rotate bytes in array

[LinkMTech]

Guys,
May not be the solution for Art's project, but this post interested me because I was also stumped trying to shift bytes of data FIFO and ended up making something work for me.
I liked the FOR/NEXT approach given here and thought I would try it since it looked faster and smaller since I'm really collecting 32 bytes instead of 12 in this example.

My kludge: This scheme uses 24 words and takes 12.5us

DEFINE OSC 8

RX      VAR BYTE        ' Buffer data
XYZ_val VAR BYTE[12]    ' Accelerometer XYZ data
TC_OUT  VAR PORTC.4     ' Test point to measure loop time

Load_array:
TC_OUT = 1
    XYZ_val[0] = XYZ_val[1]     ' Oldest data
    XYZ_val[1] = XYZ_val[2]
    XYZ_val[2] = XYZ_val[3]
    XYZ_val[3] = XYZ_val[4]
    XYZ_val[4] = XYZ_val[5]
    XYZ_val[5] = XYZ_val[6]
    XYZ_val[6] = XYZ_val[7]
    XYZ_val[7] = XYZ_val[8]
    XYZ_val[8] = XYZ_val[9]
    XYZ_val[9] = XYZ_val[10]
    XYZ_val[10] = XYZ_val[11]
    XYZ_val[11] = RX            ' Newest data
TC_OUT = 0    
RETURN

Was surprised to to find that this scheme uses 43 words and takes 216us:

DEFINE OSC 8

RX      VAR BYTE        ' Buffer data
XYZ_val VAR BYTE[12]    ' Accelerometer XYZ data
TC_OUT  VAR PORTC.4     ' Test point to measure loop time


Load_array:
TC_OUT = 1
    FOR z = 0 to 10
        XYZ_val[Z] = XYZ_val[Z+1]
    NEXT z
    XYZ_val[11] = RX
TC_OUT = 0
   
RETURN

It meant more typing but saved some space and time.

[Demon]

...It meant more typing but saved some space and time.

I'm not surprised at all. We quickly forget that FOR-NEXT commands are a combination of ADDs, IFs and GOTOs in the background. The only problem is that the compiler has to make it generic to work for many circumstances; like the STEP -1 example. So now it has to include SUBTRACTs as well, that generates even more code whether you need it or not.

I'm simplifying, but that's pretty much it.

I use the simplified commands 'cause I haven't run out of space yet, and speed hasn't been an issue yet either. But if I ever reach that point, that's when I start breaking them down to their basic elements.

Robert


EDIT: Assembler class was back in '78-'83 era, yeesh...

[LinkMTech]

Thanks for the insight Robert.
The PBP simplified commands are the very reason I started "programming" especially after seeing the difference Bruce gives on his site.

My PIC16F1823 is nearing the end with 2020 words used and just happened to need the space and faster time. Plus it was just rewarding to figure this out on my own and thought I'd share a little for some other newbie out there.

[Art]

So far our score in PBP knowledge:
Robert 1 .... Art 56,743

I wouldn't say that, and you might not if you saw what my code looks like when only written for myself!

I'll end up using it because that part of the program can be slow, so I'd rather it tight.
Just had to remember to fill up the array with an acceptable value at initialisation so the low voltage cutout
doesn't kick in straight away (values start out all zeros), so I initially fill the array with the first stable voltage reading.

[Acetronics2]

Hi, Art

a good question to ask : do you need to store the 30 values for another purpose than calculate your mean value ???

because @ each measurement you could calculate:

new_meanval = (old_meanval*29 + last_measure) /30 ... which gives you the mean value over the 30 lasts measurements ! ( called "rolling average", if I'm right )

note here 32 measures would be best for dividing time ! so :

new_meanval = (old_meanval*31 + last_measure) >> 5 ...

I think Darrel posted a while ago a nice routine about that ...

I remember he also posted about a 48 level stack ... may be ideas to take there ...

Alain

[Art]

Yes, the device is also a car alarm that is triggered by internal lights that come on when the vehicle door is opened.
The line is 12 Volts live until a door is opened & a resistor divider provides 5 Volts for the pic's input.
If ever I get a false alarm, I want to save the last thirty seconds voltage readings to see if I can prevent it.

When the car battery goes cactus my alarm might also sound since the resistor divider is at the unregulated side,
and may not produce the voltage for logic high at the pic pin.

I could have the program look at the last thirty seconds values to prevent these false triggers.

[amgen]

from your decription, the alarm part could be done with 1's and 0's, door opened/closed, no a/d overhead, just digital pin, "0" at < 2.5volts and "1" > about 2.5volts. No help checking batt voltage though...??
Don

[Demon]

Yes, the device is also a car alarm that is triggered by internal lights that come on when the vehicle door is opened.
...

Just a technicality; I'd feed directly from the door switch, not the lamp itself. You could have forgotten the interior lamp switch in the OFF position, the lamp could be burnt, or the robbers might know this trigger; they could just break the window, turn off the lamps and open the door.

I know it's highly unlikely, but it would make your device even better.

Robert