Extending RPI I2C Bus with a buffer / redriver

Summary

This blog illustrates how the I2C bus of the Raspberry PI (RPI) can be connected to an external sensor such as the Adafruit SI1145 Digital UV Index, IR, visible light sensor using a buffer/redriver. 

Raspberry PI 2 Model B (RPI)

Essential Reading

For the tests performed with the I2C interface on the RPI the NXP I2C specification is a solid read for those interested in the nuts and bolts of I2C operation.

At the hardware level the I2C.org site explains the gritty detail of the bus. What was helpful on the website, in addition to the specification, are the images showing various states of the bus and the associated operational issues.

Hardware - RPI
The GPIO header (J8) was utilisedI2C connections on the RPI.

The image below shows the data (SDA) and clock (SCL), Image courtesy Pinouts.xyz

RPI Pinouts for I2C

Hardware - SI1145
To test the I2C connection, a Silicon Labs SI1145 light sensor was connected to the RPI GPIO connector. The schematic of the SI1145 board is shown below.

Adafruit SI1145 UV Sensor

Hardware - Cable
For testing two types of cables were tried:

1. Flat 4 core telephone cable with a quoted capacitance of 670pf/100m and
2. CAT5e cable which is specified with a capacitance of 330pF/100m (not mutual pairs) to ground

Hardware - Buffer / Redriver
There have been similar designs, discussions in various blogs and on RaspberryPi.org relating to buffering I2C. While there is some truth to the point, as mentioned in one of the blogs, that I2C is meant for device to device communication on the same circuit board, the reliability of the signal and ultimately data, is a partially a combination of the hardware design and quality of equipment used. 

For a more robust implementation a traditional differential solution such as CAN (Controller Area Network) or even Ethernet would be considered for longer cable runs exceeding 5m.

In sourcing a manufacturer for the buffer the manufacturer NXP, whom have been developing I2C for some years, was chosen. NXP offer a PCA9600D buffer / redriver among many others. 

Specifications for the buffer were focused on the I2C bus speed and the current sink capability, which was 60mA for the PCA9600D. 

For driving long lengths of cable the buffers higher driver output current and associated lower pull up resistors were considered simultaneously.

Hardware Testing - Minimal Bus
To confirm operation of a short length I2C bus connected to the light sensor the Raspbian NOOBS Lite image was used for testing. All updates and upgrades were applied to this build. The Adafruit guide showing configuration of the I2C on the RPI was used. No other changes were made to the Raspbian build.

A pair of PCA9600D drivers were connected with 470R pullup resistors.

Raspberry PI 2 Model B to I2C Sensor (Buffered)

Using the terminal window the I2C bus was scanned using i2cdetect -y 1, to suite the model B RPI.

The light sensor was found at the correct address, 0x60. This address in detailed on page 17 of the Si1145-46-47 Silicon Labs datasheet.

Putty Capture of I2C Bus of RPI

To establish a baseline for the SCK and SCL signals, the oscilloscope was connected and captures taken.

Buffered SCL (CH2) and SCL (CH1) Lines

The above capture was taken using 470R termination resistors on the I2C lines (SDA, SCL) using very short wires. Even increasing the pullup resistance to 10k only introduced minor rounding to the rising edges of the signals.

Hardware Testing - Telephone Cable
Before the setup was modified with 7.5m (25ft) of flat 4 core telephone cable between the two NXP drivers. Telephone cable capacitance was measured with an average capacitance between each of the unconnected pairs of 400pF.

RPI I2C with 4 Core Flat Telephone Cable

Resistors were only fitted on the driver end directly connected the RPI. Three resistor value were tried, 10k, 470R and 220R.

SCL (CH2) and SDA (CH1) Lines on Telephone Cable 10k Pullups

The above capture shows that the bus, with 10k resistors, was driven with too little current for the high capacitance of the telephone cable.

SCL (CH2) and SDA (CH1) Lines on Telephone Cable 470R Pullups

The above capture shows a significant improvement in the clock and data lines after the 10k resistors were reduced to 470R. Each of the signals operate rail to rail. Visible in the capture was the crosstalk on SDA line (CH1) induced from SCK (CH2).

SCL (CH2) and SDA (CH1) Lines on Telephone Cable 220R Pullups

Driving the open collectors of the NXP driver with even more current, marginally higher than 10mA using 220R resistors, the above capture shows yet again an improved signals at the start of the transmission. Further along the transmission the crosstalk becomes emphasised so much so that requests from the sensor become corrupted.

Corrupted SCL (CH2) and SDA (CH1) Lines on Telephone Cable 220R Pullups

To test for data corruption, sent or received, the SI1145 device register (Part_ID), which contains the model of sensor, was continuously read. The returned value should always be 0x45 for this Silicon Labs device. The frequency of corrupted data was greater than 90%.

SI1145 Part_ID Register

For illustrative purposes the schematic below shows the connection for the telephone cable.

Schematic for Telephone Cable with Pullups

Hardware Testing - CAT5 cable
Again the average capacitance between each of the unconnected pairs was measured for the CAT5 cable. For 7.5m (25ft) of CAT5 cable the capacitance between different pairs was 300pF and between pairs 500pF.

RPI I2C with CAT5 Cable

Again resistors were only fitted on the driver directly connected the RPI, three resistor value were tried, 10k, 470R and 220R. For the test with the CAT5 cable the SDA line was paired with 0V and the SCL was paired with the 5V signal.

SCL (CH2) and SDA (CH1) Lines on CAT5 Cable 10k Pullups

The above capture shows that the bus, with 10k resistors, is similar to the telephone cable.

SCL (CH2) and SDA (CH1) Lines on CAT5 Cable 470R Pullups

As was shown with the telephone cable testing a significant improvement in the clock and data lines was seen after the 10k resistors were reduced to 470R. The resulting image above shows less crosstalk on SDA line (CH1) caused by crosstalk from SCK (CH2).

SCL (CH2) and SDA (CH1) Lines on CAT5 Cable 220R Pullups

Testing with the 220R resistors produced much less crosstalk issues when compared to the telephone cable.

Corrupted SCL (CH2) and SDA (CH1) Lines on CAT5 Cable 220R Pullups

Again the SI1145 devices first register (Part_ID) was continuously read. The frequency of corrupted data was no greater than 10%. Having the SDA and SCL lines separately paired with the power 0V and 5V, significantly reduced the crosstalk.

For illustrative purposes the schematic below shows the connection for the CAT5 cable.

Schematic for CAT5 Cable with Pullups

Summary
For extending the I2C bus of the Raspberry PI the use of unbalanced drivers such as the PCA9600D or a digital isolator ISO1540 should be more than suitable over short lengths of twisted pair cable. 

For transmitting I2C of distances over a few metres then balanced drivers with twisted pair cable would be highly recommended.

DS18B20 OneWire for CY8CKIT-049 PSoC

Summary

This blog utilizes a OneWire component supplied on the Cypress Forums written by Evg Pavlov to work with a Cypress CY8CKIT-049 prototyping board. The original library can be seen on Pavlov's site here. See this Cypress Thread for full details on the OneWire component and project with the Cypress Pioneer prototyping board.

Some minor changes and additions were made to the original Pioneer project. One of the changes allows the PSoC on the 049 to be returned to the bootloader mode using SW1 together with a count down timer.

OneWire Temperature Sensor (DS18B20)
The DS18B20 is a digital thermometer which communicates over a single wire bus to a micro. For this project there are the two additional wires to power the sensor. It shall be noted that OneWire devices can usually be parasitically powered meaning there are also two wire solutions being the communications / power and 0VDC power supply.

For testing of the 049 workspace the Adafruit probe MPN#381 containing the DS18B20 was used.

DS18B20 Temperature Probe

Cypress Forum Project

One of the projects in the forum was targeted at the CY8CKIT-042 prototyping board termed the Pioneer Kit. The PSoC on the Pioneer is a CY8C4245AXI-483. This project contained the all important OneWire example component with other components such as an LCD. The additional components from the original project could have been disabled in the schematics instead a new workspace was created for the Cypress CY8CKIT-049 prototyping board.

CY8CKIT-049 - Courtesy Cypress Semiconductor

CY8CKIT-049 Project

After creating a new workspace in PSoC Creator, the OneWire component was imported from the 042 Pioneer project by selecting the Components tab in PSoC Creator, clicking the Project name and right clicking to show Import Components. The component was then located in the original project and imported.

PSoC OneWire Component

A Timer component was added to the project to service the data collected from the OneWire bus, as implemented in the previous design.

Timer for Data Reporting

For the 049 board it was necessary to add a bootloadable component to the project. The dependency for this bootloadable component was located in the 049 compressed Kit Setup files supplied by Cypress. For completeness the bootloader was added to the new project.

Bootloadable with UART

Lastly the Cypress suggested solution using the on-board switch SW1 to return the bootloadable project to the bootloader was implemented using a countdown timer. This is the relevant PSoC Creator 101 Tutorial. SW1 was also used to start the OneWire process after the 049 board had been powered by the USB, just to ensure no data was missed on power-up.

Bootloadable Reset Option

Project Software

Below is the listing for main.c from the project.

/*==============================================================================
 * Digital Thermometer demo project
 *
 * uses
 * DS18x4: MAXIM DS18B20 Digital Thermometer component (v 0.0)
 * Read temperature simultaneously from up to 8 DS18B20 sensors
 * 
 * 11/05/2017
 * Example project for CY8CKIT-049
 * Built with Creator 4.0
 * Includes timer to monitor SW1 - Resets PSoC after 2 sec, puts into bootloader
 * SW1 is also monitored to initiate the OneWire comms 
 *
 * 17/03/2018
 * Project rebuilt with updated component using Creator 4.1
 *
 * ===========================================================================================
 * PROVIDED AS-IS, NO WARRANTY OF ANY KIND, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 * FREE TO SHARE, USE AND MODIFY UNDER TERMS: CREATIVE COMMONS - SHARE ALIKE
 * ===========================================================================================
*/

#include <project.h>
//#include <stdio.h> //sprintf

/*
#if defined (__GNUC__)
    // Add an explicit reference to the floating point printf library
    // to allow the usage of floating point conversion specifiers.
    // This is not linked in by default with the newlib-nano library.
    asm (".global _printf_float");
#endif
*/

static volatile CYBIT flag_Timer = 0 ;

/* Function Prototypes */
void Initialize(void);
void ReportTemperature (void);          /* convert temperature code to deg C and send to UART */

//==============================================================================
// Timer Interrupt
//==============================================================================
CY_ISR(isr_Timer)                       /* ISR Timer to report temperature at regular intervals */
{
    flag_Timer = 1;
    isr_Timer_ClearPending();
}

//==============================================================================
// Reset PSoC Interrupt
//==============================================================================
CY_ISR (isr_Reset_Butn)                 /* Made it here then call a sw Reset */
{
    Bootloadable_Load();
}

//==============================================================================
// Init PSoC 
//==============================================================================
void Initialize(void)
{
    UART_Start();                       /* UART start */
    UART_UartPutString("Temperature sensor Maxim DS18B20:\r\n");
    OneWire_Start();                    /* OneWire start */
    Timer_1_Start();                    /* Reset button timer start */
    isr_Reset_StartEx(isr_Reset_Butn);  /* Attach handler */
    Timer_2_Start();                    /* Clock report timer start */
    isr_Timer_StartEx(isr_Timer);       /* Attach handler */  
}

//==============================================================================
// Convert temperature code to degC and 
// Send result to Terminal using UART
//==============================================================================
void ReportTemperature(void) 
{
    char strMsg[80]={0};                /* output UART buffer */
    char buf[8];                        /* temp buffer */
    static uint32 counter = 0;          /* sample counter */
    float res;
    counter++; 
   
/*
    //example using GetTemperatureAsFloat() function
    sprintf(strMsg,"%lu\t %.2f\t %.2f\t %.2f\t %.2f\t %.2f\t %.2f\t %.2f\t %.2f\r\n", 
                        counter,
                        OneWire_GetTemperatureAsFloat(0),   
                        OneWire_GetTemperatureAsFloat(1),
                        OneWire_GetTemperatureAsFloat(2),
                        OneWire_GetTemperatureAsFloat(3)    
                        OneWire_GetTemperatureAsFloat(4),   
                        OneWire_GetTemperatureAsFloat(4),
                        OneWire_GetTemperatureAsFloat(6),
                        OneWire_GetTemperatureAsFloat(7)    //25675 ticks
            );
*/
             
    strcat(strMsg, itoa10(counter, buf)); strcat(strMsg, "\t");
    strcat(strMsg, OneWire_GetTemperatureAsString(0)); strcat(strMsg, "\t");
    strcat(strMsg, OneWire_GetTemperatureAsString(1)); strcat(strMsg, "\t");
    strcat(strMsg, OneWire_GetTemperatureAsString(2)); strcat(strMsg, "\t");
    strcat(strMsg, OneWire_GetTemperatureAsString(3)); strcat(strMsg, "\t"); 
    strcat(strMsg, OneWire_GetTemperatureAsString(4)); strcat(strMsg, "\t");
    strcat(strMsg, OneWire_GetTemperatureAsString(5)); strcat(strMsg, "\t");
    strcat(strMsg, OneWire_GetTemperatureAsString(6)); strcat(strMsg, "\t");
    strcat(strMsg, OneWire_GetTemperatureAsString(7)); strcat(strMsg, "\r\n"); //1910 ticks
    UART_UartPutString(strMsg);
    
}

//==============================================================================
// Main
//==============================================================================
int main()
{
 uint8_t i;
 char c;
    
 CyGlobalIntEnable;                  /* Enable global interrupts */ 
 while(SW1_Read());                  /* Wait for button press to continue (gives you time to switch to a terminal window) */
    Initialize();
    
    flag_Timer = 1;                     /* force first measument instantly */
    
    for(;;) 
    {     
        if(flag_Timer)                  /* read DS18B20 on timer, intervals >1sec */
     {   
            flag_Timer = 0;
            OneWire_SendTemperatureRequest();                                       
        }
        
        if (OneWire_DataReady)          /* DS18 completed temperature measurement - begin read data */
     {   
            OneWire_ReadTemperature();
            ReportTemperature();
        }    
        
    }  
}


/* [] END OF FILE */

Project Hardware

Shown below is the demonstration project with the temperature sensor connected. Physical connections are detailed in the PSoC Creator project.

CY8CKIT-049 with DS18B20 Sensor

The output in TeraTerm is shown below.

Term Term Output

PSoC Creator Project

An updated build for PSoC Creator 4.1 using new components is available below for download.

DS18x8_demo_CY8CKIT-049-42XX.Bundle01

For legacy below is the PSoC Creator 4.0 Bundle.

DS18x8_demo_CY8CKIT-049-42XX.Bundle00.zip

Project References

Many thanks (Спасибо) once again to Pavlov for the OneWire component!

Beta Layout (V2) Reflow Controller Double Sided PCB (Second Reflow)

Summary

This blog explores the suitability of the Beta Layout (V2) Reflow Controller for second reflow PCB's (boards). The concept behind second reflow is that the surface tension of solder retains a component during the solder reflow process. This retention of the component is due to the ratio of component weight to the total component pad surface area (aka component land pattern).

Literature
There is no shortage of literature released on the subject of double sided PCB's and the reflow process. Reading on the subject can be found in journals such as Component Candidacy of Second Side Reflow with Lead-Free Solder, in documents Weight Limits For Double Sided Reflow Of QFNS or on websites such as Surface Mount Process.

The engineer that was cited in the document "Weight Limits For Double Sided Reflow Of QFNS" is Phil Zarrow of ITM Consulting, who also authored another document Reflow Soldering of Through-hole Components. This document originally started me on the double sided reflow process and only recently did i consider using this with my home reflow oven.

Weight to Area Rule
Described in the literature by Phil Zarrow is a ratio for weight of a component to the total pad mating area. Originally listed in imperial it states that to hold the component:

Grams per square inch must be ≤ 30 (grams)

Changing to metric measurements:

Grams per 645.16 mm squared must be ≤ 30 (grams)

Reflow Testing with Small Components
Using a sample board to test solder reflow with small components, the amount of solder paste was varied. 

In the image below the resistors circled in RED had an over application of solder paste, resistors circled in yellow had a moderate to normal application of solder paste and the diode circled in blue had a normal application of solder paste.

Sample PCB With Various Solder Paste Amounts Applied

The solder paste on the board was allowed to sit for an hour, then the board was reflowed in the oven with components facing downwards.

Sample PCB After Reflow

After reflow, as can be seen in the image below, all components were located and most reflowed correctly except for the additional solder paste. The normal application of solder paste was sufficient hold both the resistors circled in yellow and small diode circled in blue.

Sample PCB Reflow Closeup

For the resistors used on the sample PCB, 0805, the mass per resistor is listed as 0.45 grams per 100 pcs. The mass of the resistor was taken from the Yageo document. 

Yageo Resistor Mass

For the resistor pads and dimensions the details were taken from another Yageo document.

Yageo Pad Size

In that same document the parameters W, width, and I2, pad depth, were used.

Yageo Pad Dimensions

For the 0805 package there are two pads of 1.25mm x 0.35mm, giving a total of 0.875mm squared. Using this value to determine the maximum mass from the metric ratio "645.16 mm squared ≤ 30 grams" yields 0.0407 grams.

For the Yageo 0805 resistor with a mass of 0.045 grams, using just the mating area of the Yaego resistor pad, the rule recommends that 0.0407 grams should be the limit. This rule appears to have some tolerance which may be due to the larger land pattern on the PCB possibly involving solder surface tension between the physical sides of the resistor pad to the land pattern.

Reflow Testing with Medium Components
Next sample board was prepared with Stannol flux for a small Panasonic inductor, ELL-6RH series.

Sample PCB with Solder Paste on Inductor

The footprint on the sample board was larger than the size recommended by Panasonic however the solder paste was added and the part was placed in position.

Panasonic Recommended Footprint

Immediately the board was placed component side down inside the reflow oven and the soldering cycle started.

Sample PCB with Failed Reflow

The inductor held its position against the sample board until the reflow cycle where it promptly fell off the board.

Summary

Although the inductor tested on the sample board in this blog was not suitable for reflow under the circumstances, this same result is not true for larger devices such as the TO-263-5 part shown below. The thermal pad beneath the body of the device (TO-263-5) is usually large in area which facilitates better adhesion and suitability for second reflow boards.

TO-263-5 Package

With a number of factors influencing double sided PCB reflow for the home hobbyist, each factor should be taken at its own merit and how it is applicable to a specific board, reflow oven and components to be populated.

Beta Layout V2 Controller RS232 to USB Upgrade

Summary

As mentioned in a prior blog involving the Beta Layout V2 Reflow Controller, the on-board RS232 can be removed and USB added using an adaptor such as the Adafruit - CP2104.

Beta Layout V2 with USB

Hardware Required
Beta Layout V2 Controller
Adafruit USB to TTL Adaptor #CP2104

1 x 1k 0805 resistor
2 x M2.5 plastic screws
2 x M3 x 15mm bolts
2 x M3 star washers
2 x M3 flat washers
2 x M3 nuts
1 x plastic L bracket (home made)
3 x small lengths of hookup wire

Opening the Controller

As detailed on a previous blog the Beta Layout case does not use fixing to hold the two part enclosure together. Instead there are a pair of opposing plastic clips on the lid and base, concealed by the ventilation holes, which were released using a flat blade screwdriver to gain access to the controller.

Beta Layout V2 Controller

There are only a handful of components associated with the existing TTL to RS232 driver (MAX232) and these components are all surface mount capacitors for the charge pump or supply decoupling. For more details on the RS232 driver see the Maxim datasheet - MAX232.

In the image below the components to be removed are circled in RED.

Logic Board RS232 Driver

To unsolder the MAX232 driver it was easier to unscrew the logic board so that the board could be worked on directly. While the 10 way header for the serial port can be unplugged, the Thermocouple wires are soldered directly to the logic board so the connector must be removed from the plastic panel.

To remove the Thermocouple connector from the panel of the unit, the bolt connected to the metal plate is first unscrewed, then the bolt holding the Thermocouple connector together is removed. This allows the connector to come apart allowing the logic board to be removed from the unit.

Thermocouple Connector Disassembly

Removing the RS232 Driver

One of the easiest methods to remove the MAX232 driver is to use two soldering irons, unless you have a tool for your soldering iron that can remove SOIC packages! Alternatively a set of side cutters to chop the legs off the driver and solder wick to clean up the mess works wonders if there is no interest in saving the driver.

Logic Board RS232 Driver Removed

In the image above the driver and four charge pump capacitors were removed. The capacitor to the lower left of the MAX232, for the power supply decoupling, was left on the PCB. For people choosing to use another flavour of USB to TTL adaptor they may find that external 5V power is required. This capacitor, even though small, may be able to provide some minimal power supply decoupling for an alternative adaptor board.

The Adafruit adaptor board which was used in this upgrade, is bus powered, so this decoupling capacitor is not used and could be removed. Below is an image of the changes to the logic board. These are explained below.

Logic Board with Adafruit USB Adaptor

Logic Board Changes

For the image above the changes listed below were made.

1. Bridge the transmit lines on the MAX232 footprint.
   
   As shown in the image below there are two pins which need to be linked 'shorted' together. These pins are actually pin numbers 7 and 10 on the actual MAX232 transceiver.
   
   

   

   Bridge MAX232 Transmit Lines

   
   
2. Connect a 1k resistor between the receiver lines on the MAX232 footprint.
   
   Since the receiver line from the USB to TTL converter can provide enough power to supply the ATMEGA a current limiting resistor was added between the boards. The rail to rail steering diodes inside the ATMEGA cause this phenomenon. See this EEVBlog #831 Episode for a detailed description on YouTube.
   
   

   

   Add 1K Resistor Receive Line

   The resistor is fitted between pin 12 of the MAX232 and the via directly adjacent to the right between the transceiver pads. As the board via's are untented the resistor can be soldered directly to the via.
   
   
3. Connect TTL transmit, receive and 0VDC to the Adafruit adaptor.
   
   The original transmit and receive lines on the PCB header were reused for connection to the USB to TTL converter. As shown in the image below the three connections are all made to the inside pins on the header.
   
   

   

   USB to TTL Connections

   The transmit connection is the TXD pin, receive the RXD pin and 0VDC is the GND pin on the Adafruit USB to TTL adaptor.

Mounting the Adafruit Board

To mount the Adafruit board a custom L bracket was made from plastic and fashioned to cover the old 9 pin serial connector cut out.

Adafruit USB to TTL L-Adaptor

Since there are only two mounting holes (ID 2.5mm) on the Adafruit board a pair of 2.5mm self-tappers were used to hold the board in position.

Mounted Adafruit Board

To hold the L-bracket in position against the panel of the Beta Layout unit, a pair of 3 x 15mm bolts were used with the usual star washers and nuts on the rear and M3 flats against the outside of the units panel.

Mounted Adafruit in Case

As a dry run the unit was assembled without fixing the power board down to check the clearances.

Final Mounting of USB to TTL Adaptor

Testing the Adafruit Board
To be prudent the USB was connected to a PC running TeraTerm. The unit was powered with the Thermocouple fitted into the connector.

USB to TTL Converter in Device Manager

Running on Windows 7 the drivers installed automatically for the Silicon Labs USB controller. Below is a capture of the USB connection running at 9600 baud.

TeraTerm - USB to Beta Layout V2

The USB to TTL adaptor was working as expected and so the Beta Layout unit was reassembled by fixing down the power board.

Power Board Fixings Under Connectors

Final Mounting 
Note that one of the plastic screws to hold down the power board was located underneath the mounted Thermocouple connector. The second screw was located on the opposite end of the power board underneath the Adafruit PCB. 

Should repairs need to be performed on the power board the Thermocouple connector and USB board would need to be removed to gain access to the unit.