asmclk and the automatic clock setter

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Introduction #

For customers writing PASM2 only programs, the compiler automatically prepends a clock-setter function to the code so that the system clock settings are automatically calculated and configured without the need for user configuration. It just works!

If you don't have an issue with code space, you probably can ignore the rest of this article, and forget all about asmclk!

The compiler handles the heavy lifting and complicated configuration for you!

But if you do come across some example code that includes the asmclk instruction at the top of a DAT block... you might be interested to know what it is for!

How to disable the clock setter #

There might be an occasion where the user wants to avoid the overhead of the start-up clock setter, and instead add the configuration inline with their PASM2 code.

The Propeller 2 asmclk instruction provides a way to setup the system clock "inline", based on the constant _clkfreq that's set in your code file, rather than by prepending the clock-setter.

To make use of asmclk, you must define a special constant _AUTOCLK=0 to disable the prepended clock setter; and in which case you MUST use asmclk (or apply manual clock settings to the appropriate registers).

What impact does the automatic clock-setter have? #

The pre-pended clock-setter uses 16 longs, and executes in milliseconds each boot up time. The total time will vary based on the length of user code. For each long in user code, the boot up sequence of the clock-setter takes 2 clocks (1 read, 1 write). After the clock-setter executes and configures the clock, it then copies the blocks of user code from memory into LUT, then blows them out into the new location. Essentially the user code is shifted up by 64 bytes (16 longs) in memory, to override the space previously taken up by the small clock-setter program.

This behaviour can explain the reason for certain code needing a way to disable this process. One obvious example is where the user code fills the entire memory, and doesn't have space even for the temporary 64 byte requirements of the clock-setter. In this case, the user could use the _AUTOCLK constant to disable the clock-setter, and instead add the asmclk command at the top of their PASM2 code.

Code Snippet #


' *** CLOCK SETTER
' *** Sets clock mode, moves application into position, then executes it.
DAT		org
'
'
' Set clock mode if externally driven
'
ext1		hubset	_clkmode1_		'@000: start external clock, remain in rcfast mode
ext2		waitx	tenms			'@004: allow 10ms for external clock to stabilize
ext3		hubset	_clkmode2_		'@008: switch to external clock mode
'
'
' Move application down to $00000+
'
		mov	ptra,#@app_start
		mov	ptrb,#0
		rep	#4,_appblocks_
		setq2	#$200-1
		rdlong	0,ptra++
		setq2	#$200-1
		wrlong	0,ptrb++
'
'
' Set clock mode if RC_SLOW
'
rcslow		hubset	#1			'@028: start external clock, remain in rcfast mode
'
'
' Relaunch cog 0 from $00000
'
		coginit	#0,#$00000
'
'
' Data set by the compiler
'
tenms		long	20_000_000 / 100	'10ms in rcfast
_clkmode1_	long	0			'@034: clkmode with rcfast
_clkmode2_	long	0			'@038: clkmode with external clock
_appblocks_	long	0			'@03C: application size in blocks of $200 longs
'
'
' Application start
'
app_start					'append application bytes after this label