It works. Thank you.

Now, if I have negative steps, if I want to drive the stepper motors in reverse what do I have to do?

Can I drive 3 stepper motors in the same time with an Arduino Uno (Atmega328p) programmed with Arduino IDE, Stepper.h library?

I have three stepper motors that have to move each a different number of steps:
StepperX 50 steps
StepperY 80
StepperZ 130

What I want to do is to determine all three move 50 steps in the same time, then StepperX stops, StepperY and Z continue for another 30 steps, after which only StepperZ moves alone for the last 50 steps?

Is it possible?

You signal seems to be a low pass filtered sum of two phase shifted different amplitude square waves. You do not need a DAC to generate a good imitation of such a signal.

The MEGA bootloader implements STK500v2 ...
It does claim to support the ATmega128 as well as a number of other "larger" ATmega chips.

My Atmega128, stand alone microcontroller, does not have a bootloader and what I want to do is to write it with a code that have nothing to do with a bootloader. I also want to use an Arduino Uno (with Atmega328p on it) ISP programmer not an Arduino Mega.

That picture is a mapping between the physical pins of an Atmega328p in DIL package and Arduino Uno pins.

Read first, a bit, the Atmega328 datasheet. It will clarify many of your questions.

As regarding power pins, there are dozens of chips with VCC and GND pins in the middle of pin rows.  

[Can Arduino Uno, loaded with Arduino ISP, be used directly by avrdude to program an Atmega128?]

A question related to this topic:

Can Arduino Uno, loaded with Arduino ISP, be used directly by avrdude to program an Atmega128? if one makes the connections:
Arduino Uno---Atmega128
SS---------------RESET
MOSI------------MOSI
MISO------------MISO
SCK-------------SCK

I know Uno works as an ordinary ISP programmer for Atmega8, Atmega328, Atmega32U4 but I do not know if same thing is true for Atmega128.

However once you decide on a family (in my case LPC) the chips are actually very easy to use, well the smaller ones anyway. The ability to MUX functions on pins is good and bad, I find that with a large chip (144 or 217 pins) I need a spread sheet to keep track of what function I have on what pin, that can quickly get to be a nightmare.

Have you tried ARMwizard?
If not (because I am seeing that you have experience with LPC family from NXP) can you try it, when you have time, and tell if this tool for setting ARM peripherals really simplify the work of programmers.

They are more powerful than AVRs but these ARM Cortex-M micro-controllers do not seem easy to work with. See this discussion: http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=127016
Too many vendors, too many types of ARMs, too many different peripherals, too many ways to set them.
The huge diversity of ARMs seems to be their main enemy.

Other posters have pointed out that a semiconductor company does not build a successful business model based on selling to hobbyists.

Possible, but just claiming that without bringing evidence does not weight too much.

Yes, the Atmel AVR processor is used in all different kinds of applications, commercial and industrial. I'm not at liberty to discuss specific customers, or numbers, but they are definitely used by professional embedded systems.

A similar topic, opened last year, ran for a few dozens posts without demonstrating that 8 bit AVRs have indeed industrial applications.
see: http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=114016&postdays=0&postorder=asc

[Avrdude and ArduinoISP for writing 8051 chips]

Avrdude and ArduinoISP for writing 8051 chips

I have some 8051 compatible controllers, AT89S52, AT89S2051 and AT89S4051 but no programmer for them. Is it possible to write their flash using ArduinoISP running on Arduino Uno and avrdude?

It looks like this is possible as long as using an usbasp and avrdude someone wrote an AT89S52,
see: http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=99161

[our 8-bit micros were down 7% sequentially, and were down 35% year-over-year]

It can be calculated from the report below how much money Atmel got from 8 bit microcontrollers (likely 8051+AVR8+others), this year and in 2011. I made the calculation and it appears that they earn two - three times more money from 8 bit families than from 32.

"Moving to a discussion of our business segments. For our Microcontroller business unit, Microcontroller revenue of $226 million increased 3% sequentially and was down 25% as compared to the third quarter of 2011.

For the second quarter in a row, both our core microcontroller revenue and maXTouch revenue grew sequentially.

By product family, during the second quarter, our 8-bit micros were down 7% sequentially, and were down 35% year-over-year. While 32-bit microcontrollers increased 33% sequentially and increased 11% year-over-year."

Source: http://seekingalpha.com/article/971731-atmel-management-discusses-q3-2012-results-earnings-call-transcript?page=3

The net revenues reported by a company or in the name of a company in the media do not weight too much. Figures like this "[Atmel] Microcontroller segment net revenues increased 95% to $892 million for the year ended December 31, 2010 from $458 million for the year ended December 31, 2009." can be misleading.

[Bad Remark:]

Beside AVRs, Atmel also manufactures Intel 8051 compatible processors and many other IC which have heavy industrial or domestic applications.

The microelectronic technology Atmel uses for all of its chips is reliable.

However, for newly released AVRs there could be problems:

Bad Remark:
Atmel AVR microcontrollers are inexpensive and have an inexpensive development tools but the AVR microcontrollers are not up to par for commercial applications. I used a AVR32 in a comical application and while the chip did have some nice features is also had several bugs in the board support package, drivers, development tools and in the chip itself. To make matters worse each shipment of parts not only had bugs but different bugs requiring a hardware and software change for each shipment. This may not be a big problem for a hobbyist but it’s a disaster for a company that wants to put its product into production. I would not recommend anyone attempt to use an Atmel part in a commercial product unless they want to lose there job.

Answer:   
Sounds like you had some trouble with AVR32, which is relatively new. However, standard 8-bit AVRs are used all over in successful industrial and commercial products, and this has been the case for many, many years. – wjl Oct 15 '11 at 1:31
http://electronics.stackexchange.com/questions/2324/why-are-atmel-avrs-so-popular

Do you want to program the ATmega48-20AU using an arduino board like Arduino Uno or you have means to program the ATmega48 and you are just looking for information about how to interface an Arduino Uno with an Atmega48, to make them talk on I2C?

If the AVR has a bootloader already uploaded then with an USB to Serial adapter like this
http://www.ebay.com/sch/sis.html?_nkw=TRIPPLITE+U209+000+R+TRIPP+LITE+USB2+SERIAL+ADAPTER+
you can program your micro controller.
You will also need (most probable) a MAX232 level convertor ( http://en.wikipedia.org/wiki/MAX232 ).