Glitch on port out?

[diltsman]

I am using an XK-1A.

I have a program that is configuring a port in C++ with inline assembly. When setc res[XS1_PORT_4F], CTRL_INUSE_ON is executed I see the wave for the port go to 0xf for 10 ns (1 clock). It is 0 before this and then returns to zero until I actually execute an out on the port.

Is this the port actually outputting 0xf (looks like a glitch to me) or is something else happening? How can I tell if a port is in input or output mode from the Waves tab? Also, if this is a glitch, is there some way for me to prevent it?

[diltsman]

I have looked at this a bit more, and the same "glitch" appears to happen when writing essentially the same program in xc.

[segher]

Is it really driving it strongly high, or is something weakly pulling
it up (either the chip, or something else on your board, or even
your measuring equipment)? I'm thinking it could be the internal
pulldown switching off.

[diltsman]

I am using the simulator in the xTools. Sorry, I guess that I forgot to mention that.

[Ross]

You don't mention the configuration of the port, but if at all possible it is recommended to configure ports using the "configure_" functions in xs1.h since they are written in a way that avoids glitches and safe against exceptions etc.

[segher]

You don't mention the configuration of the port, but if at all possible it is recommended to configure ports using the "configure_" functions in xs1.h since they are written in a way that avoids glitches and safe against exceptions etc.

That certainly is good advice, esp. for when you reconfigure ports.
The correct order to do things in can be tricky.

But the configure_* functions do not enable the ports (setc IN_USE);
that is done by the constructors, which run essentially the same
code as diltsman's as far as I can see?

[Ross]

Yeah the ports are enabled by the constructors - so when you declare a port they are switched on before main.

same "glitch" appears to happen when writing essentially the same program in xc.

Like the following you mean?

Code: Select all

port p = XS1_PORT_1A;
void main()
{ }

[diltsman]

This is the exact XC code.

Code: Select all

#include <platform.h>

out port bled = PORT_BUTTONLED;

int main (void) {
	for (int o = 0x03;; o = ~o)
	{
		bled <: o;
	}
    return 0;
}