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Sunday, January 19, 2014

Gertboard - Programming With ATmega Registers

Arduino IDE Built-in Functions Vrs. Direct Register Programming

Functionally, what is the difference between this script:
and this script:
Note: In the sketches discussed here, to see if the LEDs on the Gertboard are ON or OFF, PB0 is connected to BUF1, PB1 to BUF2, PB2 to BUF3, PB3 to BUF4, PB4 to BUF5, and PB5 to BUF6.
Both scripts turn LEDs on the Gertboard ON and OFF. First, LEDs 1, 3, and 5 are ON with 2, 4, and 6 OFF. One second later, 1, 3, and 5 are OFF, and 2, 4, and 6 are ON. One second later, the process repeats itself, on and on. The first script uses functions built into the Arduino IDE. While the second script uses the Arduino IDE delay() function, the LEDs are controlled by writing directly to microcontroller registers. The second script is sure a lot shorter and takes up about half of the space in memory. I must admit that it does use a little trick that I'll discuss later. That trick can not be used with the first script. I tried to shorten the first script by using for loops and if/else statements in lines 12 - 27. I gave up because it didn't seem to save lines of code.
The built-in functions that are part of the Arduino IDE are a convenient way to interface with the ATmega I/O pins. The second script shows another method, and that is to communicate directly with registers within the microcontroller associated with those I/O pins. Registers are eight bit bytes that are addressable within the ATmega memory space. In most cases, each bit of a register can be either read or written to. Each bit, or a combination of bits control some function or produce some effect. Before you can deal with these registers you need knowledge of what the registers do. That knowledge comes from the ATmega datasheet from Atmel.
If you are not concerned about writing a few more lines of code or how many bytes get loaded into the microcontroller, what other reasons would there be for programming ATmegta registers? Here are a few reasons:
  • IT'S EASY TO DO. There is a library include file, iom328p.h for the ATmega328P, that defines all of the registers and register pins in exactly the same way that they are named in the datasheet. For example, in the second sketch, the line "DDRB=0b00111111;" correlates directly to 13.4.3 on page 92 of the datasheet. This is the datadirection register for port B. Writing a 1 to the lower six bit positions makes pins PB0 to PB5 outputs. "PORTB=0b00101010;" correlates to 13.4.2 on page 92. This will make PB1, PB3, and PB5 logic high, turning on their respective LEDs. The first line of that second script, "#include <avr/io.h>" looks to see what ATmega chip you are using and, in our case, the ATmega328P. When the script is compiled, the definitions in iom328p.h are used to figure out the locations within the ATmega to access.
  • There are things you can do that are not supported by the built-in functions. For example, even though you can do PWM to control a motor using analogWrite(), you are stuck with one frequency - about 490Hz. If your motor would rather work at 10khz, you can program the counter registers. There are a lot more things you can do or develop yourself. For instance, I wish to develop my own 1-Wire interface for use with my DS18B20 temperature sensors.
  • I found that the function delayMicroseconds() produces delays that area about 2/3 of the correct values. I noticed this first with my camera remote control project. I was looking for a pulse width of 284usec but had to program delayMicroseconds(415) to get 284usec. Testing with an oscilloscope, from a desired delay of 10 to 3000usec, I consistently measured about 67% of the desired value (the delay() function is pretty accurate). So, I plan to write my own function for microsecond delays. There are two 8 bit counter/timers and a 16 bit counter/timer accessible by registers that can be utilized.
  • For me, I like getting closer to the hardware when programming. It makes me feel more in control.
  • Gives me a chance to use those fun bitwise operators like OR, AND, and EXCLUSIVE OR and shift bigs right and shift bits left.
  • Here are most of the ATmega functions that are accessible through registers:
    • I/O Pins, both content, data direction, and pull-up resistors for inputs
    • The various counter timer registers and prescallers for 8 and 16 bit counter/timers. These also control the PWM oscillator functions at the I/O pins
    • SPI control, status, and data registers
    • Registers for the serial communications using the USARTs
    • Registers for the 2-Wire serial interface
    • Analog comparator
    • A to D converter
    • Interrupts
I said I like to use bitwise operators, and the EXCLUSIVE OR operator is the trick I used to make that second script so short. Line 7 turned three of the six LEDs on (PB1, PB3, and PB5). Line 12 is going to toggle those LEDs that are ON to OFF and those that are OFF to ON.
Line 12 uses a compound operator. Recall that =+ is a compound operator. The expression A =+ 1 is the equivalent to A = A + 1. In our line 12, the compound operator is =^ which is an EXCLUSIVE OR. It is the equivalent of PORTB = PORTB ^ 0b001111. The EXCLUSIVE OR compares two bits. If they are the same (both 0 or both 1) the result is a 0. If they are different, the result is a 1. Let's apply that to PORTB which we will assume is 00101010. Putting one number under the other let's see what we get:
  00101010 PORTB
  00111111 EXCLUSIVE OR with this binary number
  00010101 NEW PORTB value
Let's do that again:
  00010101 PORTB
  00111111 EXCLUSIVE OR with this binary number
  00101010 NEW PORTB value
Note we are back to where we started. I hope you can see that this operation will toggle the six LEDs ON and OFF.

Another Sketch Writing To I/O Pins

I would like to introduce another sketch. This one uses four bitwise operators. Between the two sketches we will have used all of the bitwise operators except for the shift right.
This sketch uses the same six LEDs on the Gertboard except only one LED is lit at any one time. It starts with all LEDs OFF, then lights each LED for one second starting with the LED connected to pin PB0. Each LED, down the line, is ON for one second. One second after PB5 is lit, the sequence starts over again at PB0.
Let's take a look at line 12. Here we have two bitwise operators, an OR compound operator, |=, and a SHIFT LEFT, operator, <<. If you are not familiar with this type of notation it surely looks strange.
Line 12 means we are going to take the current contents of the register PORTB, the register connected to pins PB0 through PB5, and OR them with some value so that PORTB is changed. I'll talk about what OR means in a minute. For now, the question is: what is that value we are going to use to change the contents of PORTB? Why, it is (1 << PORTB0), of course. What?? What does that mean? If you look into the include file, iom328p.h, (called by avr/io.h, the first line in the sketch) you will find that PORTB0 is defined as 0. In like manner, PORTB5 is defined as 5. You can see the pattern here. So, (1 << PORTB0) becomes (1 << 0). That means we take the number 1 and shift it left 0 times. That seems to make even less sense. It begins to make sense, however, if we look at 1 as 00000001. Nothing much happens to it if we shift it 0 times as in line 12 of the sketch. But, what about similar line 32. This means we shift 1 left by 5. Just move that 1 over 5 places to the left. This new value now becomes 00100000. Get it?
Now, we can talk about the OR operator. When we OR two binary digits, if either digit is a 1, the resulting value is a 1. A 0 result only corresponds to both digits being 0. So, if we started out with PORTB as 0b00000000 (all LEDs OFF) let's apply sketch line 12 and see what happens.
  00000000 PORTB
  00000001 OR with this binary number
  00000001 NEW PORTB value
This obviously turns the LED connected to PB0 ON, and that is the only one on. Now we wait one second and move on to sketch line 15, which is similar to line 12, except that the |= is replaced by &=, the AND compound operator. And, (1 << PORTB0) is now: !(1 << PORTB0). The exclamation point is the NOT symbol, for negation, another bitwise operator. We know that (1 << PORTB0) is 00000001. What then is !(00000001)? The answer is 11111110. Every bit is changed to its complement (1 becomes 0, 0 becomes 1).
The AND bitwise operator takes two bits and if either is a 0, or both are 0, the result will be a 0. The only way to get a 1 is if both bits are 1. So let's AND the contents of PORTB, 00000001, with 11111110:
  00000001 PORTB
  11111110 AND with this binary number
  00000000 NEW PORTB value
This turns the PB0 LED OFF. Obviously we could have written PORTB = 0; with the same effect, but what is the learning value in that? If you follow the logic you can see where sketch line 16, and beyond, turns the LED connected to PB1 one, for a second, before turning it off, and turning the next LED on, etc., etc.
Controlling those LEDs the way I did in that last sketch is rather silly. Lines 12 through 35 could have been written as:
PORTB = 0b00000001;
PORTB = 0b00000010;
PORTB = 0b00100000;
So why did I write all of that extra code? I wanted to illustrate the use of those bitwise functions because they are invaluable when writing to registers and memory. They are an elegant way of accessing individual bits in a field of other bits (our field is 8 bits because all ATmega registers are eight bits long). It's a way of altering some bits while leaving other bits alone. You could keep track of all the bits in a register at all times, but sometimes that is not possible. Other processes, either hardware or software, could change some bits in a register. You only want to access the bits you want to change and leave the others alone.
As you can see from what we have done before, if you wish to make a bit a 1, or assure it is a 1, OR it with a 1. OR the bits you don't want to change with a 0. Look at the example above.
If you wish to make a bit a 0, or assure it is a 0, AND it with a 0 and AND the others with a 1 to leave them alone. If you wish to toggle a bit, changing it from a 0 to a 1, or a 1 to a 0, EXCLUSIVE OR it with a 1, and EXCLUSIVE OR the others with a 0. If you study the examples above you can see how that works.

Sketch To Read From I/O Pins

I talked about writing to I/O pins in the previous sketches, so my last sketch shows an example of reading the logic level of an I/O pin:
This sketch sets pins PC0 to PC5 as inputs. Writing to PORTC will determine if an internal pull-up resistor will be connected to the pin. Writing a 1, connects the pull-ups. So writing 3F, in hexidecimal, writes a 1 to the lower 6 bits of the register.
Every second, the script sends the contents of PINC to the serial monitor running on the Pi. If PORTC is the register that outputs a logic level to the port C I/O pins, PINC is the register that records the logic level of external inputs of the port C I/O pins. If nothing is connected to an external pin, the internal pull-up resistor will set a logic 1 in the corresponding PINC register bit. If you connect that pin to ground, a logic 0 will be set in the register bit. Using the BIN built-in constant, will send the contents of the register in binary format.
If there is nothing connected to any of the port C pins, you will see:
However, if you connect PC0 to ground, for example, you will see:
etc. for as long as the ground is connected.


I hope this has been informative and if you are a beginner that you learned somethings useful.

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