Wednesday, August 5, 2009

Programming the ATtiny13 in Assembly

The Arduino introduced me to Atmel's AVR family of microcontrollers, specifically the ATmega168, which led me to purchase an AVRISP MKII ($34 from Mouser) to program off-the-shelf ATmega168s. So when I needed an IC smaller than the ATmega168, it made sense to stick with AVR microcontrollers, rather than invest in another programmer. This is how I ended up writing assembly for the ATtiny13.

I'd programmed a PIC16F877 in college (a long time ago). AVR's flavor of assembly is a bit different from Microchip's, so there were some annoying things to learn when I started programming the ATtiny13. My goal in this post is to familiarize the reader with the AVR-specific assembly grammar.

First, here's how to get set up for burning code onto your AVR microcontroller:

A breadboard, a 5V power supply (6V is the max allowable supply voltage for the ATtiny13, 2.2V is the minimum), an ATtiny13, an AVRISP MKII, and a USB cable.

ATtiny13 datasheet, AVR Studio

Hardware Setup:
Place the ATtiny13 on your breadboard. Refer to page 2 of the datasheet for power/ground connections, and for identifying the four pins that connect to the AVRISP MKII programmer: RESET, MISO, MOSI, SCK. (The ATtiny13 only has 6 I/O pins, and I need to connect four of these pins to the programmer?!? Don't worry: the microcontroller I/O pins that connect to the AVRISP MKII can still be used for other purposes besides programming the microcontroller. In fact, as long as your AVRISP MKII is USB-powered by your computer, you can leave it connected to your microcontroller while you test out your newly burned code. But if you disconnect USB power to your AVRISP MKII, your microcontroller will play dead if the AVRISP MKII is still connected to it -- I learned that the hard way while demonstrating a prototype to investors!)

There's a lot of ways you can physically connect the AVRISP MKII to your ATtiny13. The AVRISP MKII has a 6-position ribbon cable connector (the six connections are RESET, MISO, MOSI, SCK, power, and ground). I started out by just poking solid wires into this connector, and connecting the other ends of the wires to the breadboard. I've improved on this with a neater, more reliable wiring job that I'll detail later.

Ready to burn:
(Assuming you've already installed the AVRISP MKII on your computer) connect the AVRISP MKII to your computer with the USB cable. The LED in the AVRISP MKII might be red or might not light at all. Connect 5V power to the breadboard. The LED changes to green. Start a new assembler project in AVR studio:

Project type: Atmel AVR Assembler (if you download WinAVR, you can select AVR GCC if you want to write your code in C instead -- but what's the fun in that? Actually, AVR microcontroller architecture is supposedly streamlined for C compilers, so the usual argument for programming in assembly -- faster code execution -- doesn't really apply. I guess some of us just feel more comfortable writing in Assembly.)

Project name: TestProj
Check both boxes "Create initial file" and "Create folder."

The automatically generated "TestProj" folder will hold all the files associated with your project, the two important ones being "TestProj.asm" and "TestProj.hex." A .asm file is the assembly code that you write. A .hex file is the machine-language executable that is burned onto your microcontroller. (The .hex file is a word-for-word translation, converting each assembly language word into a hexadecimal number. All readability is removed, and the order of things is shifted due to conditional branches, but if you have nothing better to do, it is possible to decipher the actual .hex file.)

Click "Next" and select "AVR Simulator 2" for the "Debug platform" and "ATtiny13A" for the device (the ATtiny13A is the new and improved version of the ATtiny13, and is the chip you're using if you bought it following the above link).

You should see a bunch of windows. The one in the middle is where you edit your .asm code. I like to begin with a few commented lines:

What this code does:
1. Light an LED as soon as the MCU has power.

Hardware Setup:
0. One MCU, one LED, and a resistor.
1. Connect LED to pin 5 of the ATtiny13A with a 1k resistor such that the LED lights when pin 5 is high.

To Do:
1. Light the LED in response to an external switch
2. Make the LED glow, Jonathan Ive style

Cut and paste the above into your .asm file. AVR Studio will color all of the text green, indicating it's all commented out. The above example illustrates two ways to make comments: the ";" for a single line, or "/*" to begin a multi-line comment, ending with "*/". You can also use "//" for single line comments.

Here's the actual code:

.include ""

.def temp = R16

//Pin assignments
.equ LED = PB0 ;pin 5 of ATtiny13

.org 0x00
rjmp setup


ldi temp, (1 << LED)
out PORTB, temp
out DDRB,temp

rjmp loop

I'll explain this code in a second. First, let's compile it and run it. Cut and paste it into the TestProj.asm file. Assemble it by selecting "Build" from the drop-down Build menu (as with all the other things we're about to do, you can also accomplish this with a keystroke or with a toolbar button; in this case pressing F7, or by pressing the "Assemble" button in the toolbar.)

To see what the code does and make sure it works, we run the debugger: select "Start Debugging" from the Debug drop-down menu. A yellow arrow appears next to the line "rjmp start" (if you're using my initial commented lines, this is line 26 of your .asm file, as indicated by the line and column number at the lower right of the screen). The yellow arrow shows you where the "Program Counter" is pointing. What's the program counter?

There are three kinds of memory in AVR microcontrollers. When you build your code, it's converted into the machine language hex that tells the chip what to do. This sequence of hex numbers is stored in the chip's non-volatile (i.e. turn off the power and the code is still there) "Flash" memory. The "Program Counter" is a counter that starts at 0x00 and counts up, unless some conditional branch tells the program counter to jump to a different value. The value of the counter tells the microcontroller which address to look at in the Flash memory. Picture the Flash memor as an array (a vertical column) of 16-bit words. The top-most word is at address 0x00, the next one down is at address 0x01. The ATtiny13 has 1K Byte of flash memory, organized as a single column of 512 rows, each row 16 bits wide; that means the Program Counter counts from 0 up to 511 (0x000 to 0x1FF -- see Figure 5.1 on page 15 of the ATtiny13 datasheet) because the memory can store 1023 bytes, or 512 16-bit words. Each instruction (a.k.a. "opcode") in your code is stored as one or two 16-bit words in this Flash memory. The opcode is the actual machine language. We'll look at a few opcodes for fun below.

A Tangent On AVR Memory:

What are the other two kinds of memory in the ATtiny13?
One kind is non-volatile EEPROM (non-volatile meaning that you don't lose it when power is lost). Unfortunately, there's a relatively small limit to the number of times you can erase/write the EEPROM. The EEPROM memory is not used, unless you make a conscious effort to do so. I've never used it, and I'm not going to mention it again in this post.
The other kind of memory is the volatile "SRAM" (static RAM, it retains its memory indefinitely while the power is on, but forgets everything when the power is turned off). SRAM stores the values of your variables (e.g. a variable that stores the state of a push-button switch). The SRAM, a.k.a the "Data Space" or "Data Memory," is divided into three physically separate parts (see figure 5-2 on page 16 of the ATtiny13 datasheet): General Purpose Registers, I/O Registers, and "internal data SRAM." Although they're all lumped under the same category, they are actually connected very differently inside the MCU, requiring you, the assembly programmer, to know which instructions apply to which parts of Data Memory, resulting in annoying things like different instructions for the same task (setting/clearing bits for instance).

The General Purpose Registers, for example, have direct connections to the ALU. Therefore, most of the assembly instructions are applicable only to these 32 registers. In other words, if you're going to do some operations to a value (add, multiply, manipulate bit values, etc.), the value must first be placed in one of these 32 registers.

The 64 I/O registers have very specific functions, and cannot be used for storing variables, so there's lots of instructions that don't apply to the I/O Registers. Each of the 8 bits in each of the 64 I/O registers has some special meaning. The hardest part of getting started programming in Assembly is learning what bits in what I/O Registers you need to set/clear/check to achieve the desired result (e.g. enable interrupts, use the counter in PWM mode, set an I/O pin as an input, enable pull-up resistors, check the result of a calculation, etc.).

The 64 bytes of "internal data SRAM" is only useful for holding values. If you want to do anything with a value, you need to copy it into the General Purpose Registers. Think of it like this: SRAM is our storage bin of 8-bit words, whereas the registers are the 8-bit words sitting on our workbench. To use anything that resides in "internal data SRAM," we have to put it on our workbench first. Indirect addressing makes this less tedious than it sounds, but more on that later. Besides indirect addressing, the most convenient, and most common way of using "internal data SRAM" is to push and pop values from "the Stack." The Stack is not physically separate from the "internal data SRAM." The Stack is nothing more than SRAM addressed by the Stack Pointer. The Stack Pointer defaults to RAMEND, the highest possible Data Memory Address. To free up register space (there's only 32 after all), a value stored in a register is pushed onto the Stack. Since the "top" of the Stack is sitting at RAMEND, and there's 64 bytes of "internal data SRAM," we can fit a max of 64 values on the Stack (assuming we're not already using some of the "internal data SRAM" for something else).

One last note: it's confusing, but when people say "SRAM" they are only referring to the "internal data SRAM" (in fact, I'm going to drop the phrase "internal data" as well). The General Purpose Registers and I/O Registers are usually referred to as "registers," not as "SRAM."

Back to Debugging:
When the MCU runs your code, an internal oscillator is clocking the Program Counter, advancing the pointer (shown graphically in the debugger as a yellow arrow) to the next address in Flash memory. When you are debugging your code, you "clock" the Program Counter manually by pressing F11.

So press F11 once. Note that the Program Counter now reads 0x01 (see the Processor window on the left). That means that the instruction that the yellow arrow is pointing at resides at address 0x01 in Flash memory. Also note that the Cycle Counter reads 2, implying that our "rjmp" command ate up two clock cycles (i.e. if our clock has a frequency of 1MHz, it takes 2 microseconds to execute "rjmp").

Continue to step through your code with F11, and note how the Program Counter changes, and how the Cycle Counter increments. Also note that the yellow arrow never points at the line "setup:" or the line "loop:". These are labels, not instructions. They do not reside in Flash memory. Labels allow us to assign a name to a location in our code, so that we can easily tell the code to jump there. This is more readable than "rjmp 0x01," and it is more flexible, allowing us to insert code before the label, without having to recalculate what Flash address the label is referring to.

When you get up to the line "rjmp loop," the Program Counter is stuck at Flash memory address 0x05, and the Cycle Counter continues incrementing by two for each execution of the rjmp command. The ATtiny13 will remain stuck in this loop forever. Let's stop debugging for now, and burn the code onto ATtiny13. Select "Stop Debugging" from the Debug drop-down menu.

Burn the code onto the ATtiny13 by clicking on the "Con" button on the toolbar (the button looks like an IC with the name Con). This opens up a dialog box to establish a connection between the AVRISP MKII and your computer. Now click on the "AVR" button (to the right of the "Con" button). Go to the "Program" tab, and in the section labeled "Flash," check "Input Hex File" and browse to the .hex file you just built (it's in the TestProj folder). Click the "Program" button to burn the code onto the chip. The actual code burning takes a fraction of a second (which feels impossibly quick if you're used to uploading code to the Arduino). Your breadboarded LED should light up, almost the instant you press the "Program" button.

Some More Debugging:
Run the debugger again. This time, as we step through the code, let's look at what's happening in SRAM. In the Processor window, expand the "Registers." We define "temp" as register 16, so keep an eye on register 16 when you step through the line "ldi temp,(1 << LED)." LDI (stands for Load Immediate) is an instruction with two operands. The first operand is the General Purpose Register where you wish to store a value. Since we defined "temp" to be register 16, we're going to store some value at register 16. When we stepped through this line of code, we saw the value stored was "0x01." How is "1 << LED" = 1? As we'll see later, the .equ statement sets LED = PB0, which is set = 0 in a .equ statement in the .include file. So 1 << LED is translated to 1 << 0. The operator “<<” means bit shift to the left. “1 << n” means bitshift 1 n times to the left. For example, 1 << 0 = 1, 1 << 1 = 2, 1 << 2 = 4, 1 << 3 = 8, etc. So if we want to store the value “1” in temp, why didn't we simply write:

ldi temp, 1

Believe it or not, “1 << LED” is infinitely more readable than “1.” Say I want to connect my LED to PB2 (pin 7) instead of PB0 (pin 5). I only need change my .equ directive from .equ LED = PB0 to .equ LED = PB2.

The next line outputs the value in "temp" onto PORTB. In the I/O View window, expand "PORTB". In the lower part of the I/O View window, watch the LSB in register PORTB get shaded in. Similarly, the LSB of register DDRB is shaded in as you step through the next line of code. Independent of what code is executed next, the LSB of register PINB is shaded in after the next clock cycle.

This is the essence of debugging. Step through the code line by line, and satisfy yourself that the correct bits are being set and cleared in the correct registers.

A Full Explanation of the Code:

.include ""
Include the file "" This file is mostly just a compilation of .equ directives. Like labels, directives are not instructions stored in Flash memory. They are interpreted by the assembler when you compile the code. ".include" is a directive. Here's another directive:

.def temp = R16
We're assigning the name "temp" (for temporary variable) to register 16, just to make the subsequent code more readable.

.equ LED = PB0
Everywhere the word "LED" appears in our code, the compiler replaces it with the value 0 (because of the statement ".equ PB0 = 0" in the .include file).

Yet another directive. Recall we said that there's three kinds of memory: Flash, SRAM, and EEPROM. Well, CSEG (stands for Code Segment) defines the start of a code segment, and code, as we know, resides in Flash (Program memory). To explicitly store stuff in SRAM, we'd use a ".dseg" directive (stands for Data Segment). More on that later.

.org 0x00
This directive tells the location counter that the next line of code belongs at the address indicated. Since our .org statement appears after ".cseg", we're in the code segment, which means that our location counter is the Program Counter, and the address is a location in Flash memory. And 0x00 means that the next line of code is to be stored at Flash address 0x00.

This is a label. It's just a way for us to name a location in memory, making the code more readable, and making it easy to program conditional jumps to this location in memory. Since the "setup" label is appearing in a Code Segment (.cseg), the label refers to a location in Program Memory. If the label were part of a Data Segment(.dseg), then it would refer to a location in SRAM (that's how you create variables).

ldi temp, (1 << LED)
out PORTB, temp
out DDRB, temp
We covered LDI above. OUT, as the name suggests, takes the value of "temp" and outputs it to I/O Registers PORTB and DDRB. This is a good example of the annoyance of AVR's Data Memory. When copying values from one General Purpose Register to another, you use the MOV instruction. But to copy a value from an I/O Register to a General Purpose Register, you use the IN instruction, and to go the other direction, you use OUT. Examples:

mov r16, r17 ;copy r17 to r16
out PORTB, r17 ;copy r17 to PORTB
in r17, PORTB ;copy PORTB to r17

The statements:
mov PORTB, r17
out r16, r17
are nonsensical and will generate errors when you compile.

/* Another example of a label. This label refers to Program Memory address 0x05. Obviously, if we decide we need another line of code in setup, then "loop" refers to memory address 0x06. You get the idea.

rjmp loop
Go to loop (go to memory address 0x05). And what's at memory address 0x05? A instruction to go to memory address 0x05. The ATtiny13 spins its wheels indefinitely. There's more elegant ways to make the ATtiny13 stop doing stuff. And soon we'll be adding code to the loop routine.

Since .cseg and .org 0x00 are defaults for the compiler, you could erase the ".cseg" and ".org 0x00" from your code, though it won't make your .hex file any smaller. Anyway, it's a good idea to get used to these directives for when your code starts using more memory. For example, here's a code snippet that defines a location in SRAM:

;DSEG = Data Segment, so the code that follows
;applies to Data Memory (SRAM)

.org 0x60
;The next line of code shall reside at SRAM address 0x60.
;0x60 is the lowest address we can use for SRAM,
;(the registers reside below 0x60).

;myData is the label for this Data Memory address.

;reserve 1 byte at the location labeled "myData."

Machine Language:
What about the opcode machine language stuff. How does the "rjmp loop" opcode actually look in Flash memory? The opcode for the rjmp instruction is "1 1 0 0 k k k k k k k k k k k k" where 1100 is the code for rjmp, and the 12 k's are the 12 bits of the address to jump to. (Of course, only nine of these 12 bits are necessary to address the 512 rows of 16-bit wide Flash memory.) So, for example, the opcode for "rjmp setup" is:

1100 0000 0000 0001

because the label "setup" refers to Program memory address 0x01. How do we know that "setup" labels address 0x01? the .org 0x00 statement sets the Flash address to 0x00, so the "rjmp setup" command resides here. The very next line of code, then, resides at address 0x01, which is where the "rjmp setup" jumps us to. (So why'd we even bother with the "rjmp setup"? Why not just begin the code with the "ldi" instruction? This question will be answered in our next program, where we'll see that the first seven or so lines of Flash memory have a very special purpose).

Similarly, "rjmp loop" is:

1100 0000 0000 0101

because the label "loop" refers to Flash memory address 0x05. Again, just count lines of actual code, and you'll see that the line "rjmp loop" belongs at address 0x05, (as the Program Counter indicated while debugging).

Let's take a look at this in the actual .hex file. Expand the "Output" folder in the file tree at the left, and double-click "TestProj.hex". Here's what's in my .hex file:


1100 (rjmp) is represented in hexadecimal as C. Ignore the first line of the .hex file. Can you figure out where the "rjmp setup" instruction is? Remeber that every four bits of opcode is represented by a single hex number (1100 = C). You can obtain the opcodes (and much more useful information) from the Assembler help file, availble in the Help drop-down menu. You're looking for 1100 0000 0000 0001, which is C001. This appears in the middle of the second line. But we don't have to dig this way. Go back into debug mode. From the View drop-down menu, select "Memory." A window pops up that allows you to view any part of the ATtiny13 memory. Select the "Program" memory. You can directly read the four hex numbers sitting at each 16-bit memory address. This is where I throw my hands up -- the numbers seem a bit jumbled compared to the opcode formats described in the help file.

Next post, we'll introduce interrupts to make the promised code revisions in our "To Do" list.

Saturday, July 4, 2009

AVRISP MKII Frustration and Workaround

Following the instructions from the site for using the AVRISP MKII to burn the Atmega168 bootloader through the Arduino IDE yields the error message:

avrdude: usbdev_open(): did not find any USB device "usb"

Burning the bootloader through the AVR Studio IDE does not output any error messages in AVR Studio. But the Arduino on-board LED (Arduino pin 13) does not light after reset, suggesting that the bootloader was not correctly loaded. Not suprisingly, subsequent attempts to upload sketches through the Arduino IDE fail, yielding the error message:

avrdude: stk500_getsync(): not in sync: resp=0x00
avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51

This issue is discussed a lot on the web, but I've yet to find a solution. The following is a workaround I developed after days of frustration.

Connecting the AVRISP MKII:
If you have an Arduino board, and wish to program the Atmega168 through AVR Studio using the AVRISP MKII, simply:
1. power the Arduino board externally (a standard 9V wall-wart works fine),
2. connect the AVRISP MKII to your USB port,
3. Connect the 6-pin ribbon cable connector from the AVRISP MKII to the 6-pin header on the Arduino board. The header is labeled "ICSP." The correct orientation is that the red wire of the ribbon cable should be on the side of the header labeled "1." (There is also an inconspicuous raised triangle on the top of the connector that indicates which position is pin 1.)

(Of course, you can also program the Atmega168 with a simple breadboard and an AVRISP MKII. All you need to do is provide 5V (max) to the Atmega168, and connect the six pins from the AVRISP MKII ribbon cable to the appropriate pins on the Atmega 168. There are tutorials on the site.)

You can write your code in AVR Studio. For people with an embedded systems background who are just getting started with the Arduino, and are looking for a way to program the Atmega168 for use off the Arduino board, I recommend that you do not waste your time with the Arduino IDE. Like the Arduino IDE, AVR Studio is free. Unlike the Arduino IDE, AVR Studio is very flexible, allowing you to program the Atmega168 in Assembly or C (you need to download WinAVR to program in C), and does not tie you to the Arduino bootloader.

For everyone else, the Arduino is a great tool for starting out with microcontroller programming. Easy to learn, and tons of support, perfect for quick embedded systems prototyping without having to know Assembly, or even the ins and outs of your microcontroller.

The problem I've run into is that I've been developing my microcontroller code using the Arduino interface, but I am using the Atmega168 off the Arduino board, on a custom printed circuit board. Therefore, I need to burn my Arduino code onto off-the-shelf Atemga168 chips. At this point it's a bit of work to rewrite everything in AVR Studio without the Arduino functions. Eventually I'll buy an STK500. They're a bit expensive, but they make burning the Arduino bootloader hassle-free.

The workaround described below allows you to create the .hex file in the Arduino IDE, and burn it onto the Atmega168 in AVR Studio.

This is the workaround I've found:
Attemping to upload your code to the Arduino board does not work, but it does generate the .hex file for your code! The .hex file is saved in the "applet" folder in your sketch folder (the Arduino IDE creates an applet folder the first time you attempt to upload your sketch; each sketch has its own applet folder).
1. Work on your code in the Arduino interface.
2. When you are ready to upload, attempt an upload. This generates the .hex file. (You don't need to connect the Arduino board to attempt the upload).
3. From AVR Studio, program the Atmega168 (using the connections described above). On the "program" tab, in the "Flash" box, browse to the .hex file you just created. (You only need to do this once. The program tab points at the .hex file, so each time you update the .hex file in the previous step, just click the "Start" button in the Auto tab, or the "Program" button in the Program tab to program the device.)

That's it! If anyone can tell me how to burn the bootloader using the AVRISP MKII the way the Arduino team intended, it'd be much appreciated.

Even for those of you who had no trouble burning the bootloader, the method described above is helpful if your microcontroller is on a custom PCB, and you wish to update the code on your microcontroller without prying it off the board and sticking it into the Arduino board. Just compile the .hex file in the Arduino IDE, connect the AVRISP MKII directly to your PCB (of course, you'll need a 6-pin header on your PCB to plug the connector), and burn the .hex files through AVR Studio.

Monday, November 17, 2008

Changing PWM Frequency on the Arduino Diecimila and the Atmega168

Atmega168 pins 12, 11, 15, 16, 17, and 5 can be configured for PWM output. On the Arduino Diecimila, these are pins 6, 5, 9, 10, 11, and 3 respectively.

The 8-bit PWM value that you set when you call the analogWrite function:

analogWrite(myPWMpin, 128); // Outputs a square wave

is compared against the value in an 8-bit counter. When the counter is less than the PWM value, the pin outputs a HIGH; when the counter is greater than the PWM value, the pin outputs a LOW. In the example above, a square wave is generate because the pin is HIGH from counts 0 to 127, and LOW from counts 128 to 255, so it is HIGH for the same amount of time it is LOW.

It follows logically that the frequency of the PWM signal is determined by the speed of the counter. Assuming you are using an Atmega168 with the Arduino Diecimila bootloader burned on it (which is exactly what you are using if you bought an Arduino Diecimila), this counter's clock is equal to the sytem clock divided by a prescaler value. The prescaler is a 3-bit value stored in the three least significant bits of the Timer/Counter register: CS02, CS01, and CS00. There are three such Timer/Counter registers: TCCR0B, TCCR1B, and TCCR2B.

Since there are three different prescalers, the six PWM pins are broken up into three pairs, each pair having its own prescaler. For instance, Arduion pins 6 and 5 are both controlled by TCCR0B, so you can set Arduino pins 6 and 5 to output a PWM signal at one frequency. Arduino pins 9 and 10 are controlled by TCCR1B, so they can be set at a different frequency from pins 6 and 5. Arduino pins 11 and 3 are controlled by TCCR2B, so they may be set at a third frequency. But you can't set different frequencies for pins that are controlled by the same prescaler (e.g. pins 6 and 5 must be at the same frequency).

If you use the default values set by the Arduino Diecimila's bootloader, these are your PWM frequencies:

Arduino Pins 5 and 6: 1kHz
Arduino Pins 9, 10, 11, and 3: 500Hz

How do you change the PWM frequency?
In the void setup() part of your Arduino code, set or clear the CS02, CS01, and CS00 bits in the relevant TCCRnB register. For example, if you want to set pins 5 and 6 to output a PWM signal at the highest possible frequency (see "List of Possible Frequencies" at the end of this post), insert the following code in the void setup() section of your Arduino code:

//First clear all three prescaler bits:
int prescalerVal = 0x07; //create a variable called prescalerVal and set it equal to the binary                                                       number "00000111"
TCCR0B &= ~prescalerVal; //AND the value in TCCR0B with binary number "11111000"

//Now set the appropriate prescaler bits:
int prescalerVal = 1; //set prescalerVal equal to binary number "00000001"
TCCR0B |= prescalerVal; //OR the value in TCCR0B with binary number "00000001"

The above code cleared bits CS02 and CS01, and set bit CS00.

Another way to do this, is with the following code:

//First clear the three prescaler bits
TCCR0B &= ~(1<<cs02);>
                                                   Invert that to get "11111011". AND that with TCCR0B.
TCCR0B &= ~(1<<cs01);>
TCCR0B &= ~(1<

//Now set the appropriate prescaler bits
TCCR0B |= (1<<cs00);>
                                                OR that with TCCR0B.

If the above code is confusing, try the Bit math explanation here:

List of Possible Frequencies:

For pins 6 and 5 (OC0A and OC0B):

If TCCR0B = xxxxx001, frequency is 64kHz
If TCCR0B = xxxxx010, frequency is 8 kHz
If TCCR0B = xxxxx011, frequency is 1kHz (this is the default from the Diecimila bootloader)
If TCCR0B = xxxxx100, frequency is 250Hz
If TCCR0B = xxxxx101, frequency is 62.5 Hz

For pins 9, 10, 11 and 3 (OC1A, OC1B, OC2A, OC2B):

If TCCRnB = xxxxx001, frequency is 32kHz
If TCCRnB = xxxxx010, frequency is 4 kHz
If TCCRnB = xxxxx011, frequency is 500Hz (this is the default from the Diecimila bootloader)
If TCCRnB = xxxxx100, frequency is 125Hz
If TCCRnB = xxxxx101, frequency is 31.25 Hz