Logic High Input Min is not met - Works but!

Thread Starter

RamaD

Joined Dec 4, 2009
328
A microcontroller is connected to a port IO Expander, ADC and DAC chips through SPI. All of them use +5V supply except the uC at 3.3V and is 5V tolerant. The port IO expander VIH Min is 4V and the ADC & DACs VIH Min is 3.5V, figures taken from datasheets at 0.8VDD and 0.7VDD. But the uC can output a max. of 3.3V only. A few test boards have been working without any issues for about an year. They are about to ship some boards, I found this one out on a casual review and said they are not meeting the specs. My comment was not well received!
Please suggest me whether this is ok.
 

BobTPH

Joined Jun 5, 2013
3,777
Are you sure it is actually using a 3.3V high output? Many micros can optionally use open drain outputs. As long as it if 5V tolerant, the output can use a pullup to 5V. This would work correctly.

Bob
 

JohnInTX

Joined Jun 26, 2012
4,561
It's not OK. Those minimum specs say that the manufacturer guarantees operation at the specified voltage but not below. While the inputs may appear to function at lower voltages there is no guarantee that they will do so from device to device, over the full temperature range, full speed etc. Noise margins are also degraded which may make the data you send to those DACs error prone.

@Papabravo is absolutely correct. Time to re-work or scrap and don't fall into the trap of saving money or schedule. Things like this only get more expensive the further down the line they get. If you can scab some pullups onto the lines, @BobTPH 's suggestion might get you out of the woods (with some firmware changes).

At any rate, now that you have admitted your errors on the intertubes, shipping the product with known and admitted defects leaves you open to serious liability issues if you have problems down the road.

All of sudden that scrap/rework seems pretty cheap, doesn't it?
 

Thread Starter

RamaD

Joined Dec 4, 2009
328
These were in a friend's place and did this review over a coffee! I was blunt in saying that they don't meet the specs which invited their wrath!
@BobTPH & @JohnInTX
This is C8051F360, and used push pull outputs for chip selects & SPI. Speed is a lowly 2.5MHz, so was also thinking on similar lines of pull-up resistor.
It was haunting me whether I was overly concerned about nothing! Thank you all for assuring me.
 
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