmware engineer firmly believes that the code that they wrote is 100% bug-free. Somehow they feel that it’s not needed because they have put the code through tons of testing and still couldn’t crash the firmware. Seasoned folks will tell you that this is absolutely wrong, users on the field will definitely find ways to break your system. WDTs are an absolute necessity for anything which you ship out.

So what are they basically? A WDT is a type of timer that is used to detect and recover from system hangs. It works by continuously monitoring the system and resetting itself after a certain period of time. If the system fails to reset the timer(meaning it has gotten stuck somewhere) before it expires, the WDT will trigger a system reset or other recovery action. It’s analogous to you petting a dog at a constant interval and the moment you stop or miss the petting, the dog will bark/bite to alert you. Hence the name “Watchdog”.

There are diff variations of WDT. Internal WDTs are something which you find commonly in new-age controllers wherein a certain section on the chip is a dedicated timer to perform the WDT. External WDT variant usually utilises an external chip to perform the same. The general consensus is that for anything mission-critical you use an External WDT, just avoid the hardware locking up in some extremely brutal edge cases if you can afford to have that extra part. WDTs in general are not only used for a hard reset, sometimes can be multi-staged wherein you can trigger Action#1 when Timer#1 runs out via interrupts and so on and give the system to correct it and the last one being a hard reset. Some internal WDTs can be used to trigger an interrupt and then give certain processing clock cycles so that some debug state info can be saved before issuing a hard reset.

It’s an understatement to mention that WDTs are essential. NASA Clementine moon spacecraft in 1994 was a casualty of badly implemented WDT in software whereas Mars Pathfinder had a well-built WDT which saved millions of dollars.

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Teardown of Akash Tablet

Posted under DIY, Hacks, Repair, Teardown on December 18th, 2022 by Amaldev

Found this old tablet lying around in our labs. It’s an old Akash tablet (2013 Model). It was designed and manufactured by the Indian company DataWind. It was a $50 tablet launched by the Indian Govt to provide low-cost tablets to students in order to increase access to educational resources and improve learning outcomes. IIT Bombay procured around 100k tablets for distribution across the country.

Coming to the internals and specs. It’s a capacitive 7-inch screen tablet with a front-facing VGA camera. The main processor is an Allwinner A13 SoC (ARM Cortex-A8 1GHz) which runs Android 4. It has a nifty Power Management IC in AXP209, which has 2 DC-DC converters and 5 adjustable LDOs and can support Lithium battery charging upto 1.8A. The I2C interface connects it to the main processor. The device supports WiFi connectivity with a prebuilt Realtek module, RTL8188CTV. It has a pair of 256MB RAM modules(256X8DDR3-WT, I think from HMD). This model seems to have a NAND Flash memory(MT29F32G08CBACA) of 32GB from Micron with a possibility of extending it to 64GB which remains unsoldered on PCB. SSD2532QN6 is the capacitive touch panel controller.

Well, the device is almost 10yrs old, I tried to turn it ON and it doesn’t turn ON without external power plugged in. Seems the battery is in deep discharge(2500mAh capacity) and the protection circuit is preventing charging it up even when I desoldered and tried to charge it separately. I have to remove the protection and bring it to a level then do the normal charge. There is no bulging or anything so I think the battery is fine but as always there is a risk with deeply discharged cells.

Overall the Akash project was an ambitious exercise but the hardware wasn’t upto the mark with a lot of complaints of overheating, boot crashes, software glitches, older specs etc. It never truly reached its potential of being a game-changer for students before the smartphone wave hit the country.

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