Peter P. wrote:
> Since now most CMOS microprocessors are again sensitive to conduction of their
> body diodes (as they were in CMOS CD4xxxA times), and are no longer immune to
> body diode currents (as they were in CMOS CD4xxxB times, i.e. for 25 to 30
> years), the time has come to look the problem in the face and find common
> solutions, as I find that this 'return to the past' can be quite costly.
>
> The problem is that many (most) CMOS chips are now specified for Vdd-0.3 to
> Vcc+0.3 maximum limits *operating*. This means that:
>
> a - body ESD diodes cannot be used for anything at all
> b - internal MOSFET reverse diodes must be ignored
> c - external clamps which are able to clamp at Vf=0.3V for usual surge currents
> must be provided externally. Silicon clamps and tranzorbs do not work.
> d - the cost of doing this must be assessed.
> e - leakage from Schottky clamps compromises some of the advantages of CMOS
> inputs. Its strong temperature dependence can make certain A/D schemes stop
> working altogether (e.g. R/C a/d converter on external pin)
>
> For a 13 IO pin part, this means that:
>
> e - 26 diodes which were previously implied present are no longer present
> f - the actual cost of the part is the cost of the part plus the cost of the
> external suppressor plus the cost of the board space consumed, plus the cost of
> extra routing (which can mean vias or a double sided board where one layer was
> enough before).
>
> The problem gets worse with more IO pins.
>
> So according to my simple calculations, taking an 18-pin low cost PIC as example:
>
> - budgetary price:                             1 x $ 0.5  = 0.5
> - add 13 double external Schottkys in SOT-23: 13 x $ 0.03 = 0.39
> - add board area to accomodate Schottkys, 
> and 2nd layer and vias to route network,
> add assembly and stock costs for added parts:    + $ 2    = 2.0
>                                                            -----
>                                                             2.89
>
> Cost of MCU out of this: ~ 17% (less than a fifth)
>
> Thus, there are no $ 0.5 microcontrollers if they only accept +/- 0.3 V limits,
> operating. $ 0.5 MCUs are a myth in the same class as 1000 Watt PMPO
> specifications for pocket iPod speakers under these conditions. The 'break even'
> price, when a MCU costs as much as the external suppression network needed to
> protect it, is then about 5 cents/IO pin that needs protection for the first few
> pins, and after that the price jumps when the board must be augmented to add all
> the new parts. Thus a $ 0.5 MCU 'breaks even' when about 10 IO pins are used,
> and the cost roughly triples afer that (accounting for the 2-sided board). The
> parts count jumps from one to 14.
>
> Questions:
>
> Q1 - what is the general take on this. I understand that the revolutionary
> 'return to the past' represented by return to CMOS 4xxxA series 'performance'
> happened quietly and without any howls of opposition. Probably helped by $ 0.5
> MCU budgetary pricing, attractive to beancounters and innocent, credulous young
> men who believe that migration documents are written to ease their pain.
>   

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.

> Q2 - is there a list of standard clamp (row) parts that can mitigate the 
> problem without breaking the bank ?
>   

Luckily, the costs of small MOVs and TVSs are falling, and some are 
multiples. I especially like the
Cooper-Bussman devices.
> 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.

> Q4 - are mcus with origin China etc affected by this ? (i.e. Holtek, and many
> others ?)
>   

no idea.
> Candidate parts:
>
> Single device: BAT42  <tbd>
> Double device: BAT54S $ 0.02 / 3k
>
> Multiple devices: Semiconwell etc, 6 channels -> $ 0.30+ / 5k etc
>
> Any comments are welcome.
>
> thanks,
> Peter P.
>
>
>   

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