Friday, 6 July 2012

RFnode 1.2b PCB update

RFnode PCB version 1.2b almost ready for printing and have made many changes with this version (and a few more to come).

Major changes so far:

  • Added SDcard slot for storage on the under side. This will allow the RFnode to be used as a data logger. The problem with SDcard slot is that takes up a large chunk of PCB space and the SDcard libraries take up a lot of programming space so if using a Atmega168 you need to keep a close eye on your program size.
  • Replace Red&Green LEDs with a RGB LED. This opens up the full spectrum of colors. This also means I loose 3 analog I/Os but gain 2 digital I/Os. This leaves me with -3 Analog I/Os (includes 2 for i2c bus)
    -6 Digital I/Os (Includes 2 for serial and 1 for SDcard if used)
  • Breadboard friendly pin header. Now all of the I/Os are on a single 11pin breadboard friendly header. The boards were going to be just for another project I am working but have decided to open it up a little. So that means making the board more general purpose and Arduino hacker friendly.  
  • Better power consumption. I ended up caving in and adding a switch to select power source. It seemed to be the most simplest and cost effective solution to the power consumption issue I was having with the power regulator.
  • Optional pull up resistor pads added for D6 & D8. Makes it easy to add 1wire digital sensors like the DHT11 & DHT22 temperature and humidity sensors.    

Future Changes:
The board still has room to grow so there are some more changes to come.

  • Size. The board has grown a little in this version while I sort out and move features around but is still only approximately 45mm X 35mm. Most of that space is due to connectors and a few parts.
  • Lithium ion support. Looking to add a charge and protection circuitry for Lithium ion battery packs. Looks like a MCP73831T could handle the charging of the battery but now need to source the cut off protection side of things to protect the battery voltage from dropping too low. The problem with this I am running into is cost. It is cheaper to but a phone battery that has this built in then it is to buy the circuit by it's self. Currently I do lithium ion battery protection via software were the software will check the battery voltage then if it is too low will go to sleep (but still consumes a little less then 0.1mAh) 
  • More I/Os. The Radio uses 6 digital I/Os and the RGB LED uses 3 analog I/Os. Then if Using SDcard, serial and i2c bus you only get left with about 4 I/Os so I want to add more I/Os. I know there are chips that allow using the i2c bus to give you an extra few I/Os but I am thinking about something a little more powerful then one of these guys. I am thinking about adding a whole second processor. If I add a second Atmega168/328 I could off load all of the sensor work to the second processor leaving the first processor to handle the communications (USB, Radio, SDcard, RGB LED) then the second processor to handle everything else with almost all of his I/Os (13 digital and 4 Analog).
  • More memory. One of the problems I ran into with creating a meshing protocol with the RFnode beside battery life was lack of memory, routing tables take a lot of space. I did come up with a few memory tricks like creating a u_int8[256] array using the index as the remote RFnode ID then the value being where to address the packet so the next node can follow the same process and pass it on. This allowed to have a routing table of less then 256 nodes in 256bytes of memory. With a Atmega168 this is still about 25% of ram of 50% of EEPROM space and does not leave much space for routing data like number of hops and route health but still possible.
    So to fix the memory issue I am thinking about adding a i2c based ram/flash chip. This will also allow data logging without using the SDcard. 

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