Note: I only write about what I know. Although there are many Arduinos and Arduino type products, I only have experience with the Arduino Uno Rev3. Likewise, the only Atmel microcontroller I know is the ATmega328P.
A Little Background (well not so little)
I have Atmel ATmega328P microcontrollers on two devices: An Arduino Uno (Rev 3), and a Gertboard. The Gertboard connects to, and is programmed by, my Raspberry Pi. In both cases the microcontroller is in a socket, so is removable. I have developed applications for both where I needed to remove the ATmega328P and install it in another device. I use the Arduino IDE system to develop sketches on both the Arduino and the Gertboard/Raspberry Pi. There is more on the Gertboard at the end of this post.
I developed a camera remote control using the Gertboard and removed the programmed ATmega chip and installed it on a circuit board. The circuit board, a 434MHz transmitter, and a motion detector were mounted in a box. I reported on that project in this blog entry.
I am completing a project that uses both the Arduino and the Gertboard/Raspberry Pi. In this case, it is the sketch developed on the Arduino that must run on an ATmega328P that is removed from the Arduino. This project, that I will report on later, requires the ability to communicate with the ATmega328P once it is installed in my mobile device.
Both projects required me to program a new ATmega328P IC. However, you can not just insert a new chip on the Arduino or Gertboard and upload sketches to it. Out of the box, the microcontroller will not use the external 16MHz crystal of the Arduino or the 12MHz resonator of the Gertboard. It will use its internal 8MHz RC oscillator frequency reference and then divide that frequency by 8. The resulting clock frequency will be 1MHz.
In order to change the clock source, you must change the logic level of a couple of "fuse" bits. I use "" around the word fuse, because the term is actually a carryover from the time when ICs, like microcontrollers, had actual fuses. Each fuse would control the logic level of one bit. If the fuse was blown, the bit would be a 0, if not blown it would be a 1. You could always change a 1 to a 0, by blowing the fuse, but you had no way to change a 0 to a 1. If you wanted a 1, but programmed a 0, in error, you had to replace the chip. That is not the case today.
Now, "fuse" bits can be changed from 1 to 0 and 0 to 1 anytime. The ATmega328P has three, eight bit bytes of these "fuse" bits. They control different functions and leave the factory in a default configuration. In addition, there is a byte (8 bits) of lock bits. More on these later. To setup a new ATmega328P for the Gertboard, you run a program on the Raspberry Pi called avrsetup. All this program does is to use avrdude to set the lock bits and the fuse bits. It leaves all bits not concerned with the clock frequency to their default settings. The four bits that select the clock source are changed from "Calibrated RC Internal Oscillator" to "Full Swing Crystal Oscillator". And, the divide by 8 function is turned off. Avrdude is like an operating system for the Atmel microcontrollers. All programming of the microcontrollers are done through avrdude even if you use the Arduino IDE.
For the Gertboard/Raspberry Pi, setting those fuse bits is all you have to do to use a new ATmetga328P. That is not the case with the Arduino. A new ATmega328P for the Arduino needs those same fuse bit changes and needs one more thing: a bootloader installed. What is a bootloader, and why doesn't the Gertboard need one?
When you upload a sketch to the flash memory of the ATmega328P, there are two direct ways. One is to use a parallel configuration, which I will not mention further (just going to forget it exists). The other is to use the chip's SPI (Serial Peripheral Interface) bus. This synchronous, serial, bus consists of four lines: SCK (Serial Clock), SS (Slave Select), MOSI (Master Out Slave In), and MISO (Master In Slave Out). I upload sketches to the Gertboard via the SPI bus. The Raspberry Pi is the Master, the ATmega328P on the Gertboard is the Slave. Since the direct programming method is employed, no bootloader is necessary.
Atmel, in their wisdom, realized that not everyone would like to use the SPI bus, but would wish to use the more universal serial bus (or something else). The serial bus interface has several names such as RS232 and UART. It consists of several signals: TX (transmitted data) and RX (received data). The TX of the one device is connected to the RX of the other device, and RX of the first device is connected to TX of the second device. There are more signals, but, typically, they are not used. However, a signal, DTR (data terminal ready), for example, may be used to reset the ATmega328P. Once the ATmega328P is reset, it will start running any sketch loaded in flash from its beginning.
Atmel provided an area of flash memory for the user to install code allowing the rest of the flash memory to be uploaded via the serial bus. This code is the bootloader. For the ATmega328P, it can take up as much as 2 Kbytes of the 32 Kbytes of flash memory. The flash memory, not reserved for the bootloader, is the Application memory. Once the bootloader is loaded, it can load a new sketch into the Application memory via the RX and TX connections of the serial bus.
So, how do you get the bootloader loaded in a new ATmega328P? It requires hardware and software. Since a new ATmega328P has no bootloader, the bootloader must be installed from the SPI bus - the direct, primary, interface to the chip. That means you need an in-circuit programmer. There are inexpensive, commercial, units available like the USBtinyISP. They have USB on one end and the four SPI connections on the other end (SCK, SS, MOSI, and MISO), plus power and ground. They have a 6 pin ribbon cable and connector that plugs into your target device that contains the microcontroller chip you want to access. There is a standard 6 pin SPI configuration that receives the ribbon cable connector. My Arduino Uno has two of these 6 pin connectors, one for the ATmega16U2 and the other for the ATmega328P. You can install a new ATmega328P on your Arduino Uno and connect a programmer to it to upload the bootloader. But, I don't do that.
I have no experience with the aforementioned programmers so I will not discuss them. There is another programmer that works really well - and I have experience with that one. This programmer is the Arduino itself. You can't use it to load a bootloader on its own ATmega328P, but it can be used to install a bootloader on another Arduino's ATmega328P. However, I do a variation of that. I use the Arduino Uno to install the bootloader on an ATmega328P that is on a jig I made. This is the jig that is the subject of this blog post.
Now we need some software. There are two pieces of this. One piece is the bootloader itself. The Arduino IDE package has copies of the bootloaders. There are different bootloaders for the different ATmega devices and the different Arduinos. The other piece of software is the software to load the bootloader. This software is a sketch running on my Arduino Uno that is used as a programmer. This software must do several things. I have not studied this software to know how it works, but from my knowledge of the ATmega328P, I know it must do the following:
- Program the fuse bits to select the clock source to "Full Swing Crystal Oscillator" and turn off the clock divide by 8.
- Set the Lock bits to allow programming of the bootloader in the Boot Flash Section
- Set the fuse bits to the amount of flash memory devoted to the Boot Flash Section (up to 2 KBytes)
- Upload the bootloader program to the Boot Flash Section
- Set the lock bits so the Boot Flash Section can no longer be written to, protecting it from being overwritten.
There is one last point to consider before moving on to the details of my programming jig. Could there be an Arduino that programs the microcontroller directly over the SPI bus? Certainly. There are ICs that convert USB to SPI. That would eliminate the bootloader. We are forgetting one thing though: How do you communicate with a sketch? For example, a sketch that requires you to answer some questions before proceeding. The answer is the Serial Monitor. Access to the Serial Monitor is via the same interface the Arduino uses to upload sketchs - the same TX and RX connections to the ATmega328P.
For the Gertboard, the Serial Monitor button exists on the Arduino IDE running on the Raspberry Pi. However, it does not work. A separate connection to the ATmega328P's UART must be made. Consequently, the Raspberry Pi/Gertboard requires six signal connections: SCK, MOSI, MISO, SS, and TX and RX. The Arduino UNO only has two ATmega328P signal connections: TX and RX. For the Gertboard, I have to open a separate terminal window and run a terminal program to communicate with a running sketch. All-in-all, the loss of a little flash memory to the bootloader seems worthwhile.
An ATmega328P Programming Jig
The following is the schematics and board layout diagram for my jig:
Schematics For ATmega328P Programmer
Here is the layout of the board with components and wiring:
Board Layout For The ATmega328P Programmer
Looking at the board layout, the ATmega328P sits in a 28 pin ZIF socket (all that green in the diagram). This allows an IC to be quickly inserted and removed. Check out the product page from Sparkfun.
The connector pins labeled SPI are for female - female jumper wires. This interface connects to the Raspberry Pi or to the Arduino Uno. Connections to the Raspberry Pi are to set the fusebits on an ATmega328P, and to load sketches developed on the Gertboard. The connections to the Arduino UNO are only to install a bootloader on an ATmega328P.
Note the interface labeled "FTDI". This interface is used to upload sketches to an ATmega328P (must have the bootloader installed) that I have developed on my Arduino. In addition, it provides access to the Arduino IDE Serial Monitor, allowing us to communicate with a program running on the jig's ATmega328P. FTDI is the name of a chip manufacturer that makes USB to Serial ICs. The FTDI interface on my jig connects to a Sparkfun FTDI Basic breakout board. This small board has a mini USB on one end and a six pin female connector on the other that plugs directly onto my jig.
There is a three pin female connector labeled resonator. Since the Gertboard and the Arduino Uno operate on different frequencies, the connector allows a 12MHz resonator to be connected for the Gertboard sketches and a 16MHz resonator for the Arduino operations.
There are two LEDs on the jig. The green one lights whenever power is applied to the jig. 5V can come from the FTDI board or the Arduino Uno. The Gertboard will supply 3.3V. The red LED is a sanity check and is connected to PB1 of the jig (this corresponds to pin 9 of the Arduino). After loading a bootloader, I can check if I can upload a sketch to a new ATmega528P by running a version of Blink. Anytime I need to check the jig or my connections, I can program that red LED.
Parts List
- Prototype Breadboard PCB, half size: Adafruit 1609, $4.50
- ZIP Socket, 28 pin: Sparkfun PRT-09175, $2.95
- Right Angle Header Strip for FTDI and SPI: Mouser 571-9-103323-0, $2.24 for a strip of 40 pins
- ATmega328P Microcontroller: Mouser 556-ATmega329K-PU, $3.55
- Socket strip for Resonators: Digikey 1212-1125-ND, $3.89 for a strip of 64. Perfect socket for resonator pins
- 16 MHz Resonator: Mouser 520-ZTT1600MX, $0.50
- 12 MHz Resonator: Mouser 520-ZTT1200MX, $0.50
- Red LED - clear housing: Digikey C503B-RCN-CW0Z0AA1-ND, $0.15
- Red Green - clear housing: Digikey C503B-GCS-CY0C0791-ND, $0.24
- 10μF, 16V Tantalum capacitor: Mouser 581-TAP106K016CRW, $0.53
- 0.1μF 100V Ceramic capacitor: Mouser 1C20X7R104K100B, $0.18 ea.
- 1N4148 diode: Mouser 512-1N4148, $0.10
- 10KΩ Metal Film Resistor: Mouser 71-RN55D-F-10K, $0.10
- 1KΩ Metal Film Resistor: Mouser 71-RN55D1001F/R, $0.10 ea.
I use LEDs with clear bodies. I anticipate using LEDs in battery powered projects so I want to minimize the current consumed by LEDs. Normally, you calculate the series resistor to deliver about 10ma. of current to drive the LED on. A resistor of about 330Ω is commonly used. The LEDs with the clear housings require much less current so I typically use a 1KΩ series resistor (about 2ma.). Even that can seem too bright, I might try 2KΩ on my next project.
Using the Jig
Diagram used for all situations follows:
Connecting The Programming Jig
Installing a Bootloader
Make connections as in the diagram above from the jig's SPI connector to the Arduino. Make sure there is a 16MHz resonator installed on the jig. Install an ATmega328P on the fixture. By the way, I probably should have mentioned it before, but do NOT use a ATmega328 unless you are capable of adding support for the device in avrdude and the Arduino IDE. The two devices have different signatures so the ATmega328 will not be recognized unless you are capable and willing to make the software changes I mentioned. The two devices are about the same but the "P" version runs at lower power.
Do the following from the Arduino IDE:
- Upload the sketch ArduinoISP: Files\Examples\ArduinoISP.
- Select the Arduino Uno board (selects the bootloader for the ATmega328P because that is the microprocessor for the Uno): Tools\Board\Arduino Uno.
- Select the Arduino as ISP as the programmer: Tools\Programmer\Arduino as ISP.
- Install the bootloader: Tools\Burn Bootloader.
That's all there is to it. It takes less than a minute for the process to complete. Once the TX and RX LEDs, on the Arduino, stop blinking, the bootloader is loaded.
Uploading Sketch Via FTDI
Once the boatloader is installed, you can upload a sketch to the jig using the FTDI Basic board. When I plugged my FTDI Basic into my USB port, for the first time, I found that the drivers had already been installed on my Windows 8.1 system. They must have been part of the Arduino IDE installation. If you do not have the drivers, check out Sparkfun's product page for instructions on getting them. The FTDI Basic costs about $15 at Sparkfun.
Raspberry Pi/Gertboard
The Gertboard is a expansion board that connects to the Raspberry Pi's GPIO pins. It has several different circuits useful for interfacing the Raspberry Pi to the outside world. The Gertboard works with all thee versions of the Raspberry Pi (A, B, B+). Here is what the Gertboard offers:
- ULN2808 containing six open collector outputs, each capable of driving a 500ma. load up to 50V (from a separate power source)
- MCP4802 dual channel D>A (digital to analog) converter
- MCP3002 dual channel A>D (analog to digital) converter
- ROHM BD6222HFP H bridge motor controller (need a separate power source)
- 12 output buffer circuits
- 12 input buffer circuits
- 12 red LEDs
- 3 momentary switches
- ATmega328P microcontroller. Running on 3.3V with a 12MHz resonator. All IO pins are available.
The documentation, unbelievably not provided when you purchase the product, is excellent. It can be found here. The manual references C and python programs that can be used to test out each section of the Gertboard. In order to use the ATmega328P, a bit of software must be loaded in the Raspberry Pi. This is also referenced in the manual and is available from Gordon Henderson. Here is a link to his blog. It all just works great. Newark Element14 has the Gertboard for $40. Inexplicably, Sparkfun sells it for $60. On Sparkfun's product page it lists the microcontroller as an ATmega328. It's an ATmega328P.
My jig will look to the Raspberry Pi just like the ARmega328P on the Gertboard. Therefore, if you want to interface an ATmega328P to a Raspberry Pi without using the Gertboard, the same software from Gordon will do the job. Just make the connections as in the diagram above.
Once you upload a sketch, you may need to communicate with the ATmega328P, such as you do with an Arduino. For the Arduino, you use the Serial Monitor function of the Arduyino IDE. You will not be able to do that from the Raspberry Pi, but you can still communicate with the ATmega328P. You connect RX and TX on the FTDI side of the jig to TX and RX, respectively, on the Raspberry Pi. You connect the FTDI TX to pin 10 (RX) of the Pi and FTDI RX to pin 8 (TX) of the Pi. The alternative to the Serial Monitor is to use a terminal program like minicom within a terminal window on the Raspberry Pi. The installation and use of the minicom program is covered in the Gertboard User's Manual.
Going On From Here
Obviously, the jig as it stands, does not have much utility as it only uses one of the many ATmega328P's IO pins. However, the concept can be used to make something more useful. Here is an example of a project in the works:
All of this goes into an enclosure. I will add temperature sensors and a RF transmitter, with antenna. The ATmega328P is mounted in a standard 28 pin socket instead of the Zif socket. A 16MHz crystal provides the clock reference.
Note the FTDI basic board. It's used to communicate with the ATmega328P. This is necessary to tell the LCD what text to display. Once power is applied, the display is blank. I attach the FTDI Basic board, connect it to the PC, via a USB cable, and bring up the Arduino IDE. The sketch is already running on the ATmega328P so I click on the Serial Monitor and am greeted with a request to type a message. Once that is done, the message is displayed in a Time Square Scroll. I can then remove the FTDI Basic board.
Please check out my blog entries of March 7, 2013 to April 18, 2013 for discussions of my LCD projects for the Raspberry Pi. The last two posts, specifically, are about my Times Square Scroll software. I have since converted the software from python to C to run on the Arduino. I'll report on that effort at a later time.
