Getting started with Xmega: differences from ATmega (part 1)


To start out this series on getting started with Atmel’s new Xmega chips, I first need to explain what it is that makes it an upgrade from the original AVR ATmega chips.  While there are a lot of common elements, the combination of a large number of peripherals and the mechanisms Atmel provides to connect them all together makes for a very powerful chip.  The Xmega are capable of things that an ordinary AVR can only dream about.

For reference, let’s start with the configuration of the ever-popular ATmega*8, the core of the Arduino series:

Here we have a color-coded diagram showing the pins with all their alternate functions.  There are a total of 3 ports, only two of which have all 8 bits.  Port C is missing PC7 entirely, and PC6 is generally unavailable as it is multiplexed with the RESET pin, required to reprogram the device.  Port B will be lacking PB6 and PB7 in most applications, as they are multiplexed with the crystal driver.  In addition, notice that the pins of a given port are not only scattered around in various places on the chip, but not necessarily even in order.  The ATmega*8 does better than some, and certainly light-years better than any PIC I’ve seen, but it’s still a routing challenge waiting to happen.

Even more than the pin orderings, notice the fact that there’s only one serial port, one I2C (sorry, TWI) port, and one SPI port.  Three timers give a total of 6 potential PWM outputs if you don’t need the timers for anything else, and you don’t need the SPI port that overlaps 2 of them.  Analog is spread between the 6 ADC pins on Port C (two of which are lost if you need I2C), and the comparator steals another PWM output from a different port entirely.  However a major upgrade to the ATmega*8 series versus previous generations is the addition of the PCINT* capability.  Instead of being stuck with just INT0 and INT1 for external interrupts, every single pin can be configured to trigger one of a cluster of interrupts.

Now let’s look at another popular AVR in a bigger package, the ATmega1284:

This looks a lot better, due in part to the larger package.  Not only do we get all 8 bits of every port, but they’re actually all in order.  We gain an additional serial port (TXD/RXD1) though without synchronous capability (no XCK1).  A couple more ADC pins are available, since Port A is complete and the TWI pins have moved elsewhere.  We’re still stuck with only 6 PWM’s, but only one of them is potentially unavailable, and only if the SPI module is used in slave mode (since SS# can be moved anywhere when the chip is in master mode).  The RESET# and XTAL pins have also moved to their own dedicated pins, so that’s even fewer lost pins, though with the drawback that we end up “losing” two pins if there’s no crystal attached.

Now let’s take a look at the ATxmega*A4, the smallest of the new line:

Right off the bat we notice a slight change: the package is no longer DIP, but TQFP.  This is the main drawback of the chips: they’re only available in surface-mount package.  However, I’ve rectified that by developing (and selling) adapters that convert the chips into standard DIP pinouts: (insert link here).

The next thing you should notice is a preponderance of highlighted pin functions.  Instead of 9 or 11 “major” alternate functions (serial, TWI, SPI), we have 27.  This chip has 5 serial ports, 2 TWI ports, and 2 SPI ports.  Even better, every single one is identical from a software perspective, but more on that later.  We also see a total of 12 ADC inputs, and even two DAC outputs!  Spread between ports C through E we find 16 PWM outputs, and the diagram doesn’t even bother showing the “PCINT” functionality, because every single pin of every single port is capable of various types of interrupts.  The crystal pins are available for use as a normal port (R) if you only need the internal oscillators.

A key feature is the fact that the programming pins are completely dedicated to the task.  Marked in purple above, RESET# and the CLK/DATA pins are all that are needed to program the Xmega chips (besides reference power and ground).  These pins are never multiplexed with anything else, so no more careful wiring of the SPI port so you can still flash the chip…

On the bigger end of things, we have the ATxmega*A1 chips:

Being the largest chip in the series it has 100 pins.  You should be able to click on the above image to get a larger one you might be able to read the labels on…

Working from ports A to R, this chip has: 16 inputs on 2 separate ADCs, 4 outputs on 2 separate DACs, 8 serial ports, 4 TWI ports, 4 SPI ports, 24 PWM outputs, and a memory interface capable of both SRAM and SDRAM up to 16MB.  A “timer” crystal connection is available on the extra 4 pins at the top just in case.

The pin arrangement is very clean, with every port in order around the chip, all contiguous, and all running in the same pin order (though the same can’t really be said of the BGA version, Atmel has been made aware of the serious flaws in pin placement there…).  There are power and ground pins for every port, capable of 200mA each.  In particular, that makes the chip capable of driving 20mA on every single pin simultaneously, a potential boon for those using discrete LEDs.

(Part 2: structural differences in how registers are managed make the plethora of peripherals more manageable)


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