LTC6802 based 12 cell slave.
Posted: Sun Jun 10, 2012 9:56 pm
This topic is for a slightly different approach to cell monitoring. Using a LTC6802-1 chip we can monitor up to 12 cells. While the chip is designed for stacking, I have decided not to go this route, but instead mount each one on a pcb with a 16F886 pic to control it and get the cell data out of it. This approach makes the system very modular because it uses the same topology as the present setup with a Master sending out commands on the slave bus and receiving data back on the master bus. Just daisy chain as many slaves as required. Aside from the use of the 6802 for cell measurements instead of generic pics, the other main difference is where isolation from the traction pack occurs. In this design the 6802's spi bus is isolated from the pic by U2-U5. The pic will control the data rate which I plan to run very slow, like 1200 baud which will be more than enough for measuring 12 cell voltages once per second. The pic can anticipate when will will ask it for a measurement and get all the readings ready for when it receives the request on the slave bus.
All the slaves will be powered from the master. While the buses aren't isolated there is nothing stopping us from powering it with a isolated 5 volt supply and using optos at the master. This trick works well for the existing master bus and would add an extra level of protection. For master-slave (-slave...) connections I have chosen to use off the shelf A-B usb cables. These are available any where in any length from 0.2 meters on up. While the signals won't be USB compatible, it shouldn't cause any damage trying to connect usb devices to it, they just wont work.
I've put 2 leds on the schematic. Command 7 is very useful for diagnosing problems, so I want to keep it. The other led can indicate that the slave has initialized the 6802 and all self tests are o.k. (the 6802 can carry out self test on itself and also detect open cell connections).
Each slave will monitor from 4 to 12 cells with a total stack voltage of 10 to 60 volts.
Previous schematics and initial discussion of this slave board can be found here.
Here's my latest schematic, main changes are as follows;
J1 is now a DB15 socket.
Added D3 for input polarity protection, just in case.
Added a prototyping area to help with fine tuning bus communications.
Any comments, questions or errors that I may have made?
I am going to work on footprints and then do a pcb layout. For the initial build I will only populate the right half of the board and test master and slave bus communications before adding on the 6802.
All the slaves will be powered from the master. While the buses aren't isolated there is nothing stopping us from powering it with a isolated 5 volt supply and using optos at the master. This trick works well for the existing master bus and would add an extra level of protection. For master-slave (-slave...) connections I have chosen to use off the shelf A-B usb cables. These are available any where in any length from 0.2 meters on up. While the signals won't be USB compatible, it shouldn't cause any damage trying to connect usb devices to it, they just wont work.
I've put 2 leds on the schematic. Command 7 is very useful for diagnosing problems, so I want to keep it. The other led can indicate that the slave has initialized the 6802 and all self tests are o.k. (the 6802 can carry out self test on itself and also detect open cell connections).
Each slave will monitor from 4 to 12 cells with a total stack voltage of 10 to 60 volts.
Previous schematics and initial discussion of this slave board can be found here.
Here's my latest schematic, main changes are as follows;
J1 is now a DB15 socket.
Added D3 for input polarity protection, just in case.
Added a prototyping area to help with fine tuning bus communications.
Any comments, questions or errors that I may have made?
I am going to work on footprints and then do a pcb layout. For the initial build I will only populate the right half of the board and test master and slave bus communications before adding on the 6802.