pasm question

mikea · 2013-03-03 12:59

I'm trying to ramp down a led as a pasm lesson, but can't seem to get it right. I've been copying examples from the manual and slightly changing them. I have a couple questions...
is... "delay long 0" near the bottom considered an unchangeable constant or is it just the starting point and will change if added to? If the "0" is left off is it assumed as a starting point and delay is more like a variable. The main problem here is i'm trying to increment delay with a limit of 80_000_000, so the rate of toggle slows to one second on one second off. Seems pretty straightforward, but it's not working.Any advice would be appreciated

{{ AssemblyToggle.spin }}
CON
_clkmode = xtal1 + pll16x
_xinfreq = 5_000_000
PUB Main
{Launch cog to toggle P16 endlessly}
cognew(@Toggle, 0) 'Launch new cog
DAT
{Toggle P16}
org 0                         'Begin at Cog RAM addr 0
Toggle
 mov dira, Pin            'Set Pin to output
 mov Time, cnt           'Calculate delay time
 
:loop
 waitcnt time, Delay     'Wait
 xor outa, Pin              'Toggle Pin
 max Delay, #apex      'limit delay value to 80_000_000
 add Delay, #1             'increment delay value
 jmp #:loop 'Loop endlessly

 Pin long |< 16            'Pin number
 Delay long 0              'Clock cycles to delay
 apex long 80_000_000
 Time res 1                 'System Counter Workspace

MagIO2 · 2013-03-03 13:14

waitcnt time, delay

would add delay to time and in the next Loop wait for that counter-value. But adding 0 means that you already missed the counter-value and that you have to wait for ~50 seconds until the counter passes the value again.
So, in your code you also have a minimum delay which makes the code run as expected (not missing the counter value). This minimum value also depends on the runtime of instructions after the waitcnt,

Delay is a variable which is initialized to be 0, but you can change it anywhere in your code!

Mark_T · 2013-03-03 16:10

The idiom in PASM is

                mov     time, cnt
                add     time, delay
                waitcnt time, delay2  ' wait for delay
                waitcnt time, delay3  ' wait for delay2

In other words the delay in the instruction sets up the next waitcnt, not this one, so you need to have special case for the first use.

The waitcnt instruction behaves like a wait, then an add.

kuroneko · 2013-03-03 16:20

Also, max Delay, #apex will limit the maximum to 9 (which is the current cog register location for apex). You want max Delay, apex instead and possibly move the add before the max. Minimum value for Delay will be 22 for the current setup (4*4+6).

DAT             org     0

Toggle          mov     dira, Pin
                mov     Time, cnt
                add     Time, #9        ' minimum delay to not block

:loop           waitcnt time, Delay
                xor     outa, Pin
                add     Delay, #1
                max     Delay, apex
                jmp     #:loop

Pin             long    |< 16
Delay           long    22
apex            long    80_000_000
Time            res     1

                fit

pasm question

Comments