We have an application where we use the GPIO I/O pins on Port 1 of 
LPC2138 to an LCV244 buffer/driver.  We found that randomly, some pins 
on the buffer driver were high or low at power-up with no program 
loaded to control the pins.  We added 32k pull downs between the 2138 
and the buffer/driver inputs to pull the lines low because the GPIO 
lines are floating inputs before program changes them into outputs.  
However, we are still randomly seeing some outputs on buffer/driver 
high and some low on power up.  We measured between 0.4 and 1.2 volts 
on those lines. Why are 32k pull downs not working for us ?

Regards,
Nick
	

---- Original Message ----
From: "nknight62" <justlooking@just...>
To: <lpc2000@lpc2...>
Sent: Monday, March 06, 2006 6:44 AM
Subject: [lpc2000] LPC2138 GPIO Pull Downs ?

> We have an application where we use the GPIO I/O
pins on Port 1 of
> LPC2138 to an LCV244 buffer/driver.  We found that randomly, some pins
> on the buffer driver were high or low at power-up with no program
> loaded to control the pins.  We added 32k pull downs between the 2138
> and the buffer/driver inputs to pull the lines low because the GPIO
> lines are floating inputs before program changes them into outputs.
> However, we are still randomly seeing some outputs on buffer/driver
> high and some low on power up.  We measured between 0.4 and 1.2 volts
> on those lines. Why are 32k pull downs not working for us ?

Remember that all P1.x pins have internal pull-ups of 60-300 kohm.

Karl Olsen
	

>Karl Olsen wrote
>
> Remember that all P1.x pins have internal pull-ups of 60-300 kohm.
> 

It seems there is and error in the data sheet or I have measured the 
chip wrong. I have tested on a 2148 and a 2142 and found the 
following.

The Data sheet states "P1.29/TCK 56 *[6] I/O P1.29 - General purpose 
digital input/output pin"

"*[6] - 5 V tolerant pad with built-in pull-up resistor providing 
digital I/O functions with TTL levels and hysteresis
and 10 ns slew rate control. The pull-up resistor's value typically 
ranges from 60 k to 300 k"

Basically this claims that the P1.29 has a minimum pull up of 300k. 
My measurements show a pullup of around 5 megs which is probably just 
leakage. Am I the only one that has found this ? This seems to be the 
case on all the JTAG pins.

Keith
	

Hi Nick, something similar happens in LPC2292.

Although user manual says "All Port 1 pins are 5V tolerant with built-in 
pull-up resistor that sets input level to high when corresponding pin is 
used as input", I found this not to be the case because of "external
boot" 
feature provided by the boot loader that is mentioned elsewhere as BOOT1:0 
signals.

What this "feature" does is configures external memory to 8/16/32 bit
mode 
depending on P2.26 and P2.27 at reset.  When this happens, some pins of P1 
assume external memory interface functions and thus become outputs.  The 
code implementing this feature in LPC2292 boot loader V 1.64 appears strange.

Try making sure that P2.26-27 are not connected (and depend on internal 
pull-up) to see if you still get this behaviour.  If you have things 
connected to these pins, the other option active pull-up during reset.

Hope this helps.

Jaya

>Message: 6
>    Date: Mon, 06 Mar 2006 05:44:25 -0000
>    From: "nknight62" <justlooking@just...>
>Subject: LPC2138 GPIO Pull Downs ?
>
>We have an application where we use the GPIO I/O pins on Port 1 of
>LPC2138 to an LCV244 buffer/driver.  We found that randomly, some pins
>on the buffer driver were high or low at power-up with no program
>loaded to control the pins.  We added 32k pull downs between the 2138
>and the buffer/driver inputs to pull the lines low because the GPIO
>lines are floating inputs before program changes them into outputs.
>However, we are still randomly seeing some outputs on buffer/driver
>high and some low on power up.  We measured between 0.4 and 1.2 volts
>on those lines. Why are 32k pull downs not working for us ?
>
>Regards,
>Nick

Send instant messages to your online friends http://au.messenger.yahoo.com 

*       We have an application where we use the GPIO I/O pins on Port 1 of
> LPC2138 to an LCV244 buffer/driver.  We found that
randomly, some pins
> on the buffer driver were high or low at power-up with no program
> loaded to control the pins.  We added 32k pull downs between the 2138
> and the buffer/driver inputs to pull the lines low because the GPIO
> lines are floating inputs before program changes them into outputs.
> However, we are still randomly seeing some outputs on buffer/driver
> high and some low on power up.  We measured between 0.4 and 1.2 volts
> on those lines. Why are 32k pull downs not working for us ?

Remember that all P1.x pins have internal pull-ups of 60-300 kohm.

Karl Olsen

All,

I appreciate all who replied. Because we needed to ensure that all of our
outputs were held low through reset, we utilized the gate enable pins on the
LVC244 buffers to hold buffer output low through reset, then programmed an
output pin to drive gate enable pins low once our program is up and running.
	Regards,

Nick
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