help! Keep the state of a port pin constant when I/O ing

[tomasch]

Excuse my silly question but - no sleep - and I can't figure it out.

I am trying to use Port A for input AND output. I'd like to set a pin (let's say pin_a7) to a value of my choosing - i.e. on or off - for an LED, let's say.

I want to futz with the port, but when I write back to the port I do not want to change the state of Pin_a7.

I know it's a combination of fast_io or standard_io and set_tris_a because of read-modify-write, but head hurts from goofing with it too long.

oh yeah... I do not want to take up an extra spot of RAM for keeping a lookup for the state of pin_a7

16F628 and CCS v 3.112 (I think!)

Your expert help is much needed and much appreciated

Here's the idea in pseudo code

#use fast_io(a)
set_tris_a(0b01110000);

output_high(pin_a7); // I'd like pin_a7 to stay at whatever I set it at - could be hi - LED OFF
//
// or it could be low!!!

output_low(pin_a7); // I'd like pin_a7 to stay at whatever I set it at - could be low - LED ON

// do some stuff

val=(input_a() & 0b01110000); // REading the input pins

// do some stuff with val which may change bits 0 to 4
// here comes tricky part
val=val & 0b01111111; // strip the high bit of val to not influence pin_a7 when writing
output_a((input_a() & 0b10000000) | val); // re-read Port A keep pin_a7 only and output new value but keep pin_a7 in original state (H or L)

// now what sherlock?

[Steve H]

Keep the LED pin state in a variable, then when writing port wise to the port - AND the led bit wioth a zero to clear it, then OR in the variable value.

Steve H.

[tomasch]

Like I said - I don't want to use any RAM (variable) Thanks anyway

[prwatCCS]

Why the concern over a RAM variable. Surely is not that tight?
_________________
Peter Willis
Development Director
Howard Eaton Lighting Ltd UK

[Neutone]

I don't think you can avoid having the compiler use strach variable. You can see what gets used in the list file.

[Guest]

So then what good is read-modify-write. Surly the port latch is a "register"

It's mostly an elegance thing based on how tight the code is - just my comulsive side coming out.

Thanks tou all thus far