Wired has a great article by Pete Lomas, a co-founder and trustee of the Raspberry Pi Foundation, where he describes the design process for the Raspberry Pi.
http://www.wired.com/opinion/2012/09/raspberry-pi-insider-exclusive-sellout-to-sell-out/?pid=45&viewall=true
Tuesday, September 25, 2012
Saturday, September 22, 2012
So Many Inputs, So Few GPIO Pins
I keep coming up with so many improvements to my interface design (I'm up to version 6 now) that I'm not getting much built. I decided that I had to make the pins for the SPI and I2C buses available. But that means using five GPIO pins that I had other plans for. I have to have at least eight inputs to handle the motion detectors and door sensors for the alarm system. At least four relay outputs would be nice too, so the Raspberry Pi is quickly running out of GPIO pins.
Time to get a little help from another useful IC - the multiplexer. There are several possible choices, but the obvious one is the 74151, of which I conveniently happen to have a few.
This diagram shows how this needs to be connected. GPIO pins 1, 2, and 3 are set to output mode and used as address lines to select which input to read. GPIO pin 0 is set to input mode and connected to the output of the multiplexer.
(Note: I always refer to WiringPi pin numbers, not the standard BCM numbering.)
The result is that the interface can still have 8 inputs, but only use 4 GPIO pins to do it.
I will probably do the same thing (in reverse) to allow two GPIO pins to provide four select lines for the SPI bus. It may be a while until I do that since the SPI bus is for future expansion. I don't have any devices for it yet, just ideas.
This diagram shows how this needs to be connected. GPIO pins 1, 2, and 3 are set to output mode and used as address lines to select which input to read. GPIO pin 0 is set to input mode and connected to the output of the multiplexer.
(Note: I always refer to WiringPi pin numbers, not the standard BCM numbering.)
The result is that the interface can still have 8 inputs, but only use 4 GPIO pins to do it.
I will probably do the same thing (in reverse) to allow two GPIO pins to provide four select lines for the SPI bus. It may be a while until I do that since the SPI bus is for future expansion. I don't have any devices for it yet, just ideas.
Thursday, September 20, 2012
GPIO Output Relay Interface
Note: See my revised relay circuit here.
To allow the greatest flexibility, my outputs are all relays. This allows me to switch a variety of voltages and provides protection to the GPIO pins of the Raspberry Pi. A relay is just a switch that is controlled by an electromagnetic coil. Powering the coil will make the switch turn on. The relays I used are made to mount on a circuit board and can be driven by 5V. They can easily switch 12V or more at a moderate current level. They are NOT meant to control house current! That can be the topic of another post.
The Pi GPIO pins will only output 3.3V at a few milliamps. This is not enough to drive the relay directly, but is is enough to switch a transistor on and off. A common NPN switching transistor handles that job nicely. I used the 2N2222 which is highly available (i.e. even Radio Shack carries it.)
Note: NO SPST is a switch type designation and means Normally Open, Single Pole Single Throw. This is the simplest type of switch.
A 1K ohm current limiting resistor is attached to the base of the transistor. Power is connected to the relay coil which is then connected to the collector of the transistor. The emitter is connected to ground. When the GPIO pin is low, no current will flow from the collector to the emitter and the relay will be off. Setting the GPIO pin to high will "turn on" the transistor, power will flow and the relay will turn on.
Notice that there is a diode attached across the coil of the relay in a reversed orientation. There is magnetic energy stored in the coil while it is energized and holding the relay closed. When power is removed, the collapsing magnetic field in the coil causes a brief but powerful surge of reverse voltage which can damage the switching transistor and cause premature failure. The diode is there to stop this reverse voltage and protect the transistor. I prefer to use relays that have internal surge suppression diodes and they are becoming more common now. If your relay does not have the diode internally, then it is highly recommended that one be added.
To allow the greatest flexibility, my outputs are all relays. This allows me to switch a variety of voltages and provides protection to the GPIO pins of the Raspberry Pi. A relay is just a switch that is controlled by an electromagnetic coil. Powering the coil will make the switch turn on. The relays I used are made to mount on a circuit board and can be driven by 5V. They can easily switch 12V or more at a moderate current level. They are NOT meant to control house current! That can be the topic of another post.
The Pi GPIO pins will only output 3.3V at a few milliamps. This is not enough to drive the relay directly, but is is enough to switch a transistor on and off. A common NPN switching transistor handles that job nicely. I used the 2N2222 which is highly available (i.e. even Radio Shack carries it.)
Note: NO SPST is a switch type designation and means Normally Open, Single Pole Single Throw. This is the simplest type of switch.
A 1K ohm current limiting resistor is attached to the base of the transistor. Power is connected to the relay coil which is then connected to the collector of the transistor. The emitter is connected to ground. When the GPIO pin is low, no current will flow from the collector to the emitter and the relay will be off. Setting the GPIO pin to high will "turn on" the transistor, power will flow and the relay will turn on.
Notice that there is a diode attached across the coil of the relay in a reversed orientation. There is magnetic energy stored in the coil while it is energized and holding the relay closed. When power is removed, the collapsing magnetic field in the coil causes a brief but powerful surge of reverse voltage which can damage the switching transistor and cause premature failure. The diode is there to stop this reverse voltage and protect the transistor. I prefer to use relays that have internal surge suppression diodes and they are becoming more common now. If your relay does not have the diode internally, then it is highly recommended that one be added.
Wednesday, September 19, 2012
GPIO Input Circuit
After holidays and many other distractions, I am finally able to get back to work on my Raspberry Pi interface.
Here is the GPIO input circuit that I came up with using an opto-coupler for protection. The opto-coupler that I chose is the LTV-847 (Jameco part number 878286) which provides 4 opto-couplers in a 16-pin DIP format.
Power applied to the anode and cathode will cause the internal LED to emit light. This is detected by the internal photocell which controls the output. Because there is no electrical connection between the input and output sides, opto-couplers are handy for connecting between very different voltage levels. They are also excellent at preventing the introduction of electrical noise into a system. For this application, the isolation will provide electrical protection to the Raspberry Pi.
The complete circuit for using this is shown below. The 1KΩ resistor on the input is for limiting the current that can flow through the LED. The 10KΩ pull-up resistor is internal to the Raspberry Pi. Be sure to set the pull-up option when you set the pin to input mode. Using a separate 5V power supply for the interface provides greater protection than powering this all from the 5V line on the GPIO header. If that line gets shorted to ground, or even if it just draws too much current, it can cause the Pi to suddenly reboot.
When the input is open, no current will flow through the detector and the Raspberry Pi will see the pin hi due to the pull-up resistor. When the the input is connected to ground, current will flow and the Pi will see the the pin as low, since it is effectively connected to ground now.
Here is the GPIO input circuit that I came up with using an opto-coupler for protection. The opto-coupler that I chose is the LTV-847 (Jameco part number 878286) which provides 4 opto-couplers in a 16-pin DIP format.
Power applied to the anode and cathode will cause the internal LED to emit light. This is detected by the internal photocell which controls the output. Because there is no electrical connection between the input and output sides, opto-couplers are handy for connecting between very different voltage levels. They are also excellent at preventing the introduction of electrical noise into a system. For this application, the isolation will provide electrical protection to the Raspberry Pi.
The complete circuit for using this is shown below. The 1KΩ resistor on the input is for limiting the current that can flow through the LED. The 10KΩ pull-up resistor is internal to the Raspberry Pi. Be sure to set the pull-up option when you set the pin to input mode. Using a separate 5V power supply for the interface provides greater protection than powering this all from the 5V line on the GPIO header. If that line gets shorted to ground, or even if it just draws too much current, it can cause the Pi to suddenly reboot.
When the input is open, no current will flow through the detector and the Raspberry Pi will see the pin hi due to the pull-up resistor. When the the input is connected to ground, current will flow and the Pi will see the the pin as low, since it is effectively connected to ground now.
Tuesday, August 28, 2012
Always Sage Advice - Use Protection
If you read any about interfacing to the Pi GPIO pins you are bound to come across the dire warnings - Be Careful. These pins connect directly to the microprocessor without any protection. You can destroy the Universe if you wire something wrong.
OK. Maybe not that dire, but you get the idea. You may get away with interfacing directly with the GPIO pins (I know I have) but even if your circuit is designed perfectly, accidents still happen. Something can fall across the circuit board and short things out. So it is wise to protect your Pi.
I am building an interface that will connect to hard-wired alarm circuitry. That means lots of lines running lots of places and just that many more opportunities for bad things to happen. This protection is also a good idea since a lot of people using the Pi will likely be from the younger and less experienced crowd. This is what the Pi was developed to encourage, so having a safe way to interface to the most flexible I/O on the device is critical.
Here are some of the options for protecting a logic circuit.
Zener Diode - A Zener diode is one that allows no reverse current to flow until a threshold is reached. Above that threshold, current will flow. A 3.3V Zener diode between a GPIO pin and ground can protect it from any over voltage that is applied. Any voltage above 3.3V will just be shunted to ground.
Mike Cooke has provided a design for a screw terminal break-out board for the Raspberry Pi GPIO that uses Zener diodes for protection. The design may be found here: Raspberry Pi Breakout Box.
Transistor - A transistor can be used to switch a known safe logic level.
Line Driver or Buffer - Chips that contain multiple transistor switches internally. These are easier to install and provide a cleaner design when you have many lines to protect.
Opto-Isolators - Chips similar to Buffers but these use pairs of internal LEDs and optical sensors instead of transistors. This provides total circuit isolation and can be useful in a noisy electrical environment.
A Bi-directional Logic Level Converter seemed like the perfect thing to use. However, I tried the TXB0108 provided by Adafruit and had problems. It seemed to work fine in my prototyping board, but when I put things together, I found that it had problems driving some TTL chips. I checked it with a multimeter and found it only put out 2.5 volts when asserted on the 5V side. This wasn't enough for an input into the MAX232N that is used for the RS-232 interface.
I am interested in hearing from anyone else who has tried this chip.
The GPIO pins that are used for output on my interface are protected by transistors which control relays. That is more than adequate protection. I am still pondering what route to take now for protecting the inputs. Opto-isolators are something I am already familiar with (and probably already have some) so I am leaning toward that option.
Eventually, I will build a new version of my interface so that I can apply all that I have learned along the way. I already regret not leaving pins open for I2C. That will have go in the next revision.
OK. Maybe not that dire, but you get the idea. You may get away with interfacing directly with the GPIO pins (I know I have) but even if your circuit is designed perfectly, accidents still happen. Something can fall across the circuit board and short things out. So it is wise to protect your Pi.
I am building an interface that will connect to hard-wired alarm circuitry. That means lots of lines running lots of places and just that many more opportunities for bad things to happen. This protection is also a good idea since a lot of people using the Pi will likely be from the younger and less experienced crowd. This is what the Pi was developed to encourage, so having a safe way to interface to the most flexible I/O on the device is critical.
Here are some of the options for protecting a logic circuit.
Zener Diode - A Zener diode is one that allows no reverse current to flow until a threshold is reached. Above that threshold, current will flow. A 3.3V Zener diode between a GPIO pin and ground can protect it from any over voltage that is applied. Any voltage above 3.3V will just be shunted to ground.
Mike Cooke has provided a design for a screw terminal break-out board for the Raspberry Pi GPIO that uses Zener diodes for protection. The design may be found here: Raspberry Pi Breakout Box.
Transistor - A transistor can be used to switch a known safe logic level.
Line Driver or Buffer - Chips that contain multiple transistor switches internally. These are easier to install and provide a cleaner design when you have many lines to protect.
Opto-Isolators - Chips similar to Buffers but these use pairs of internal LEDs and optical sensors instead of transistors. This provides total circuit isolation and can be useful in a noisy electrical environment.
A Bi-directional Logic Level Converter seemed like the perfect thing to use. However, I tried the TXB0108 provided by Adafruit and had problems. It seemed to work fine in my prototyping board, but when I put things together, I found that it had problems driving some TTL chips. I checked it with a multimeter and found it only put out 2.5 volts when asserted on the 5V side. This wasn't enough for an input into the MAX232N that is used for the RS-232 interface.
I am interested in hearing from anyone else who has tried this chip.
The GPIO pins that are used for output on my interface are protected by transistors which control relays. That is more than adequate protection. I am still pondering what route to take now for protecting the inputs. Opto-isolators are something I am already familiar with (and probably already have some) so I am leaning toward that option.
Eventually, I will build a new version of my interface so that I can apply all that I have learned along the way. I already regret not leaving pins open for I2C. That will have go in the next revision.
Monday, August 27, 2012
Raspberry Pi Serial Port
 |
| Prototyping the serial port interface. |
I need to use this serial port to interface to my X10 system via a CM11A computer interface module. That can be a topic for several future posts.
First lets cover some important facts about RS-232 and voltage levels. The Pi uses levels that are 0V to represent a zero and 3.3V to represent a one. RS-232 uses -3V to -15V to represent a zero and 3V to 15V to represent a one. Thus, a level converter is required to create this interface. The MAX232 series of chips was designed for this exact purpose.
External capacitors are needed to drive the charge pumps inside the chip. Note: There are several variations of the MAX232 chip which have different requirements. The one shown in the circuit here uses 0.1uF capacitors. The ones I have use 1.0uF and some versions even have the capacitors built in. When in doubt, check the datasheet for the chart that shows the requirements for each variation.
Data Sheet for MAX232 family
If, like me, you want to take complete control of the serial port for your own uses, there are two configuration changes to make:
First, disable the boot up and diagnostic output to the serial port.
sudo vi /boot/cmdline.txt
and remove the two options referring to the serial port.
So, this
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
becomes this
dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait
Second, disable the login prompt
sudo vi /etc/inittab
find the line near the end
T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
and delete it or comment it out by putting a # at the start of the line.
Reboot and the serial port will now be free for your exclusive use.
Saturday, August 25, 2012
My Pi Has Arrived
It's Chrismas in July! My Raspberry Pi has finally arrived. Setup was pretty easy:
- Standard cell phone charger (micro USB) at least 700mA
- either an HDMI or composite video connection
- USB keyboard and mouse
- Cat5 cable run to my network switch
- Compact Flash card, 4GB, loaded with Raspbian “wheezy” OS
Ran the configuration program
sudo Raspi-config
set timezone and enable SSH
Set the IP address to static (default is automatic)
sudo vi /etc/network/interfaces
and change the eth0 section to
iface eth0 inet static
address 192.168.0.6
netmask 255.255.255.0
gateway 192.168.0.1
Then restart eth0 to take the change
sudo ifdown eth0
sudo ifup eth0
Create a user for myself and set the password
adduser ted
passwd ted
Change the default "pi" user's password while I'm at it.
passwd
Update the package manager and install Samba to create windows shares.
apt-get update
apt-get install samba
apt-get install samba-common
Configured and created the /pihome network share (google "Samba Howto")
vi /etc/sambe/smb.conf
I am using my preferred development environment on my Windows system and access the files via the network share. The command line via SSH is used to build it.
I installed the WiringPi library and got the GPIO part working.
My next post will describe the hardware interface I an building - serial port, 8 inputs, 6 relay outputs.
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