Sunday, September 14, 2014

On-chip decoupling capacitors

In virtually all of my micro-controller projects, I'll use 0.1uF ceramic capacitors between Vcc and Gnd.  Depending on the power draw of the MCU and inductance on the power lines, they may not be necessary, but at a cost of a penny or less each there's little reason not to use them.

I remembered seeing CPUs that have on-chip decoupling capacitors, and thought it would be nice if the MCUs I'm using had the same.  When working with small projects on mini breadboards, not having to find space for the decoupling cap would be convenient.  It would also save me the trouble of digging through my disorganized collection of components looking for that extra capacitor.

My first idea was to glue a 0805 (2mm x 1.25mm) MLCC to the top of the chip, and then solder 30AWG wire-wrap to the power and ground leads.  I used contact cement, and although it seemed secure after drying for about 30 minutes, once I added flux and touched it with my soldering iron tip it moved freely.  Then I tried a small drop of super glue, but for some reason it wasn't dry after an hour; maybe it was defective.  If someone knows of a glue that would work well, let me know in the comments or send me an email.

Even after drying for a day, neither the contact cement nor the super glue would securely hold the capacitors while I tried to solder them.  With the help of a pair of tweezers I was able to solder a MLCC to the top of an ATtiny85 as shown in the photo above.

For 28-pin DIP AVR MCUs that have ground and power on adjacent pins, the job is a lot easier.  Here's a ATtiny88-PU with a 0805 MLCC:

The easiest method I came up with doesn't require any glue.  I trimmed, then soldered the leads of a ceramic disc capacitor to the power and ground pins of an ATtiny84a:

2014/10/20 Update:
I found information about a high-temperature component adhesive which indicates typical cyanoacrylate adhesive is stable only up to 82C.  For something that is readily available in hardware stores, I may try silicone adhesive, or even BBQ paint.

Friday, September 12, 2014

Inside the "$10" Rockchip TV dongle

Last year Rockchip demoed a "$10" Miracast/DLNA TV dongle.  The $10 was not a target retail price, but probably the BOM cost.  They can be found for under $20 including shipping from China.  I bought one and posted a review of the Miracast and DLNA functionality.  In this post I'll document a basic teardown of the dongle, along with instructions on setting up a root console connection.

There are no screws holding the dongle together; the top and bottom of the case simply snap together.  The RK2928 is underneath a small heat spreader.  Next to it is a Spectek PE937-15E 256MB DDR3 chip. the rest of the components appear to be for power regulation.

On the bottom of the board is a Winbond serial flash chip and a Realtek rtl8188ETV USB wifi module.  Small pads labeled TX, RX, and GND are visible near the edge of the board.  They are obviously for a serial console.  I soldered some 30AWG wire-wrap wire to the pads, and to some 0.1" header pins.  With my soldering iron I melted an opening in the plastic case where I hot glued the header pins.

After putting the dongle back together, I connected the serial console to a USB-TTL serial adapter, and powered up the module.  I could see the Rx LED on the serial module flickering, indicating it was receiving console output data.  I started a terminal program, and was not seeing anything when I tried 9600 and 19,200bps.  Then I remembered my logic analyzer has a serial baud rate detection.  I hooked it up, ran a capture while the dongle was booting, and found the baud rate was 115,200kbps.

After setting the terminal program to 115,200, I could see what I recognized as the console output of a Linux kernel.  When the output stopped, I saw a "#" indicating a root shell prompt - no password required.  However the prompt wouldn't last long before the dongle seemed to reboot and repeat the start-up console output.

I first checked the board to make sure nothing was shorting from the connections I made to the serial port pads.  I then used a home-made USB cable with exposed power connections to measure the voltage.  I found the voltage was briefly dipping below 4.5V when the dongle was rebooting.  The dongle's peak power draw was too much for the PC USB port.  My solution was a 220uF capacitor connected to my home-made USB cable.  With the capacitor added, the voltage stayed above 4.8V, and there were no reboots.

The Linux installation seems to be a stripped-down android image rather than a standard linux distribution.  While the 256MB of RAM is ample for an embedded linux distribution, the 16MB of flash makes installing something like Picuntu Linux very difficult.