Jan 5, 2008

A couple of months ago I discovered the newly released Silicon Labs Si570. It seemed like a possible solution to the Softrock receivers being limited to the frequency of commonly available crystals.

I was able to get some samples and am very satisfied with the results.

The Si570 can be ordered with a start-up frequency so that it can be used just like a crystal oscillator with only a 3.3 volt regulator and a bypass and a coupling capacitor. I ordered 54.6 MHz to work on 14.1 MHz and 28.224 MHz to work on 7.056 (for easy comparison to the crystal). I am still thinking what an “ideal” startup frequency would be.

But, more important, the Si570 can be programmed to wide range of different frequencies via I2C bus. I used a PICAXE 18X which is cheap and very easy to program and I had one. If I had to buy one, I would probably get the 28X1 which is only a couple of dollars more and much more capable.

Example using the PICAXE:

main:

              i2cslave $AA,i2cslow,i2cbyte        ‘initialize

               readi2c 137,(b6)

               let b6 = b6 | $10

writei2c 137,(b6)                             ‘freeze DCO

               pause 10

               writei2c 7,($05,$42,$b5,$a2,$02,$85)       ' set new frequency for 14.08

               pause 10

 readi2c 137,(b6)

               let b6 = b6 & $EF

               writei2c 137,(b6)              ‘unfreeze DCO

               writei2c 135,($40)

               Pause 10

The new frequency will stay until frequency data is changed.

I wrote an EXCEL spreadsheet to generate the data to set a new frequency. It can be done by hand with the help of a hex calculator (one is included in Windows). The spreadsheet is better. The part that I ordered can be programmed from 10 MHz through 215 MHz.  As you can see this would work thru 6 meters. One size fits all for Softrock.  

Of course, the programming is open to many possibilities. A simple push button added to the PICAXE could increment up the band by 25 kHz or 40 kHz or whatever each time it is pressed. Any processor can be used.  Or, a visual basic program could generate the frequency word and download it to the PICAXE. One could probably implement I2C on the PC and use RS232 control lines to go to the SI570 directly. The possibilities are endless.

Some may be concerned about the quality of the signal out of the Si570. It seems to be very clean and quite to me. But, I have no way to measure it. I have screen prints of Rocky using the crystal that came with the RXTX and one with the Si570 programmed to show the same frequency as the crystal. 

I wrote to Silicon Labs asking about the specification comparison to the ICS601 which I had studied and got the following response:

This is the minimum circuit to use the 18X from the PICAXE manual. I ran the 18X at 3.3 volts so that I would not have any level shifting issues to the 3.3 volt Si570. On another version, I used a couple of 2N7000’s, to shift from 5 volt to 3.3. But, I like the simpler solution.  

Out 1(pin7) is the I2C serial data and Out 4(pin 10) is the I2C serial clock. They connect directly to the Si570 pins. Each needs a pull-up.  I used 3.3k in my prototype but value is not critical.

The pins at the left are for programming via RS232. I use a USB to serial adapter. See note below.

The programming editor for the PICAXE can be downloaded free and includes the ability to download the software to the PICAXE.

The package is not very convenient (5x7mm), but I mounted it with the pads up and it is easier to solder than SMD.

Only 5 pins are needed.

3.3 v                     SDA (serial data)

Gnd                       SCL(serial clock)

CLK+  (output)

I used a .01 bypass on pin 6 (3.3 v) and a .1 on the output (CLK+). SDA and SCL go to the corresponding pins of the PICAXE. Very easy! 

So far I have only used this with a RXTXv6.2 20/30. I connected CLK+ in place of X101 on the daughter board.  I can easily change the jumper to compare results with the crystal X100. I can’t see any difference between the Si570 and the crystal.

I sent this to Tony and he suggested that it would be pretty easy to make a board with the SI570 in place of the crystals. I am sure that he will be keeping an eye out to see if this is of interest to anyone.

 

A group buy would be less than the cost of making one custom crystal.  This will put a Softrock on any frequency.

 

 

 

Si570 data sheet

PICAXE website to get free software and documentation:

http://www.rev-ed.co.uk/picaxe/

Sparkfun.com has the 18X for $9.95 and the 28X1 is $11.95.

Wulfden.org has 18X with basic parts for $11

Wulfden.org also has a programming adapter P2 for only $3 which contains the common programming parts on a RS232 connector for PICAXE’s. Highly recommended if you have any interest in using a PICAXE for anything.

USB to RS232 adapter that I use:

 http://www.parallax.com/detail.asp?product_id=28030