While at it, I experimented further with inputs. I placed a 1k/1k divider from
VDD to an input (set to hi-Z, no pullup/pulldown) and a 10k potentiometer in
parallel to ground, plus a 0.5uF ceramic capacitor as a filter. The input was
seen as logic 0 below 1.45V, as 1 above 1.51V and changing in between (this is a
devboard so surely there is a couple of tens of mV noise around).
However, the datasheet claims a hysteresis of Vhys(min)=0.4V, which I don't
see to happen (I tried two pins on the LPC1768, P0.25 and P1.22).
Can anybody please try a similar experiment and confirm or reject my
findings?
Thanks,
Jan Waclawek
LPC17xx: Voltage on input pin when pull-up is enabled
Started by ●April 12, 2011
Reply by ●April 14, 20112011-04-14
Reply by ●April 15, 20112011-04-15
--- In l..., Jan Waclawek wrote:
> However, the datasheet claims a hysteresis of Vhys(min)=0.4V, which
> I don't see to happen (I tried two pins on the LPC1768, P0.25 and
> P1.22).
I teach a class on microcontroller systems and during one of the lab
sessions the students measured the DC parameters of one of the IO pins
of an LPC1343. We were able to verify all of the parameters except the
hysteresis voltage, which was typically about half of the data sheet
value. We measured VIH and VIL both with and without hysteresis enabled and there is definitely some additional hysteresis, just not as much as
we expected.
I tried to find a specification for the test method that NXP used for
this parameter but I was unsuccessful.
Joe
> However, the datasheet claims a hysteresis of Vhys(min)=0.4V, which
> I don't see to happen (I tried two pins on the LPC1768, P0.25 and
> P1.22).
I teach a class on microcontroller systems and during one of the lab
sessions the students measured the DC parameters of one of the IO pins
of an LPC1343. We were able to verify all of the parameters except the
hysteresis voltage, which was typically about half of the data sheet
value. We measured VIH and VIL both with and without hysteresis enabled and there is definitely some additional hysteresis, just not as much as
we expected.
I tried to find a specification for the test method that NXP used for
this parameter but I was unsuccessful.
Joe
Reply by ●April 15, 20112011-04-15
--- In l..., "kjoehass" wrote:
> --- In l..., Jan Waclawek konfera@ wrote:
> > However, the datasheet claims a hysteresis of Vhys(min)=0.4V, which
> > I don't see to happen (I tried two pins on the LPC1768, P0.25 and
> > P1.22).
>
> I teach a class on microcontroller systems and during one of the lab
> sessions the students measured the DC parameters of one of the IO pins
> of an LPC1343. We were able to verify all of the parameters except the
> hysteresis voltage, which was typically about half of the data sheet
> value. We measured VIH and VIL both with and without hysteresis
enabled and there is definitely some additional hysteresis, just not as
much as
> we expected.
>
> I tried to find a specification for the test method that NXP used for
> this parameter but I was unsuccessful.
>
> Joe
>
This is definitely puzzling as are the voltages we measured.
NXP, if you are monitoring, could you try to explain these anomalies to
us?
Larry
> --- In l..., Jan Waclawek konfera@ wrote:
> > However, the datasheet claims a hysteresis of Vhys(min)=0.4V, which
> > I don't see to happen (I tried two pins on the LPC1768, P0.25 and
> > P1.22).
>
> I teach a class on microcontroller systems and during one of the lab
> sessions the students measured the DC parameters of one of the IO pins
> of an LPC1343. We were able to verify all of the parameters except the
> hysteresis voltage, which was typically about half of the data sheet
> value. We measured VIH and VIL both with and without hysteresis
enabled and there is definitely some additional hysteresis, just not as
much as
> we expected.
>
> I tried to find a specification for the test method that NXP used for
> this parameter but I was unsuccessful.
>
> Joe
>
This is definitely puzzling as are the voltages we measured.
NXP, if you are monitoring, could you try to explain these anomalies to
us?
Larry
Reply by ●April 15, 20112011-04-15
Joe,
Thank you for your comment.
> I teach a class on microcontroller systems and during one of the lab
> sessions the students measured the DC parameters of one of the IO pins
> of an LPC1343.
Didn't you by chance measure also the load characteristics of the pin set to input with pullup? The LPC13xx datasheet appears to have the same graph in this regard than the LPC17xx datasheet, but according to our measurements the real voltages are around 1V lower.
> We were able to verify all of the parameters except the
> hysteresis voltage, which was typically about half of the data sheet
> value. We measured VIH and VIL both with and without hysteresis enabled and there is definitely some additional hysteresis, just not as much as
> we expected.
I have not seen even the 0.2V hysteresis, which would be half of the datasheet value (which is specified as "min"). Although the LPC17xx, contrary to the LPC13xx, can't have the hysteresis switched off, I wonder whether there couldn't be an undocumented register controlling the hysteresis, set to "off" by default after reset; or is this simply just a design error.
> I tried to find a specification for the test method that NXP used for
> this parameter but I was unsuccessful.
I too would like to hear NXP to comment in a professional way on these findings. I was never very confident in these hastily concocted new chips, but after issues like these surface, I become increasingly more hesitant to recommend and use them for our new designs. I'd hate to come across some show-stopping error after having spent considerable time with development; mysterious intermittents due to chips not working as promised in the (also not very carefully prepared) datasheets would be even more detrimental.
Jan Waclawek
Thank you for your comment.
> I teach a class on microcontroller systems and during one of the lab
> sessions the students measured the DC parameters of one of the IO pins
> of an LPC1343.
Didn't you by chance measure also the load characteristics of the pin set to input with pullup? The LPC13xx datasheet appears to have the same graph in this regard than the LPC17xx datasheet, but according to our measurements the real voltages are around 1V lower.
> We were able to verify all of the parameters except the
> hysteresis voltage, which was typically about half of the data sheet
> value. We measured VIH and VIL both with and without hysteresis enabled and there is definitely some additional hysteresis, just not as much as
> we expected.
I have not seen even the 0.2V hysteresis, which would be half of the datasheet value (which is specified as "min"). Although the LPC17xx, contrary to the LPC13xx, can't have the hysteresis switched off, I wonder whether there couldn't be an undocumented register controlling the hysteresis, set to "off" by default after reset; or is this simply just a design error.
> I tried to find a specification for the test method that NXP used for
> this parameter but I was unsuccessful.
I too would like to hear NXP to comment in a professional way on these findings. I was never very confident in these hastily concocted new chips, but after issues like these surface, I become increasingly more hesitant to recommend and use them for our new designs. I'd hate to come across some show-stopping error after having spent considerable time with development; mysterious intermittents due to chips not working as promised in the (also not very carefully prepared) datasheets would be even more detrimental.
Jan Waclawek
