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[Aljosa]

Zdravo,

uporabljam PIC16F627 in imam zelo čuden pojav. Pina RA1 in RA4 imam definirana kot izhoda. RA4 je open drain zato imam še dodaten pull up upor. Ko na RA1 spreminjam stanje iz 0 .. 1 mi to vedno vpliva na stanje na RA4. Na RA1 in RA4 imam vezani ledici in ko v kodi nastavim RA4 = 1 in potem RA1 = 0 mi bo zadnja komanda RA1 = 1 vplivala na RA4 in mi izklopili ledico. CMCON sem nastavil na 0x07 kar naj bi pomenilo izklopljene komparatorje na kar sem najprej posumil vendar mi ta problem ni čisto logičen. Če recimo RA1 nastavim pred nastavljanjem RA4 pa je ok.

Kakršenkoli namig bi mi prišel prav. Hvala.

Lp

[Peter123]

Predlagam da prilepiš kos kode.

[Aljosa]

Rutine pokličem v tem vrstnem redu:

1. inicializacija I/O:

[bferlic]

Vrstni red ne bi smel biti problem, razen če je kakšna napaka v prevajalniku. A lahko prilepiš še del kode, kjer spreminjaš stanje na RA1 in RA4?

LP

[Aljosa]

Preklop je čist preprost.

Najprej recimo takole:



Če naredim takole je vse ok:

[MvE]

Na hitro, če se mudi bi šlo rešiti tako, da vpišeš binarno vrednost v PORTA:

[Aljosa]

Ja sej zdej se poslužujem metode, da RA4 nastavim nazadnje. Tako dela ok, mi pa vseeno ni najbolj všeč ker še vedno ne razumem zakaj prihaja do tega.

[igo]

@MvE je že prav nakazal rešitev.
PIC pini imajo zanimivo lastnost beri/spremeni/piši . To pomeni, da se za spremembo enega pina prebere zunanje stanje celega PORTa, nato se vrednost spremeni, potem se cel PORT vpiše nazaj. Pri zunanjii pull-up/down uporih je lahko to prebrano stanje drugačno od notranjega stanja istega porta.

Zato se pri varnostno pomembnih programih uporablja zaporedje

[Aljosa]

Good point ja. Iz headerja je razvidna definicija pina, ki je vezana na port. Opazujem diagram pinov in se mi svita za kaj se gre, ja. Sedaj se mi zdi logično zakaj pride do tega. V bistvu je vedno najboljše da v takšnem primeru kar na celem portu delam operacije kot na posameznem pinu.

Hvala za info in razlago!!!

LP
Aljoša

[Aljosa]

Evo še razlaga, če bo kdo še imel podobno težavo:

When you perform any operation, apart from a MOV on a register ... the PIC {or SX} first reads the register, then it performs the operation on the number it has just read and finally it write the number back to the register. This is fine when dealing with normal registers and most special function registers. However, if you perform RMW (read modify write) operation on a port register (PORTA PORTB etc) [ed: or some timer / counter registers] then you are heading for trouble. Why? Because when the uP reads a port register, it reads the actual state of the pins, rather than the output latch. This can cause two problems:

If the pin is an input, then the input pin state will be read, the operation performed on it, and the result sent to the output latch. This may not immediately cause problems, but if that pin is made into an output later on, the state of the output latch may have changed from the time it was deliberately set by the code.
Now, if the pin is defined as an output, the output latch and the actual pin *ought* to be in the same state. In practice sometimes they aren't. If you are driving a capacitive load, the pin will take time to respond as it charges and discharges the capacitor. A common problem occurs when using the bit set (bsf) or bit clear (bcf) directly on a port.

bsf portb, 0 -- don't do this!
bsf portb, 1 -- don't do this!
bcf portb, 1 -- don't do this!
or
setb portb.0 -- don't do this!
setb portb.1 -- don't do this!

Which *might* work. However, if pin B0 is loaded in any way, then it may not have time to respond to the first instruction before the second one is executed. The second instruction reads the port and sees that the pin B0 is low (because it hasn't got time to go high) and writes the low state back into the output latch. The result would be that B0 never gets set.

How do you avoid this issue? Well, it's bad practice to use RMW instructions directly on a port. So you use whats known as a shadow register. The shadow register is simply a ram location you reserve. All operations are performed on this register, and when you are finished, you copy it to the port register. It's a bit more trouble, and it can slow things down a tiny bit, but the effort is worth it for reliable operation.

Note that it's not only bsf and bcf that are RMW instructions, although they are the most common. Other examples are

andwf portb,f -- don't do this!
iorwf portb,f -- don't do this!
xorwf portb,f -- don't do this!

(Instructions that merely *read* from a port are safe, as long as you stay aware of the fact that the value you read may be different from the value you just wrote:

andwf portb, w ; this is fine
iorwf portb, w ; this is fine

)

[Alex17]

Če hočeš spreminjati posamezne bita, pa jim vrednosti nastaviš V LATA / LATB registru.

[snow]