Softrock RX + Xtall V9.0 USB Control Stage

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Introduction

Theory of Operation

This stage provides the control interface between the hardware SDR and a PC that is running the appropriate SDR software which can provide I²C (Inter Integrated Circuit) bidirectional control signals over a USB connection. There are two lines in the I2C bus: the clock (SCL) and the date (SDA). For a more in-depth discussion of the I²C protocol, see the formal Specification.

The heart of the control circuit is U3, an ATTiny45-20PU 8 bit AVR Microcontroller (caution! the pdf for this device is over 4MB). The unit is powered off of the PC's USB 5 Vdc bus and provides a 6 bit bi-directional I/O port. It draws less than 10mA. The zener diodes in the schematic help ensure that the USB data lines (D+ and D-) are at 3.3V only.

U3 uses the AVR firmware by DG8SAQ to perform the following functions:

• Accept control signals via the pins 2 and 3 of the Universal Serial Bus (USB)
• Translate input signals into I²C control signals (bi-directional SDA and input-only SCL lines) for the Si570 programmable Oscillator
• Translate output I²C signals from the Si570 back to USB signals to the PC
• Translate incoming bandswitching commands into appropriate signals to J3 (for control of the new HF-BPF board.
• Future:  new firmware for U3 is being developed to implement automatic band switching of the new HF-BPF switchable BPF.

Update

Several builders have experienced issues with the voltages on the USB-2 and USB-3 lines and diodes D2 and D3. Following a long series of messages on the Softrock Yahoo Group, Jan G0BBL and Tony KB9YIG have decided to address the issue as follows:

• Each new kit will be packed with two each of the BZY55 3.3 volt and 3.6 volt zener diodes in place of the two 1N5227B zener diodes for the D2 and D3 locations.
• Builders are advised to try the 3.6 volts BZY55 zener pair for D2 and D3 first, (marked on the glass body with 3V6), and if they still have USB communications reliability problems then go to the 3.3 volt BZY55 zener pair.

Schematic

This is a subset of the overall schematic. Note: red dot indicates resistor testpoints (hairpin, top, or left-hand lead)

Bill of Materials

Designation	Value	Color/Code	Orientation	Category	Notes
C06	0.1uF			SMT 1206	black marked strip
C07	0.1uF			SMT 1206	black marked strip
C08	0.1uF			SMT 1206	black marked strip
FL1	ferrite filter			SMT 1206
FL2	ferrite filter			SMT 1206
FL3	ferrite filter			SMT 1206
FL4	ferrite filter			SMT 1206
FL5	ferrite filter			SMT 1206
D2	1N5227B,3.6v		N-S		do not use
D3	1N5227B,3.6v		S-N		do not use
D2	BZY55,3.3v		N-S		See "Update" note in Introduction Section, above.
D3	BZY55,3.3v		S-N		See "Update" note in Introduction Section, above.
D2	BZY55,3.6v		N-S		Use this. See "Update" note in Introduction Section, above.
D3	BZY55,3.6v		S-N		Use this. See "Update" note in Introduction Section, above.
C05	4.7uF	475		ceramic
R01	68 1/6W		S-N
R03	68 1/6W		S-N
R05	4.7k 1/6W		E-W
R02	1M 1/6W		S-N
R04	2.2k 1/6W		S-N
R06	1k		S-N
R07	1k		S-N
U2	ATTiny45-20PU				with socket
J3	2 pin header
cable1	USB				4 wire shielded USB cable with USB male on 1 end

Summary Build Notes

• Install Zener diodes D2 and D3 (top)
• Install resistors  and C5 ceramic cap (top)
• Install U2 and socket (top)
• Install J3 (top)
• Install and connect USB cable (top)
• Test the Stage
• Install SMT capacitors and ferrite filters (bottom)

Detailed Build Notes

In other stages, we prefer to begin with the bottom side of the board. However, in this stage, the thru-holes for the topside coponents are very close together and are just begging to get solder splashed into them if we were to install the SMT components first. Thus, in this stage, we have reversed the bottom-then-top sequence.

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