Beagle MID

Note: This document is not complete. It is being slowly updated as time permits. Please send mail to [email protected] if you are interested or have comments. If there is sufficient interest, the LCD interface and expansion board can be made available. Please send a mail if you are interested in the interface! HY Research is available for consulting and designs of custom expansion boards.

HY Research builds the first Beagle board based MID! Unlike other designs, this one actually uses a Beagle Board.

Features of it include:

5 way input, a "joystick"
3 way input, a "slider"
Buttons
480x272 4.3" LCD with a touch screen
Li Ion battery
Bluetooth (not installed; provisions for attachment and power are part of the design)
Blue LED for signaling events.

This is loosely modeled after the Nokia N800 I carry daily. The inputs have been moved around to better places. It is composed of a stock Beagle board with headers populated for the LCD signals and the expansion signals. A custom interface board was built to break out the signals and interface it. The case is custom CNC machined from a combination of ABS and acrylic to accomodate the LCD, BeagleBoard, and interface boards.

This device was implemented during "off hours" but an estimate of normal work hour time used was about 2 weeks (80 hours) to do both the mechanicals and the electronics. As built the device is roughly: 6.25"x3.25"x1.25" which can be considered large for a PDA/hand held device. The thickness dimensions can be reduced with minimal work. In particular, it can be reduced down to 1.0" thick by adjusting the machining. It was built to have extra thick walls to simplify machining. The ABS is 0.375" thick stock; 0.250 would be sufficient. It can be further reduced in thickness by rebuilding the LCD interface board. The LCD interface board has a higher profile then needed due to the lack of mechanical information for the location of the LCD headers at the time the board was designed; to work around the lack of information a ribbon cable setup was used. Since the mechnical information is available now, a redesigned board would make it overall thinner.

Pictures

This is the first fitting of the peices straight out of the CNC machine. The extra side peices is used to secure plastic for machining. The plastic peices are held by 2 10-24 socket head cap screws on each side bolting it to a fixture. There are 2 dowel pins used to locate the peice so the peice can be removed for inspection prior to completion. A fixture is used instead of a vise to simplify relocating and to allow the outside contour to be machined. Unfortunately the fixture mounting holes are not visible due to the angle/light of the assembly.

Case, first fitting.
Here is the inside guts.

Note the color wiring harness. This is the "ribbon cable" referred to earlier. It is there solely because pinout and mechanical details were not available at design time. The current plan is to spin a second rev of the LCD interface board to directly plug into the Beagle board and eliminate the wiring hardness. The LCD interface board is wrapped in paper as an insulator to avoid shorts with other components when it is fully assembled. The Lithium Ion battery has been removed for clarity.

The expansion header has a right angle 0.1" pitch header installed and mates directly with the board.

Right side. This is a 1.3mm jack for power input and charging.

Left side. Only the USB OTG Mini-AB jack is exposed. The power jack is intentionally blocked to prevent plugging in a power cable and conflicting with the on board power supply.

Top view. This shows the EHCI host only USB port and the SD slot. Also note the button input and the 3way input on the right side.

Bottom view. This shows the audio input, output, S-Video jack and the HDMI jack. The S-Video jack is responsible for quite a bit of the thickness. One way around it is to not expose it. The other way is to remove the jack and replace it with something thinner. Prehaps a RCA jack.

Videos

A video of the Beagle mid running is on YouTube.

Electronics

The Beagle MID contains 2 main custom boards. One serves as an LCD interface board. The other serves as a expansion board. The expansion board contains circuitry to generate all assortment of voltages needed, level shifters, and the touch screen controller, and the battery charger. Touch screen functions is handled by a TSC2046 from TI. This controller is widely used in OMAP boards. It handles a basic 4 wire resistive panel and is connected to the McSPI4 functionality on the expansion port. Level shifting is handle by a TXS0406e bidirectional converter. It converts from the 1.8V used by the OMAP to 3.3V used by the UART interfaced bluetooth module. A bidirectional converter was choosen even though the direction of each line is known ahead of time because this avoids having yet another package to deal with.

All the button inputs, 5way input, and the 3 way input is implemented by a direct connection to the GPIO inputs. They all rely on the internal weak pull up for functionality. As such, the software MUST enable the weak pull up.

The power section generates 5V @ up to 600mA with the use of 2 charge pumps. 3.3V is generated with a LDO against the 5V line. A future rev of the board is likely to run the LDO against the raw battery input for added efficiency. A second LDO generates 1.8V for local use. This avoids loading the 1.8V line provided by the Beagle. A power mux is used to select between power from the battery or power from the AC input. The reason for this is a simple slow charger based on the MAX1555 was choosen. By muxing out the battery during charging, we avoid discharging the battery while it is plugged in.

The LCD board is designed to interface with a Sharp 4.3" 480x272 WQVGA LCD display. This is the display used on the Sony PSP. This LCD adds a few complications to the design. The first is it requires 2 voltages, 5V and 2.5V. In addition the 5.0V needs to be applied after the 2.5V is available. To address this, a LDO is used to generate 2.5V from the 5.0V line provided by the LCD header. The only other complication is the LCD backlight consist of several LEDs wired up in series. This means it needs around 24V. The 24V is generated by a Linear Tech switcher, LT1932.

The rest of the LCD interface board are level shifters to convert from the 1.8V logic used by the OMAP and the 2.5V logic used by the LCD. The 1.8V reference voltage is from the LCD header. 4 74AVCH8T245 are used to convert the RGB and clock signals.

A second revision of the LCD interface board has been made. This board is improved in size and has an improved wiring harness.

Software

This Beagle hand held platform is a convenient device to proving out various userland environments. As demostrated, a basic matchbox based build from Angstrom was used. As time permits, other userland environments will be ported.

Android
Mer (planned, no action taken).

HY Research