Fast Pin-Toggling

MXMike · 2009-05-20 09:08

Hi everyone,

i have some questions.

I need to toggel some pins with a speed about 25 Mhz.

Is this posible with a parallax sx?
And if yes, with which one... are the eval-boards for it?

Regards

MXMike

Bean · 2009-05-20 11:38

Mike,
The SX48 can do this with one of it's internal timers by running the SX at double the required frequency.
So by running the SX48 with the 50MHz resonator you should be able to get a 25MHz output.

If you are just getting started with the SX, I would recommend the SX Tech Tool Kit ($90)
http://www.parallax.com/Store/Microcontrollers/SXProgrammingKits/tabid/140/ProductID/364/List/1/Default.aspx?SortField=ProductName,ProductName

Then you'll need the SX48 protoboard (only $15) http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/SX48/List/0/SortField/4/ProductID/362/Default.aspx

If you need a more flexible frequency generator, then you might look into the Propeller chip. It can create a wider range of frequencies because it has built-in PLL's to increase the counter frequency.

Bean.

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There is a fine line between arrogance and confidence. Make sure you don't cross it...

Post Edited (Bean (Hitt Consulting)) : 5/20/2009 1:00:37 PM GMT

MXMike · 2009-05-20 12:40

Hi Bean,

thank you for you answer.

You say the SX can do this with it's internal timers.

I need this frequency in the main loop. I need to fill a few SPI devices.

Is the Pin-Toggeling faster with the propeller?

Remember please, i don't ne a special frequency. I need to fill a few SPI devices and an FPGA.

Regards

Mike

Bean · 2009-05-20 13:01

Mike,
The timer will toggle the pin regardless of what the main code is doing. You can turn the timer on and off to start or stop the counter output, but you will not be able to send data at 25MHz.

Are you just sending zeros ? Or do you need to send actually data to the SPI device ? More info please....

Bean.

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There is a fine line between arrogance and confidence. Make sure you don't cross it...

·

CounterRotatingProps · 2009-05-20 17:58

> but you will not be able to send data at 25MHz

Hi Bean,

related questions, if you please. If you double the freq as you mention (50MHz yields 25MHz pin toggle), would there be enough code-speed headroom left to bit-bang serialized data ?

[noparse][[/noparse]note the thread where I asked about 100MHz clocking] This is interesting, if the calculation is that simple --- or wait, er no, it's just the way the timer works, e.g. if the chip is clocked at 100MHz, then the fastest timer would be 50MHz (Y/N)? At 100 could a I2C-like setup (1 pin SCL, 1 pin SDA) with the ISR overhead drive the two pins at 25MHz using the internal timer to trigger the interupt ? I know it can be setup that way, but the speed issue is really what I'm asking. (And perhaps MXMike will have to deal with similar issues. --- Hey, Mike, I'm just jumping in a sec .. don't want to highjack your thread

)

thanks,

Howard in Florida

PS Bean *thank you* for all your support and work!

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Buzz Whirr Click Repeat

Bean · 2009-05-20 18:26

This program:

DEVICE SX28, OSCXT2, BOR42 
FREQ 50_000_000 

 
value VAR BYTE 

ShiftReg PIN RC OUTPUT ' Data is output on pin RC.0 
ClkPin PIN RB.0 OUTPUT 

 
PROGRAM Start 

 
Start: 
  FOR value = 0 TO 255 
    ASM 
      CLRB ClkPin 
      MOV ShiftReg,value ' Output bit 0 of value 
      SETB ClkPin 
      CLRB ClkPin 
      RR ShiftReg ' Output bit 1 of value 
      SETB ClkPin 
      CLRB ClkPin 
      RR ShiftReg ' Output bit 2 of value 
      SETB ClkPin 
      CLRB ClkPin 
      RR ShiftReg ' Output bit 3 of value 
      SETB ClkPin 
      CLRB ClkPin 
      RR ShiftReg ' Output bit 4 of value 
      SETB ClkPin 
      CLRB ClkPin 
      RR ShiftReg ' Output bit 5 of value 
      SETB ClkPin 
      CLRB ClkPin 
      RR ShiftReg ' Output bit 6 of value 
      SETB ClkPin 
      CLRB ClkPin 
      RR ShiftReg ' Output bit 7 of value 
      SETB ClkPin 
    ENDASM 
  NEXT 
END

Sends 2048 bits in 153.58uSec or 13.33 MHz.

P.S. This probably wouldn't work in the real world because of the back-to-back I/O on the ClkPin.

Bean.

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There is a fine line between arrogance and confidence. Make sure you don't cross it...

·

MXMike · 2009-05-21 06:38

Hi Bean,

i need to send Data.

Your example is that what i need but its to slow. Do you know if this will work faster with a propeller ?

Regards

MXMike

Leon · 2009-05-21 09:35

It's slower than an SX - 100 ns (10 MHz) just to toggle an output pin, IIRC. If you need 25 MHz you will have to use something a lot faster.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 5/21/2009 9:41:25 AM GMT

Bean · 2009-05-21 11:12

The propeller might be able to do it by using the Video Generator to send the data.
I would try using VGA 2-color mode, one bit would toggle each "pixel" for the clock, the other bit would be the data, and would update every other "pixel". The other 6 bits wouldn't be used.

Bean.

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There is a fine line between arrogance and confidence. Make sure you don't cross it...

·

pjv · 2009-05-21 14:34

Hi MXMike;

By cranking the SX up in frequency to 80 Mhz (or more) and with interleaving code, you might be able to do what you need. Unfortunately you have inadequately descibed the "other things" the SX needs to do, either simultaneously, or consecutively with generating the 25 MHz pulse train, so it is impossible to say for sure.

A complete description will make that assessment possible.

For what its worth, I have many systems where I burst data out of an SX at 10 Megabits/second with a 50Mhz resonator. I know that isn't 25 MHz, but simultaneously it also does other things.

Cheers,

Peter (pjv)

CounterRotatingProps · 2009-05-21 15:01

...      MOV ShiftReg,value ' Output bit 0 of value 
         SETB ClkPin 
         CLRB ClkPin 
         RR ShiftReg ' Output bit 1 of value 
         SETB ClkPin 
         CLRB ClkPin 
...

P.S. This probably wouldn't work in the real world because of the back-to-back I/O on the ClkPin

A while back,·having problems with SETB/CLRB rapid fired, a look in the 'manual' said something to the effect that toggling a pin this way
could cause uncertain results.· Sorry, I can't find the quote at the moment. Think it was in the area about an external device pulling a pin low when the SX was trying to push it high.· Am also wondering if, at these speeds,·there could be a capacitive/impedance factor.

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No matter how much you push the envelope, it'll still be stationery.

Fast Pin-Toggling

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