How are flocks connecting there RPI with external world(Other embedded circuits)? any standards.. if not which 1 you will pick?

Thank you all for your views. I knew this post would be little bit controversial, and eventually it went little bit ofcouse but the fun part is its still knowledgeable.

Being and embedded person and my exploration on RPI has made me to conclude that all the lower levels interface like more led's, switches other sensors should not be connected to RPI directly. To better understand this I would like to bring an example.

Suppose I want to make a robot using RPI, I can make a motor driver breakout and connect it directly to RPI and ofcourse code on RPI is no big deal, But RPI is much more computational power to just drive a motor and thus i want to put all this motor control and small sensor stuff to a companion board attached to my RPI. Thus RPI can focus on algorithmic claculation part (Putting ROS or Open CV).

Now with 8 bit microcontroller i would not have much option with USB and Ethernet, I now there are alternative avaiable but i feel its overkill for 8 bit. RS232 comes handy in this case. I can convert  RS232 to USB CDC using FT232RL or CP2102 or some other option.

I would futher like to create an open interface for RPI to communicate with expansion board.

Hence a good interface

I actually don't have a problem with using either interface. As I said at various times in this thread I have used both with no problems other than the

normal new interface learning curve.

The use of the FT232RL is a good solution for an easy interface to just about any microcontroller if it meets your bandwidth needs. And it is supported

by the Linux kernel.  The project I used it that I spoke of earlier was connected to the serial port of an 89C51RD2 which could supply eight 10 bit

temperature readings (sent as 2 bytes with 10 bytes of sensor number/sensor status/protocol info), and eight 16 bit RPM measurements (again sent

with 10 bytes of RPM  sense input number/input status/protocol info) using two commands with both sent once a second over a 9.6K USB "serial"

interface with no dropouts. I later increased it to 19.2K because I had planned to increase the number of temperature sensors to 16, and the number

of RPM sense inputs to 10 and it reduced the overhead used by the Linux serial interface. That was the normal operating mode for the design. Other

commands could be sent between the temperature/RPM update commands for getting/setting configurations and reading status when needed. I had

planned to release the command/communications protocol as a completely open specification if it sounds like something you might be able to use.

There are quite a few 8 bit microcontrollers with USB interfaces now available from Microchip and others if you want to go that route. They are usually

limited to the USB 1.1 12 Mb/s or less but given what you said above I don't see that as a problem.

The ability to supply the expansion board with 5 volts from the USB interface is also very useful. There are a couple of things to take into consideration

when doing this with the Rapsberry Pi. The USB spec. allows the host interface (Raspberry Pi or PC) to supply up to 500 mA on the 5 V line. Most

desk PCs can supply that  but laptops and other portables may not. The Rapsberry Pi has a 1A polyfuse on its 5V power line. I have measured the

current being used on a R Pi under a medium to heavy loads at between 630 and 700 mA which is consistent with what the RPF says it should be.

This of course only allows 150 mA per USB port (300 mA if only one port is used) so I have removed and replaced the 5V fuse with a jumper, or just

bypassed the fuse with a jumper on all three of the R Pi's I have. Of course the best course of action would be to replace it  with a 2A polyfuse. A

5V 2A wall supply should allow for a full 500 mA per port. Another thing to be aware of is that power/ground pins on the USB connector used to

connect the R Pi to the power supply are only rated for 1.8A. Of course (500mA x 2) + 700mA is 1.7A so it should work just fine with a little surge

current for short periods of time available when needed.

I actually don't have a problem with using either interface. As I said at various times in this thread I have used both with no problems other than the

normal new interface learning curve.

The use of the FT232RL is a good solution for an easy interface to just about any microcontroller if it meets your bandwidth needs. And it is supported

by the Linux kernel.  The project I used it that I spoke of earlier was connected to the serial port of an 89C51RD2 which could supply eight 10 bit

temperature readings (sent as 2 bytes with 10 bytes of sensor number/sensor status/protocol info), and eight 16 bit RPM measurements (again sent

with 10 bytes of RPM  sense input number/input status/protocol info) using two commands with both sent once a second over a 9.6K USB "serial"

interface with no dropouts. I later increased it to 19.2K because I had planned to increase the number of temperature sensors to 16, and the number

of RPM sense inputs to 10 and it reduced the overhead used by the Linux serial interface. That was the normal operating mode for the design. Other

commands could be sent between the temperature/RPM update commands for getting/setting configurations and reading status when needed. I had

planned to release the command/communications protocol as a completely open specification if it sounds like something you might be able to use.

There are quite a few 8 bit microcontrollers with USB interfaces now available from Microchip and others if you want to go that route. They are usually

limited to the USB 1.1 12 Mb/s or less but given what you said above I don't see that as a problem.

The ability to supply the expansion board with 5 volts from the USB interface is also very useful. There are a couple of things to take into consideration

when doing this with the Rapsberry Pi. The USB spec. allows the host interface (Raspberry Pi or PC) to supply up to 500 mA on the 5 V line. Most

desk PCs can supply that  but laptops and other portables may not. The Rapsberry Pi has a 1A polyfuse on its 5V power line. I have measured the

current being used on a R Pi under a medium to heavy loads at between 630 and 700 mA which is consistent with what the RPF says it should be.

This of course only allows 150 mA per USB port (300 mA if only one port is used) so I have removed and replaced the 5V fuse with a jumper, or just

bypassed the fuse with a jumper on all three of the R Pi's I have. Of course the best course of action would be to replace it  with a 2A polyfuse. A

5V 2A wall supply should allow for a full 500 mA per port. Another thing to be aware of is that power/ground pins on the USB connector used to

connect the R Pi to the power supply are only rated for 1.8A. Of course (500mA x 2) + 700mA is 1.7A so it should work just fine with a little surge

current for short periods of time available when needed.

Gary Stewart wrote:

 

... The Rapsberry Pi has a 1A polyfuse on its 5V power line. I have measured the current being used on a R Pi under a medium to heavy loads at between 630 and 700 mA which is consistent with what the RPF says it should be.   This of course only allows 150 mA per USB port (300 mA if only one port is used) so I have removed and replaced the 5V fuse with a jumper, or just  bypassed the fuse with a jumper on all three of the R Pi's I have. Of course the best course of action would be to replace it  with a 2A polyfuse. A  5V 2A wall supply should allow for a full 500 mA per port. Another thing to be aware of is that power/ground pins on the USB connector used to  connect the R Pi to the power supply are only rated for 1.8A. Of course (500mA x 2) + 700mA is 1.7A so it should work just fine with a little surge  current for short periods of time available when needed.

Personally, I'd be leery of supplying more than 1A through the RasPi Micro USB power jack.  Not only is the USB connector limited as you pointed out, but we don't know the current capacity of the 5V traces on the RasPi PCB.  When RPF releases PCB artwork we'll have a better idea.  Also, most Micro USB cables can just barely supply RasPi's 700 mA.  Above that the voltage drop across the Micro USB cable could easily be too much.

If I had an add-on board that needed more than 300 mA at 5V I'd consider putting a 5.5mm barrel connector on the add-on board and letting the add-on board power RasPi through the GPIO connector.  The FTDI RasPi expansion module does this: http://www.ftdichip.com/Products/Modules/RPi.htm