Stupid simple question.....

[chien_fu]

So I tried driving it with two pins but something else is screwy now....

This is what I have:

OUTPUT Pin1
output Pin2

Startup:

for startupalarm = 1 to 4
high 2 
high 1 
pause 5000
low 2 
low 1 
pause 5000
next startupalarm

It seems that "high 2" does not actually switch the pin high, but just ticks it high for a split second. "high 1" switches the pin high and it is held high for 5 seconds until it is switched low...

This is the same regardless of which pins I use. The line of code immediately preceding the pause command works, all others do not. Nothing else is connected to the chip, it's all by itself on a breadboard.............

[HenrikOlsson]

Hi,
You're suffering from what is called a RMW problem (read-modify-write). The way the PIC works when setting a single bit in a register, like the GPIO-port in this case, is to first read the whole register, then set the bit in question and finally writing the whole register "back".

Because your motor presents a large a load to the pin (especially at startup) the voltage on the pin will sag or be low. So what is happening is this:
First you have the High 2. This reads the port (as a byte), sets bit 2 and writes the whole byte back to the port.
Now you have High 1 which does the same thing but because pin2 is heavily loaded it will read as 0 when the port is read, the PIC then sets bit1 and write back to the port resulting in bit 2 now being reset to 0.

The best bet is to use a transistor to drive the motor. If that's not an option then write to the port directly, setting both pins at the same time. You do this in the same way as Darrel showed you how to write to CMCON etc - look up the physical adress of GPIO and use POKE to write to it.

[Melanie]

Remember also, you must treat your Motor as an INDUCTOR or RELAY. The back emf could destroy your PIC pin or Transistor switching it. Put that all important Diode reverse-biased across the Motor.

[Acetronics2]

+1 .....

Good answer from Henrik ...

[HenrikOlsson]

Nah, if you look at section 9.1 of the mid-range manual you'll find the following:

All write operations (such as BSF and BCF instructions) are read-modify-write operations. Therefore a write to a port implies that the port pins are read, this value is modified, and then written to the port data latch.

So I'll stick to my theory ;-)

[Acetronics2]

Ok Henrik ...

One red tomato for me ... ( was the question ...)

this also explains why it acts the same whith heavy capacitive loads ... driving directly Big Mosfets gates i.e.

Alain

PS: but what an Idea µChip had to proceed like that ???

[chien_fu]

Why is it that when I enter:

GPIO = %11111111

Picbasic says "Undefined variable"?