Fastest way to flip a bit...

dkemppai · 2009-07-02 19:21

OK, I'm having a mental block...

What's the fastest way in ASM to toggle an I/O pin or register bit, knowing only the pin or bit·in question.

For example, RA.4 needs to be toggled every time I run a sub. What's the least amount of·code to do this.
I can read the state of the pin from the I/O registers or register.· The pin/bit·doesn't change often, but when I need
to change it·I need to conserve clock cycles...

Also, Jitter is not a concern.

Thanks,
Dan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

"A saint-like quantity of patience is a help, if this is unavailable, a salty vocabulary works nearly as well." - A. S. Weaver

dkemppai · 2009-07-02 19:52

·This is what I have:

jb···· MyBit,$+4
setb MyBit
skip
clrb· MyBit

Anyone have anything faster/shorter...

Thanks,
Dan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

"A saint-like quantity of patience is a help, if this is unavailable, a salty vocabulary works nearly as well." - A. S. Weaver

Peter Verkaik · 2009-07-02 20:01

If the bitposition is known,

mov w,#1<<bitpos
xor targetvar,w

This is assumed the bank is already set, or targetvar is global,
as ports are.

regards peter

JonnyMac · 2009-07-02 20:31

Since you know it's RA.4 that you want to toggle you can hard-code Peter's example:

mov w, #%0001_0000
xor ra, w

dkemppai · 2009-07-03 00:17

Peter Verkaik said...
If the bitposition is known,

mov w,#1<<bitpos
xor targetvar,w

This is assumed the bank is already set, or targetvar is global,
as ports are.

regards peter

I didn't think of xoring...·· ...I'll have to remember that. (Although, I do use XOR gates to buffer/invert data streams via logic control...)

The problem is that the pin/bit is an assigned name·variable, and may move depending on where I use this code. For example:

MyPin· equ·· RA.1 in one example...
MyPin· equ·· RE.3 the next time I use it.

I good example would be a macro called "FlipBit", where I call the macro, and pass·"MyPin" to the macro...

Is there any way to determine the bit position of a bit in a variable in the macro language?

Thanks!
Dan

P.S. I'm really starting to like macros...· [noparse]:)[/noparse]

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

"A saint-like quantity of patience is a help, if this is unavailable, a salty vocabulary works nearly as well." - A. S. Weaver

Zoot · 2009-07-03 01:36


MyPinPort PIN RA ' could be RA through RE
MyPinNum CON 3 ' could be 0 through 7, will be pin num on pin port
MyLed PIN MyPinPort.MyPinNum OUTPUT


' now to flip the bit....

ASM
MOV W, #1<<MyPinNum
XOR MyPinPort, W
ENDASM

'and some pin var stuff:
MyLed = 1
PAUSE 100
MYLed = 0

So you can change the pin assignment to any pin any port w/o rewriting code.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php

CounterRotatingProps · 2009-07-03 17:15

> For example, RA.4 needs to be toggled every time I run a sub.
The code sample is a near call, but I'm wondering about the below·for my own thing... should just hook it up and see...

If that were·a far call instead, wouldn't that force the pipeline to flush, and increase the toggle cycles, decreasing the toggle speed?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

dkemppai · 2009-07-08 00:57

CounterRotatingProps said...
> For example, RA.4 needs to be toggled every time I run a sub.

The code sample is a near call, but I'm wondering about the below·for my own thing... should just hook it up and see...

If that were·a far call instead, wouldn't that force the pipeline to flush, and increase the toggle cycles, decreasing the toggle speed?

Guys,

All I'm after is as short a command as possible to flip the current state of a bit. It would be really nice to have an assembly instruction that was 'toggle x' where x is the bit. We have setb and clrb, but no way to just toggle the bit, so it must be done with some sort of a branch. I'd really like as short a chunk of code to be placed in a macro ·so it could be called without having to think about the garbage invoved in flipping the bit. (Trying to reduce the mundane tasks to simple macros...)

I assign logical pin names. Such as "RunLED" to the pin. That way, if I end up moving the·physical location of·"RunLED", then I don't have to recode everythig in the program, and I don't have to think about what's mapped where. I just know that "RunLED" is my·Run LED...

Unless there is a way to get the port and pin number from only the logical name "RunLED", I don't think there is anything shorter than what I currently use. (Is there a way in the macro language to get the register and bit number?) Also, I believe the XOR option may set you up for the read/write pipeline bug, no?

Again, I like the xor option, as it reduces clock cycles and is short, but is a little harder to apply in a macro as I would like to do...

Thanks!
Dan

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

"A saint-like quantity of patience is a help, if this is unavailable, a salty vocabulary works nearly as well." - A. S. Weaver

Zoot · 2009-07-08 01:45

If you want a ready "macro" that tracks any bit/pin by name, then just use SX/B:

ASM
' lots of assembly
ENDASM
RunLed PIN RA.3 OUTPUT
ASM
' lots of assembly
ENDASM
RunLed = ~RunLed ' this will generate code automatically
ASM
' lots of assembly
ENDASM

Not sure where you get a "branch" with a MOV W, #1<<x, XOR Port, W, it's two instructions every time. The r/w/m pipeline error only kicks in with SETB and CLRB, not on a whole port instruction, like the XOR Port. Macro:

MACRO TOGGLE_BIT ' use: TOGGLE_BIT PinNum, PinPort
     MOV W, #1<<\1
     XOR \2, W
     ENDM

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php

Post Edited (Zoot) : 7/8/2009 1:51:14 AM GMT

Fastest way to flip a bit...

Comments