Propeller system clock performance

Scott Lewis

I am not sure if the performance I am seeing is accurate. I have the propeller education board with a 5.0 MHz crystal and am using the 16x PLL to get a clock of 80 MHz. Is this clock split between all the cores, resulting in 10 MHz per core, or does each core get the whole 80 MHz.

I ran a test to see how fast a core could switch an output pin and do nothing else and i got about 43 kHz, I think it should be more than this. Here is the code I used and a screenshot from my oscilloscope.

CON
  _xinfreq = 5_000_000
  _clkmode = xtal1 + pll16x
  
var
long stack[noparse][[/noparse]20]
PUB main
  
  cognew(start(15),@stack[noparse][[/noparse]0])
    
Pub start(Pin)
  dira[noparse][[/noparse]Pin]~~
  repeat
      !outa[noparse][[/noparse]Pin]

Paul Baker

Everyone gets the 80MHz, each assembly instruction takes 4 clock cycles to execute for a throughput of 20MIPS per cog. If you want to output a fast signal you will need to program in assembly, use a counter, or use an already written object that has an assembly driver for generating frequencies.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Mike Green

1) All of the cores run off the system clock, so, unless they're stopped or waiting for something, they're executing most instructions every 4 clock cycles.

2) Your program is written in Spin which is compiled into byte codes that are interpreted by a program running in a cog. The speed you're seeing is about right. If you rewrote the program in the Propeller's assembly language, you'd be able to toggle an I/O pin every 8 clock cycles (about 100ns).

3) As Paul mentioned, you can use the cog counters (see application note #001) from either Spin or assembly language to generate pulse trains of at least 128MHz

Post Edited (Mike Green) : 11/14/2008 10:23:57 PM GMT

Andy Lindsay (Parallax)

You'll also get to generating higher frequency signals with counters in the Counter Modules and Circuit Applications PE Kit Lab.

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Andy Lindsay

Education Department
Parallax, Inc.

Paul Baker

A follow-up to Mike's #2:
If you use the following code:

CON
  _xinfreq = 5_000_000
  _clkmode = xtal1 + pll16x

  pin = 15
  
var
PUB main
  
  pinum := |<15
  cognew(@start,0)
    
DAT
start mov dira, pinum
loop  xor outa, pinum
      jmp #loop
pinum LONG 0

it will toggle the output pin @ 10 MHz, this is the fastest you can drive a pin from code. Using the counter can achieve frequencies up to 128 MHz when using the PLL mode of operation.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Scott Lewis

thanks for all the replies

Andrew E Mileski

Mike Green said...
3) As Paul mentioned, you can use the cog counters (see application note #001) from either Spin or assembly language to generate pulse trains of at least 128MHz

You can also use the counters in a pseudo "one-shot" mode to generate single pulses from 1 to 2^31 cycles long (I don't think the app note mentions this specifically).

Example, say a N cycle positive pulse:
- set FRQ to 1
- set PHS to 2³² - N
- set counter to NCO mode %00100 (PHS bit 31 = output)
- waitcnt for longer pulses > 4 cycles (calculate beforehand)
- stop the counter (CTR = 0) or reload PHS

How it works: this sets the counter to output a very slow square wave of 2^32 cycles (about 54 seconds at 80 MHz), but more importantly starts the counter N cycles before it is about to transition from 1 to 0. There is lots of time after the transition to stop the counter (pseudo "one-shot") or restart it.

If you want to send out back-to-back short pulses like this, then you have to use multiple synchronized cogs (waitcnt), as the counter setup takes at least 16 cycles per pulse.