Let’s work with
Now we are going to move on to
work with the system clock!
Timing Delays with the System Clock
Certain I/O operations are much easier to study with code that
controls the timing of certain events, such as when an LED
lights or how long a pushbutton is pressed. The three basic
Spin building blocks for event timing are:
1. cnt – a register in the Propeller chip that counts system clock
2. clkfreq – a command that returns the Propeller chip’s system
clock frequency in Hz.(Think of it is as a value that stores the
number of Propeller system clock ticks in one second.)
3. waitcnt – a command that waits for the cnt register to get to
a certain value.
Essentially, this is the way waitcnt works:
Lets use an example;
waitcnt(clkfreq + cnt) ‘ causes the program to
WAIT for 1 second, then move on.
To create a delay that lasts only a FRACTION
of a second, the code would look like this;
waitcnt(clkfreq/3 + cnt) ‘ this command
causes a delay for 1/3 of a second, then
moves on.
If we really want to get fancy, we write;
waitcnt(clkfreq/1000 + cnt) ‘ now it waits for 1
Millisecond before it moves on!
Lets try writing some code for this:
‘File: LEDOnOffP20.spin
dira[20] := 1
outa[20] := 1
waitcnt(clkfreq/4 + cnt)
outa[20] := 0
waitcnt(clkfreq/4*3 + cnt)
Load the program LEDOnOffP16.spin into RAM or EEPROM by
selecting F10 or F11.
So what happened?
1. Is the LED flashing?
2. Can you figure out what the flash rate is?
(for long does it stay on then go
3. Put your BRAIN in GEAR! Can you figure out a
way to make the LED Stay on for 1 second and stay
OFF for 3 seconds?
 In case you have missed it, indentation when writing
spin code is VERY important!
 Whenever you have commands that need to be linked
together such as commands that follow a repeat
command, be sure the code below is INDENTED to
avoid unexpected results!
The BLUE area shown below should be shown in
GREY in the Parallax Propeller Tool program when
outa[20] := 1
waitcnt(clkfreq/4 + cnt)
outa[20] := 0
waitcnt(clkfreq/4*3 + cnt)
Quick info on the waitcnt command:
If you write the code:
waitcnt(clkfreq + cnt) it’s the SAME as
waitcnt(12_000_000 + cnt)
Essentially, it’s the Propeller chips DEFAULT clock.
If we want to change the system clock to speed it up or
slow it down, we use special code in the CON block to
do that. Be aware that if NO Clock Config Constant
(CON) is set, the Propeller will use the default setting.
Let’s take a look in the next slide!
Let’s break down the SPECIFICS!
_xinfreq = 5_000_000
_clkmode = xtal1 + pll16x
The code is broken down like this;
_xinfreq = 5_000_000 ‘ defines the expected frequency from
the crystal (Oscillator) on the circuit board. (It’s the silver
looking can)
In our case the crystal is 5 Mhz (5 Megahertz = 5 Million cycles
per second)
_clkmode = xtal1 + pll16x ‘ _clkmode sets the chip into clock
mode. xtal1 configures the PROP chip to work with our external
crystal. Pll16x tells the PROP chip to set its Phase Locked Loop
(pll) circuitry to multiply the 5Mhz crystal value X 16. …………...
so 16 X 5Mhz = 80 Mhz!
That’s Pretty Cool STUFF!
Now you know how to change the
time on a PROP chip so that you
can make the chip turn something
ON or OFF at a rate you want! Just only takes a little math to
make this chip do some interesting
The EXTERNAL crystal is ACCURATE when used with
correctly written code….
HOWEVER, the PROP chips INTERNAL timing is NOT
very accurate.
SO….If you want to make something work within a
timing sequence and you want the time to be
accurate…you need to use the EXTERNAL CRYSTAL to
set the timing. (This means you have to write the CON
block of code to set the external crystal to work!)
Let’s write some code to try this out!
‘’File: ConstantBlinkRate.spin
_xinfreq = 5_000_000
_clkmode = xtal1 + pll16x
dira[16] := 1
outa[16] := 1
waitcnt (clkfreq/2 + cnt)
outa[16] := 0
waitcnt (clkfreq/2 + cnt)
Load the program ConstantBlinkRate.spin into RAM or EEPROM
by selecting F10 or F11.
So what happened?
1. Is the LED connected to P20 flashing?
2. Is it flashing 1 time every second?
(remember that 1 flash per second = 1 Hz)
3. Can you explain how the PROP knows to flash the
LED one time per second?
See the next slide to break it down a little further!
A little more info on clkfreq:
When you write the code;
waitcnt(clkfreq/2 + cnt) ‘ the waitcnt is like a pause
while the cnt register counts clock ticks.
The clkfreq command returns the number of ticks PER
SECOND (Based on the PROP chips clock settings)
So…even though the _clkmode = xtal1 + pll16x tells the
PROP to run at 80 Mhz, this has NO effect on the flash
rate. This is just telling the PROP chip at what speed
to run!
Let’s make a change in the code
and test it out!
‘File: ConstantBlinkRate.spin
_xinfreq = 5_000_000
_clkmode = xtal1 + pll8x
Only 1 change was
made in your previous
code! Change your 16
to an 8. (This sets the
system clock to run at
40 Mhz instead of
dira[20] := 1
outa[20] := 1
waitcnt (clkfreq/2 + cnt)
outa[20] := 0
waitcnt (clkfreq/2 + cnt)
Download the code and see if
there is a change in the flash rate
of the LED.
So what happened?
1. Did your flash rate remain the same?
2. What happens if you change the pll8x to pll4x or
Although an actual NUMBER can be used to replace
the clkfreq command its use should be limited.
clkfreq is used when PREDICTABLE DELAYS are
essential! (This is very critical when you want an
object to be used by another object!)

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