Saturday, May 21, 2011
RAM and MAC
Today I have been working on a couple of the newer features of Nanode 5 - namely the SPI SRAM and the MAC chip.
As part of the improvements over Nanode 2, it was decided to make provision for an SPI memory device - surface mounted to the underside of the pcb. This device could be SRAM, EEPROM, data flash or FRAM - the choice would be left up to the user.
Personally, I have chosen SRAM for this first board, as it is cheap and easy to use and I am using similar devices on a project at work.
However the 23K256 SRAM has a maximum voltage of 3.6V and the Nanode is 5V, in order to maintain compatibility with the Arduino. However this compatibility issue is starting to cause problems, and so I decided to reconfigure the board so that everything is powered from 3V3. I had also just received a new batch of MCP1702 250mA 3V3 regulators, so it was a good opportunity to test the board running on one of these regulators. More and more hardware seems to be going down to lower voltages in order to save power, so it is fitting that Nanode should follow suit.
Once the 3V3 rail was up and tested, it was time to solder on the tiny 23K256 SRAM chip - and the even tinier 11AA02 MAC device - which just looks like a SOT23 transistor. Above are the two devices soldered to the board underside. The thin green wire is a temporary modification to power everything at 3V3. The SRAM is fairly central, and the tiny MAC chips is to the south west of the SRAM.
Later on, I modified some example code in order to test the SRAM. It takes about 4 seconds to write to the whole 32K bytes. This isn't very fast - but quick enough for Nanode and the sort of HTML serving tasks likely to be encountered.
I was hoping to work on the MAC chip which uses a unique 1 wire protocol called UNI/O and only needs one dataline to communicate with the microcontroller. Unfortunatelythe Microchip server which serves their data sheets and application notes is on maintenance today and the code examples are not accessible. Tomorrow , I'm of to the London Hackspace with Sam Carlisle and Matt Gaffen, so I should have an opportunity to get the Nanode reading its unique MAC address.