<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
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.<br>
<br>
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.<br>
<br>
The reset-hold idea sounds good to me. <br>
<br>
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".<br>
<br>
Here's a potentially dumb answer to your question after I meditated
about just how efficient your timestamps could get.<br>
<br>
If you can trust the storage to read back what you've written you
could <a href="https://en.wikipedia.org/wiki/Huffman_coding">Huffman
code</a> 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.<br>
<br>
Remind me why ferromagnetic chips are more interesting than EEPROM
or flash?<br>
<br>
-Pete<br>
<br>
<div class="moz-cite-prefix">On 11/29/2015 11:20 AM, Chip McClelland
via TriEmbed wrote:<br>
</div>
<blockquote
cite="mid:C5D59C90-73E6-4FE8-9DDB-00FDE2E481EB@mcclellands.org"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
All,
<div class=""><br class="">
</div>
<div class="">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:</div>
<div class=""><br class="">
</div>
<div class="">1) Powering a Bluetooth LE module using an Arduino
IO Pin. I am using the Adafruit Bluetooth LE friend. According
to <a moz-do-not-send="true"
href="https://learn.adafruit.com/introducing-the-adafruit-bluefruit-le-uart-friend/current-measurements"
class="">this article</a> it should only draw 15.2mA peak
current. Looking up the <a moz-do-not-send="true"
href="http://playground.arduino.cc/Main/ArduinoPinCurrentLimitations"
class="">ATMEGA 328p specs</a> 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?</div>
<div class=""><br class="">
</div>
<div class="">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 <a
moz-do-not-send="true"
href="http://www.fujitsu.com/us/Images/MB85RC256V-DS501-00017-3v0-E.pdf"
class="">Fujitsu chip</a> 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?</div>
<div class=""><br class="">
</div>
<div class="">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?</div>
<div class=""><br class="">
</div>
<div class="">Thank you all for your help,</div>
<div class=""><br class="">
</div>
<div class="">Chip</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
Triangle, NC Embedded Computing mailing list
<a class="moz-txt-link-abbreviated" href="mailto:TriEmbed@triembed.org">TriEmbed@triembed.org</a>
<a class="moz-txt-link-freetext" href="http://mail.triembed.org/mailman/listinfo/triembed_triembed.org">http://mail.triembed.org/mailman/listinfo/triembed_triembed.org</a>
TriEmbed web site: <a class="moz-txt-link-freetext" href="http://TriEmbed.org">http://TriEmbed.org</a>
</pre>
</blockquote>
<br>
</body>
</html>