I/O line protection
2008-01-03 by Steven Hodge
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 [Non-text portions of this message have been removed]