From HF Signals
Jump to: navigation, search

Interfacing with the SI570

The SI570 is a 3.3v device. By default the arduino will pull up the i2c sda and scl lines to +5v. With a 4.7k external pull-up the line is only pulled to 3.4 v which is still in the ok range for the si570.

The internal pull ups are now disabled by default in software.


The Si570 is calibrated in factory for a specific output frequency. Radiono will use the factory calibration information so you do not need to do any calibration.

The only information required by the code is the frequency for which your module was calibrated. In most Si570, this seems to be 56.320 Mhz. If your Si570 was calibrated for another frequency, you will need to change that constant in the source code (in the function setup() of radiono.ino.

There are two ways to find this frequency:

  • Spec: Look on the marking on your Si570 (mine says CAC000141) and lookup some tables online to find which frequency corresponds to your marking (See section below on how to find you Si570 startup frequency)
  • Experimental: just remove the Arduino and apply power to the control board. Measure the frequency at the output of the Si570.

With factory calibration, the VFO error will be minimal. In theory the Si570 has a maximum error of 61.5 ppm (for the CMOS "C" grade version). It runs 20Mhz above the vfo frequency, so maximum error will be (vfoFrequencyInMegaHertz + 20) * 61.5 Hertz.

Example, for the 20m band, the maximum error would be: (14+20) * 61.5 = 2.1 kHz.

In practice, I have found much smaller values:

How to find your Si570 startup frequency

  • Go to URL:
  • Use Option 1. Then use default sub-option of "Oscillator".
  • Enter you Si570 part number starting with 570. For Thomas's chip above you would enter: 570CAC000141
  • Press to "Go" button which looks like a right arrow.
  • Results returned tell us that it was a custom ordered chip with a start up frequency of 56.32 MHz. Along with some other important and useful information:
Existing Custom Parts 1 result

Product Si570
Startup Frequency               56.32 MHz
I2C Address (Hex Format)        55
Format                          CMOS
Supply Voltage                  3.3 V
OE Polarity                     OE active high
Stability                       61.5 ppm
Frequency Range Exception Value 10 - 160 MHz
Operating Temp Range (°C)       -40 to +85

Typical Voltages found on a 3V3 CMOS Si570

1 (NC)   = 3.26v     Probably not connected internally. Silicon Labs tell us to leave it isolated. Floats at about 3V3.
2 (OE)   = 3.26v     Output Enable
3 (GND)  = 0.000v    Ground
4 (CLK+) = 1.616v    Signal output. Square wave.
5 (CLK-) = 0.000v    Not connected on the CMOS version of the Si570. Complementary signal output for other versions of the Si570.
6 (VDD)  = 3.27v     3V3 Supply voltage
7 (SDA)  = 3.26v     I2C interface Serial Data
8 (SCL)  = 3.26v     I2C interface Serial Clock