Having now had the Bubble Development Boards for a few days, I’ve tested a few more features.

Both JTAG sockets (Xilinx 14-pin and ARM 20-pin) work with their respective dongles.

The Ethernet adapter, a Wiznet W5100, is showing signs of life. If I plug an Ethernet cable into it, the ‘link’, ‘full duplex’ and speed indicator LEDs come on. Frequently the ‘link’ LED blinks continuously along with the Rx LED. I don’t know why, though searching on the web indicates that the W5100 can be fussy about the choice of transformer it’s used with, and sometimes about what kind of hub it’s plugged into – apparenly the automatic MDI/MDI-X negotiation can get confused. The tests were done with a TP-Link TL-SF1005D switch and I haven’t tried any other. Sometimes, though, the link will settle down to a steady state, then I see the Tx and Rx LEDs doing sensible things.

The W5100 chip is wired to the Samosa bus with very simple address decoding so that it should appear every 4 bytes across the whole 256-byte Samosa address range. Manually poking and peeking its registers from the Bubble board’s boot loader gave sensible results, and I even managed to get it to try and send a packet, the Tx LED blinking reassuringly.

Recent Linux kernels have a driver for the W5100. I tried kernel version 3.7.1 which has already been patched for the Bubble board. The existing drivers expect the chip to be I/O or memory mapped, which is not the case on this board because it’s on the Samosa bus. The Samosa bus is accessed using its own function calls inside the kernel. I patched the driver, and had to undo one or two assumptions made in the code about memory access, and the results are promising: the chip is found correctly, interface eth0 appears, and it even sends packets from the DHCP client attempting to get itself an address. However, the first attempt to send a packet results in an error:

NETDEV WATCHDOG: eth0 (w5100): transmit queue 0 timed out

and a stack trace, though the kernel carries on running. It seems that the Ethernet chip’s interrupts are not making it into the kernel. This turns out to be due to the offending interrupt line not being connected to anything in the Bubble board’s CPLD, so I’ll have to fix that next. That needs the Xilinx programming dongle which I haven’t got with me at the moment.

A couple of days ago I received the first examples of the Bubble Development Board and started testing them.

Much to my joy, the board fits neatly into a low-cost plastic case from Farnell, part number 1526699. With front and rear panels suitably cut out, it should be well protected and easy to connect things to.

But does it work? Well, yes, largely. Here are the results so far.

• Power: the LEDs (green for +5V input, orange for VDD_IO from the Bubble) come on and the Bubble board starts up reliably.
• Reset button: does what it says on the tin.
• Serial port: console works.
• USB host: tested to work with a memory stick and a USB-to-Ethernet adapter.
• USB slave: working. USB networking connection successfully established to a desktop PC as documented at http://www.balloonboard.org/balloonwiki/USBNetworking.
• Audio: output works, straight into a pair of headphones, and quality seems good with no unpleasant background noises. Haven’t tried input yet.
• HDMI: the Bubble board I was using was set up to drive an LCD panel, but by installing fbset and fiddling around with the settings (making them roughly 800×600, 60Hz) I managed to get a display on a monitor connected to the HDMI port. However, the image tends to disappear every few seconds and then come back again with some flickering, so something’s not quite right. I tried another monitor via an HDMI-DVI cable with the same result.
• Micro SD socket: not working yet. The kernel reports an error during boot and /dev/mmc* devices don’t appear. I’ve noticed that other implementations of the SD card socket from Bubble have pullup resistors on all the data and clock lines which I neglected to put on this board. This might be an easy fix because the wiring looks right.
• JTAG, Samosa, Raspberry Pi header, Ethernet: not tested yet.

Here’s another picture with the Bubble board turned over so you can see what’s on it.

I’ve just got V1.0 of a development board for the ‘Bubble’ embedded ARM module ready for manufacture, and will hopefully have prototypes soon.

It’s designed to make the Bubble board easy to work with, and brings out various useful connectors:

• JTAG ports compatible with Xilinx (14-pin) and ARM (20-pin) dongle pinouts
• Serial port for the console
• USB host, for memory sticks, keyboards and whatever else
• USB slave, for mass storage or USB networking
• Micro SD card
• HDMI for monitor connection
• Ethernet
• Stereo analogue audio in/out
• Raspberry-Pi compatible expansion connector with I2C, SPI and GPIO

It’s all powered from a Micro USB connector which should take a standard mobile phone charger, and is designed to sit neatly in a low-cost plastic box for protection.

Everything except the Ethernet port is already supported by Linux on the Bubble board, though making the HDMI port work will involve some fiddling with the LCD output timings. The Ethernet port is based on the Wiznet W5100 chip, which is popular in the embedded community and is supported by recent Linux kernels.