A WS2812 array driver in one line of code - no PASM required

Peter JakackiPeter Jakacki Posts: 6,618
edited February 14 in Propeller 1 Vote Up0Vote Down
I thought I'd share this as I like the simplicity of a driver that fits in one line of code. Even though I have a PASM module that can be loaded as required in Tachyon and there is probably no real need to have a one-liner replace, it is still a thing of beauty due to its simplicity.
: XLEDS ( array leds pin -- )	DUP PIN L MASK -ROT 0 -ROT 3 * ADO IC@ 8 FOR H SHROUT L NEXT LOOP 2DROP 50 us ;

The heart of this code are the fast pin operations H, L, and SHROUT where H and L set a pin specified by PIN high or low, and SHROUT outputs the next lsb of data via an I/O mask while preserving these parameters but shifting the data right. At 96MHz clock these operations each execute in 333ns while the FOR NEXT loop also executes just as fast. Using IC@ instead to fetch the next byte using the loop index but instead of pushing this onto the data stack it simply replaces what was there which initially was dummy data 0 (-ROT 0 -ROT) and the used data at the end of each eight shifts.

Setup sets the fast PIN while making sure the pin is low with L. Then a mask is created to be used by SHROUT, so there are two sources for setting a pin, either H or L, or by SHROUT ( iomask data -- iomask data/2 ). Since I want to insert dummy data into the parameter list I need the -ROT 0 -ROT where -ROT does this ( a b c -- c a b ) which is in the opposite direction to a normal ROT but equivalent to ROT ROT. The 3 * is just so we can pass the function an LED count and convert that to bytes required so that ( array bytes ) are passed to ADO which takes the starting address and count and also pushes the branch address for LOOP.
: XLEDS ( array leds pin -- )

OP		<---- stack top			DESC
------------------------------------------------------------------------------------
DUP		array leds pin pin
PIN		array leds pin			set fast pin
L		array leds pin			fast pin low
MASK		array leds pinmask		create mask for SHROUT ops
-ROT		pinmask array leds		work dummy data into the correct position
0		pinmask array leds 0
-ROT		pimask 0 array leds
3		pinmask 0 array leds 3		convert leds to byte count
*		pinmask 0 array bytes
ADO		pinmask 0			start a loop using start address and count
IC@		pinmask data			replace data with the next byte pointed to by loop index
8		pinmask data 8			for 8 times
FOR		pinmask data
H		pinmask data			output a high
SHROUT		pinmask data>> 			output a data bit
L		pinmask data>> 			output a low
NEXT		"	"			next bit
LOOP		"	"			loop for another byte
2DROP						discard parameters
50		50				delay for 50 us
us
;						return

What's the problem with this driver? It needs the bytes reversed beforehand or just work with them as is. There's no time to do a 8 REV for each byte without losing the timing.

EDIT: Here's a decompilation listing - 21 instructions or 42 bytes - I've also added a column of Spin near equivalent code as a guide
( 0084 $3002  ok )   SEE XLEDS
---				Spin near equivalent
2D0E DUP
2D10 PIN			pinreg := pin
2D12 L				outa[pinreg] := 0
			        dira[pinreg] := 1
2D14 |<				pinmask := 1 << pin
2D16 -ROT
2D18 $0000 (0)			data := 0
2D1A -ROT
2D1C $0003 (3)			bytes := leds*3
2D1E *
2D20 ADO			repeat i from array for bytes
2D22   IC@			  data := rdbyte[i]
2D24   $0008 (8)		  repeat 8
2D26   FOR
2D28     H			    outa[pinreg] := 1
				    dira[pinreg] := 1
2D2A     SHROUT			    outa[pin] := data&1
				    data := data >> 1
2D2C     L			    outa[pin] := 0
				    dira[pin] := 1
2D2E     NEXT
2D30   LOOP
2D32 2DROP
2D34 $0032 (50)			delayus(50)
2D36 GOTO us
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Comments

  • 5 Comments sorted by Date Added Votes
  • That's very cool. I'm going to take this as a challenge to try to understand that one-liner! :-)
  • Wow! What next, VGA driver?
  • IC@		pinmask data			replace data with the next byte pointed to by loop index
    

    I always seem to find new words I haven't used in my code yet!
  • D.P wrote: »
    IC@		pinmask data			replace data with the next byte pointed to by loop index
    

    I always seem to find new words I haven't used in my code yet!

    IC@ has existed before but V3 didn't have room for it and many other functions. This version though does a replace rather than a push, as it is more useful and faster like that in a loop. Besides, it required only one extra PASM instruction to implement before falling through into C@.
    7A28(0084) CA 1B BF A0 | ICRD                    mov     tos,loopstk+1
    7A2C(0085) 8D 1B BF 00 | CFETCH                  rdbyte  tos,tos
    7A30(0086) 83 01 3C 5C |                         jmp     unext
    
    Tachyon Forth - compact, fast, forthwright and interactive
    useforthlogo-s.png
    Tachyon Forth News Blog
    TACHYON DEMONSTRATOR
    Brisbane, Australia
  • Peter JakackiPeter Jakacki Posts: 6,618
    edited February 15 Vote Up0Vote Down
    D.P wrote: »
    IC@		pinmask data			replace data with the next byte pointed to by loop index
    
    I always seem to find new words I haven't used in my code yet!

    Just by way of illustrating how useful it is to have words that use the stack as fixed registers rather than pushing or popping, especially in a loop, this is an SPI block write method written both with I C@ and IC@. Have a look at the timings.
    ( 0003 $2D0E  ok )   0 100 LAP ADO I C@ SPIWRB DROP LOOP LAP .LAP
    46736 cycles at 96000000Hz  or 486.833us 
    
    ( 1579 $2D0E  ok )   0 0 100 LAP ADO IC@ SPIWRB LOOP LAP .LAP
    30736 cycles at 96000000Hz  or 320.166us
    

    Tachyon Forth - compact, fast, forthwright and interactive
    useforthlogo-s.png
    Tachyon Forth News Blog
    TACHYON DEMONSTRATOR
    Brisbane, Australia
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