[TriEmbed] A Few Questions - Hopefully not stupid ones

[Pete Soper]

The Arduino gate IS a FET. :-) The concern would be if the gate's output 
voltage is high enough. It won't be the same as the Atmega's supply 
voltage. A bypass cap may be called for if the BLE board is far away and 
connected with a thin wire. A decent meter or storage 'scope could 
determine whether the peak current spec reflects reality and whether the 
supply voltage droops with a given setup.

I know this isn't likely to be relevant to your case, but folks should 
know that there are layers of 'absolute max' specs for Atmega, so, for 
example, you can't expect to drive all eight pins of one register at 
40ma, power dissipation and max temperature may be issues, etc.

The reset-hold idea sounds good to me.

The four byte epoch date/time encoding is very well understood and 
supported. If you want to get a head start on 2038 you could have a 
single bit in your flash to say "if set then these 32 bit timestamps are 
relative to 1/1/1970 and otherwise time zero is expressed in the 
null-terminated string following this bit".

Here's a potentially dumb answer to your question after I meditated 
about just how efficient your timestamps could get.

If you can trust the storage to read back what you've written you could 
Huffman code <https://en.wikipedia.org/wiki/Huffman_coding> the 
intervals between timestamps and make them just long enough to express 
the elapsed time from one measurement to the next.  If your "regular" 
interval between measurements is fixed and you can hit each reporting 
time within a second or less then you only need one bit to express the 
new date/time, and other intervals would be expressed with longer bit 
strings you would form as a side effect of traversing your "interval 
dictionary" that for the best case efficiency you would create on the 
fly. Your first timestamp after power up would be degenerate unless you 
got lucky, and you would necessarily have to run through the samples to 
establish the date/time of the last pre-power-cycle timestamp before 
proceeding if you don't want to just spit out a full 32 bit timestamp 
each time you start. A hugely cheaper way to go is to subscribe one bit 
of your logging data to indicate whether the timestamp is the full four 
bytes or some smaller sequence of bytes expressing one or more flavors 
of interval relative to the last full timestamp. Finally, if the logging 
data has unused bits such as the high order bit of seven bit ASCII 
characters you could perhaps express a timestamp with zero overhead in 
the nonvolatile storage.

Remind me why ferromagnetic chips are more interesting than EEPROM or flash?

-Pete

On 11/29/2015 11:20 AM, Chip McClelland via TriEmbed wrote:
> All,
>
> I am building a new board and have to make some design decisions. 
> Before sending to OSHPark, I wanted to see if you all could tell me if 
> I am about to make any mistakes:
>
> 1) Powering a Bluetooth LE module using an Arduino IO Pin.  I am using 
> the Adafruit Bluetooth LE friend.  According to this article 
> <https://learn.adafruit.com/introducing-the-adafruit-bluefruit-le-uart-friend/current-measurements>  
> it should only draw 15.2mA peak current.  Looking up the ATMEGA 328p 
> specs 
> <http://playground.arduino.cc/Main/ArduinoPinCurrentLimitations>  I 
> see that that it can source 40mA on an IO pin.  I know that engineers 
> like to add a "safety factor" so, do you think I would be OK or should 
> I use a FET and have the Arduino simply control it?
>
> 2) I am moving away from storing data on a MicroSD card and want to 
> use a FRAM chip instead. BTW, if you have not looked at FRAM it is 
> awesome!  I will use the Fujitsu chip 
> <http://www.fujitsu.com/us/Images/MB85RC256V-DS501-00017-3v0-E.pdf> and communicate 
> using i2c.  Here is the question, what if I wanted to use an external 
> reader to extract the data from this chip, I could expose the i2c pins 
> on the boar and use a POGO pin reader to connect - that way I would 
> not need to refresh the ATMEGA on the board to change to a data dump 
> sketch.  So, I would connect this device to provide power, ground, SDA 
> and SCL.  The thing is, I don't want the ATMEGA on the board to get in 
> the way of this.  Could I simply add a pin connected to the board's 
> RESET pin and hold it low, disabling the onboard processor and leaving 
> the i2c bus available for access?
>
> 3) I want to add a date and time stamp to each event I log on the FRAM 
> chip, I am thinking the best format would be UNIX or EPoC time as it 
> will use only 4 bytes.  That way each event will use only 4 date/time 
> bytes and one data byte and I can store a year's worth of data on a 
> 256k chip.  Is there a better way to store date and time?
>
> Thank you all for your help,
>
> Chip
>
>
>
>
>
> _______________________________________________
> Triangle, NC Embedded Computing mailing list
> TriEmbed at triembed.org
> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> TriEmbed web site:http://TriEmbed.org

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.triembed.org/pipermail/triembed_triembed.org/attachments/20151130/ddcb4fdf/attachment.htm>