AVR-Chat

Hi Steven.

About the I/O lines, i've never heard about the littlefuse. Didn't 
you think to use optocouplers? Like the TIL111.

About the RS-485, you could put some passives components plus the 
LTC1480 transceivers. On the magazine, Circuit Cellar, the october 
issue an article cames talking about the subject of protection RS-
485. If you want i could provide more information for you.

Bruno

--- In AVR-Chat@yahoogroups.com, "Steven Hodge" <stevehodge@...> 
wrote:
>
> My apologies ahead of time for the length of this post, but I 
realize it
> would be hard to answer without adequately describing the operating
> environment.
> 
>  
> 
> I'd like to get feedback on ideas I have to provide protection for 
I/O lines
> for uC-based PCB's.  These will be scattered throughout a boat, so 
12 V DC
> (up to 14.6 V when the main "house" batteries are on charge) is 
the nominal
> supply voltage -- call it VB -- for all boards.  There is lots of 
this
> running around everywhere, not just to PCB's but to motors, pumps, 
diesel
> engine, solenoid coils, radios, radar, instruments, stereo, 
computer, etc.
> All grounds come back to a single common ground bus near the house 
battery
> bank.
> 
>  
> 
> Protection concerns are (1) overvoltage on an I/O pin by 
accidentally
> connecting it to VB, (2) the usual ESD, (3) protection of the uC 
and digital
> stuff from RF-induced voltages from the radar and VHF & HF radios 
(and who
> knows what else, eg, computer), and (4) protection in the opposite
> direction, of the radar/radios from RF noise from digital 
switching.
> Usually an inverter is also in use, to supply "household" 120 VAC 
from the
> 12 V house bank, so that probably adds to the noise.
> 
>  
> 
> All the uC-based boards will be linked with an RS485 bus, using 
shielded
> CAT-5 Ethernet cable and LTC1480 transceivers at each end.  These
> transceivers include ESD protection.
> 
>  
> 
> The uC will be an ATmega644P running at 3.0 - 3.3 V (still 
undecided) and
> 1.8432 MHz (for ideal RS485 and lower power consumption).  The 
power supply
> will be an LP2951.  The Atmel App Note AVR042 gives a pretty good
> description of how to protect the uC power pins, so I'm following 
the
> recommendations there (although I did have to clarify the units in 
Fig 6-1
> with Atmel tech support).  I'll also put the usual bypass 
capacitor at the
> pins of all other IC's.  At this point, I don't have any questions 
about
> power protection, and this post is just directed at I/O pins.
> 
>  
> 
> There is no high-frequency I/O requirement.  Almost all digital 
signals
> change state once in a blue moon on uC time scales.  At most I 
might have
> one at 50 Hz max, and 1 or 2 at a few Hz.  All analog signals are 
very
> slowly varying, time scales of tens of seconds at the fastest.
> 
>  
> 
> PCB real-estate is a bit of an issue but not a serious one.  At 
any rate,
> I'd like to start with what is functionally the best and only then 
adjust if
> the result makes the boards unacceptably too big.
> 
>  
> 
> All I/O signal wires will be shielded, with the shield grounded 
only at one
> end.
> 
>  
> 
> I always attempt to have input signals active-low, floating when 
not
> asserted (and pulled high at the PCB end), but there are still 
many cases
> where active-high is unavoidable.  Signal levels are either CMOS 
or VB level
> in most cases.  There are a few at 5 V level. 
> 
>  
> 
> As much as possible I'll try to use a ground plane on the uC-based 
PCB's,
> but probably not a power (Vcc) plane.
> 
>  
> 
> For digital input lines, I'm proposing:
> 
>  
> 
> (a) front end protection using a MAX681x for mechanical switch 
input (which
> the majority of input signals are) or a Littelfuse SP72x for non-
switch
> inputs.  Both have ESD & overvoltage protection. The MAX681x has 
the
> additional advantage of providing automatic high-to-low level 
shifting to
> get VB-level signals down to CMOS levels, and thus, since the 50 
ms time
> delay of this IC is not an issue in most cases, I could just use it
> everywhere except for those cases where it would be an issue (eg, 
the 50 Hz
> signal).  The trade-off is that if I used the SP72x there would be 
a
> significant number of situations where I would have to add a high-
to-low
> buffer/shifter IC.
> 
>  
> 
> (b) an additional RC low-pass filter between the above and the uC 
pin, where
> the value of R is chosen so it also acts to limit the current into 
the uC
> pin to its max of 40 mA.  C would then be selected so the -3dB 
breakpoint of
> the filter is well above 50 Hz, say 1-2 KHz.  Values I've 
computed, for
> CMOS-level voltages, are about 100 ohms for R and about 1 uF for C 
for such
> a breakpoint.
> 
>  
> 
> For analog input lines, level-shifting cannot be done so I'd just 
use the
> SP72x and RC filter.  The filter values would be adjusted to 
accommodate
> their typically higher voltages (such as VB or 5 V).
> 
>  
> 
> When it comes to output lines, things are murkier in my brain.  
For signal
> and non-inductive load outputs, I'm thinking of using the same 
SP72x's, or
> nothing at all.  Inductive loads will definitely require a flyback
> suppression diode, but it's not clear to me if I should also add 
the SP72x,
> or a DO-15 package TVS, to them as well.  I often also install a 
diode
> across the actual load (ie, actual motor or solenoid coil 
terminals).
> 
>  
> 
> Any comments on any of the above will be very appreciated.   One 
specific
> question I have is should I add anything else to the RS485 lines, 
beyond
> what the LTC1480 transceivers provide?
> 
>  
> 
> Thanks, Steve
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
> 
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