I2C Bus Introduction


This stage sets up the I2C bus to permit control and monitoring of the various functions of the MOBO 4.3. The I2C clock and data lines from the microprocessor are bridged over to the analog side of the board via the ADUN 1251 isolator and, thence, through U10's bidirectional voltage translation to U3, the PCF8574 I2C bus to parallel port I/O expander.

Basic functioning of the bus will be tested by commaning the MOBO 4.3 to switch to TX mode, which should result in a low signal at pin 12 of U3 and a high at pin 1 of P32 (Softrock RXTX V6.3 PTT-I footer).

It is always a good idea to double check the bill of materials in this set of documentation with the official BOM on the Mobo site. Discrepancies may be introduced inadvertently, as the changes are sometimes not communicated or discovered, making timely update of this site a problem.

(go directly to build notes)

I2C Bus Schematic

(Click for Full Schematic)
I2C Busschematic


(go directly to build notes)

I2C Bus Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

1header, 2-pin Header
1header, 2-pin RA Header
1header, 4-pin Header
1header, 4-pin RA Header
1housing, female, 2-pin Housing
1housing, female, 2-pin Housing
20.1 uFSMT 0805
11 uFSMT 0805
310 k ohm 5%SMT 0805 1/8W
22.2 k ohm 5%SMT 0805 1/8W
22.2 k ohm 5%SMT 0805 1/8W
1221 k ohm 5%SMT 0805 1/8W
1PCF-8574 remote I/O ExpanderPCF-8574 PCF-8574SOIC-16
1GTL2002D 2 bit bidir voltage xlatorGTL2002D GTL2002DSOIC-8

I2C Bus Summary Build Notes

I2C Bus Detailed Build Notes

Bottom of the Board

I2C Bus Bottom View

Install Bottomside Ics and Transistors

Install Bottomside Ics and Transistors photo
U03PCF-8574 remote I/O ExpanderPCF-8574 PCF-8574SOIC-16
U10GTL2002D 2 bit bidir voltage xlatorGTL2002D GTL2002DSOIC-8
Take ESD precautions
There is a beveled edge on the side that pin 1 is on.

Install Bottomside Static Components

Install Bottomside Static Components photo
C100.1 uFSMT 0805
C350.1 uFSMT 0805
R0510 k ohm 5%SMT 0805 1/8W
R0710 k ohm 5%SMT 0805 1/8W
R0910 k ohm 5%SMT 0805 1/8W
R232.2 k ohm 5%SMT 0805 1/8W
R242.2 k ohm 5%SMT 0805 1/8W
R352.2 k ohm 5%SMT 0805 1/8W
see note #4
R362.2 k ohm 5%SMT 0805 1/8W
see note #4
R27221 k ohm 5%SMT 0805 1/8W

Top of the Board

I2C Bus Top View

Install Topside Components

C411 uFSMT 0805

Install Pins

P02header, 2-pin RA Header 
External PA input must be short to ground to activate type. Max current 500 ma, Max voltage 60V
P11header, 4-pin RA Header 
I2C connector.
For connecting to Marc's Switched LPF (optional)
P02_Matehousing, female, 2-pin Housing
For keying external PA.
External PA input must be short to ground to activate type. Max current 500 ma, Max voltage 60V
P33_Matehousing, female, 2-pin Housing
P33header, 2-pin Header
Similar to P2, but this will not activate in high SWR conditions
P35header, 4-pin Header
I2C connector
For connecting to Preamp/Attenuator board (optional)

I2C Bus Completed Stage

Top of the Board

View of Completed Top

Bottom of the Board

View of Completed Bottom

I2C Bus Testing

I2C Bus Voltages

Test Setup

Without a osclloscope you could do some static DC measurements. The I2C bus is open collector (or drain) driven; i.e. each device pulls the Data or CLK bus to ground and using the I2C buss pullup resistors for the high level.

Measure on each of the I2C buses and see that the pullups are all there. Then you can try and short to ground and see the I2C signal propagate through the isolator/level translator.

Measure dc voltage with respect to ground

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
P24: Pins 1,2,3 wrt Pin 4Vdc+5TBD_______
P13: Pin 1,2,3 wrt Pin 4Vdc3.28TBD_______
P11: Pins 1,2,3 wrt Pin 4Vdc+5TBD_______

Integrity of U10 (GTL2002) and U13 (ADuM1251)

Test Setup

Once you have varified the pullup resistors are correct, short to ground each of the I2C lines (starting at the processor (digital) end) and see how it flows through the Isolator (ADUM 1251) and level translator(GTL2002D).

You should use a small resistor to ground (maybe 100 ohms) in case you have a short on one of the buss lines to power :)) Shorting to power is probably bad for any device trying to drive the buss; i.e. make sure you don't short to power by mistake. Using the 100ohm resistor will swap the level pulled up by the 2.2K resistors. Or just use a current limited supply for testing; and set current trip point just above nominal running current.

Note: From P24 to P13, through the ADUM 1251, the Clock line (SCL) is unidirectional; the data line (SDA) is bidirectional

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
(Digital side) Ground P24-2, measure P11-2Vdc0TBD_______
(Digital side) Ground P24-2, measure P13-2Vdc0TBD_______
(Digital side) Ground P24-3, measure P11-3Vdc0TBD_______
(Digital side) Ground P24-3, measure P13-3Vdc0TBD_______
(Analog Side) Ground P13-2, measure P11-2Vdc0TBD_______
(Analog Side) Ground P13-2, measure P24-2Vdc+5TBD_______
(Analog Side) Ground P13-3, measure P11-3Vdc0TBD_______
(Analog Side) Ground P13-3, measure P24-3Vdc0TBD_______
(SR side) Ground P11-2, measure P13-2Vdc0TBD_______
(SR side) Ground P11-2, measure P24-2Vdc+5TBD_______
(SR side) Ground P11-3, measure P13-3Vdc0TBD_______
(SR side) Ground P11-3, measure P24-3Vdc0TBD_______

Test Basic I2C Functioning

Test Setup

Plug the MOBO 4.3 into the SR RXTX V6.3 and power up the Softrock

Connect the MOBO.s USB cable to the PC

Run a program to "command" the MOBO to switch PTT ON (e.g., Mobo Control (see bottom of page for latest version)

Measure the voltage at the testpoints

Test Measurements

TestpointUnitsNominal ValueAuthor'sYours
/I2CPTT wrt ground (TX OFF)hi/loHIGHtbd_______
/I2CPTT wrt ground (TX ON)hi/loLOWtnd_______
P32 Pin 1 wrt P32 Pin 2 (TX OFF)hi/loLOW_______
P32 Pin 1 wrt P32 Pin 2 (TX ON)hi/loHIGH_______

Test Frequency Control

Test Setup

With the MOBO still plugged into the powered RXTX V6.3, run the programmed test to validate that the MOBO can control the RXTX V6.3's SI570 local oscillator.

Measure the frequency of the LO output (you can take a loose insulated wire out of your HF rig's antenna and place it close to the divider chip on the RXTX V6.3 and then see if you are receiving signals at the specified center frequency).

Test Progran Requirements

Test Setup

  • Program should allow user to specify a desired center frequency and then command the SI570 to generate 4X that frequency via the I2C bus.
  • Program should allow user to toggle PTT on and off via the I2C bus.