Bob Axtell <engineer <at> neomailbox.com> writes:
 
> This is similar to the reduction in utility of the 74HCxxx parts that 
> occurred in the late '90s. The semi manufacturers,
> starting with Fairchild, made the series with smaller dies, increasing 
> their profit margin. But inadvertantly, the
> change dramatically increased the speeds of the parts, requiring me and 
> many others  of redesigning the  products
> so that more bypassing is installed to maintain proper intended operation.

Yes, but that was a relatively small thing and it was not unexpected. This is
large imho.
 
> Luckily, the costs of small MOVs and TVSs are falling, and some are 
> multiples. I especially like the
> Cooper-Bussman devices.

As I found out the hard way, TVSs and MOVs do not have a low enough Vf to clamp
for the 'new' micros. In fact, at usual circuit currents in a PIC (say 10 mA -
ish inductive kickback from a relay coil), many small and inexpensive Schottkys
don't have a low enough Vf to be used. So this would mean (also for Wouter):

- suppression on any wire or PCB trace that is not shielded and longer than
about 2 inches, and may be high Z (including during reset) (to pass
susceptibility tests).
- all relay kickback catchers must be expensive, large Schottky to have Vf <
+0.3V during inductive kickback of 100 mA or more, if the relay is driven
directly by the PIC (5V, 3V etc).
- anything inductive or capacitive connected to a PIC pin needs the same
suppression. This includes piezo disks and speakers and even microphones and
'ac' inputs or inputs from analog conditioning circuits that can overshoot or
undershoot a little (including during reset/powerup/powerdown, when coupling
caps charge). Any ground bounce or glitch travelling on the supply lines and
exceeding 0.3V will cause a problem on any decoupled pins, such as timing
inaccuracy. 0.3V is 6% of 5V. Thus the supply must now be filtered to 2% too.
- any PIC pin connected to a large logic level MOSFET gate and tristated will
'see' ripple from the mosfet drain and this will cause trouble if it's larger
than 0.6Vpp at the gate (easily so if something fast is happening at the drain,
like a relay or motor kickback, or a glitch propagating through the unfiltered
supply line). This includes the time during reset when pins are necessarily
tristate. My tests with 16F84 showed that the chip won't exit reset at all if
such a condition exists.
- any floating tristated pins MUST be pulled to some logic level including at
all times during development (to permit same) AND reset (i.e. use resistors on
all pins a la MCS51).
- anything else I haven't thought about, such as small filter caps on IO pins
discharging while the power goes up/down to/from standby level and much much 
more.
- also as I said, the Schottky leakage is at least 1 order of magnitude worse
than CMOS leakage, and gets worse with heat. Multiply that by 20 IO pins and
your battery will have to grow.

My recent experience with the 'new' features on 16F54 lad me to believe that
even small pulses violating the 0.3 V rule will cause the chip to lose
instructions/timing. Any real time clock or other time dependent process should
be affected. The watchdog could be triggered periodically. When such a 
condition exists there is no way to pull the chip out of reset reliably.
Checking for oscillation with an external  watchdog does not work (the
oscillator works with reduced amplitude on 16f54 during the fault condition).

All ICSP pins that lead to a header must be suppressed as above, as they are
connected to a long cable that is floating and will rectify mains ac from the
'air' when the programmer is not driving them. Maybe the diodes can be mounted
on the cable end to avoid changing the circuit.

> > Q3 - what happens to small minimal parts count projects where the parts count
> > can double or triple due to the new clamping requirements ?
> >   
> 
> Unfortunately this will be required to meet ESD requirements of UL, CSA, 
> and EU.

I know. That's why I am asking. It looks like the people who make $ 0.5 MCUs 
are actually losing money to the people who make $0.05 Schottky suppressors.
Maybe it's a new trend in outsourcing ?

Peter P.


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