UART and Serial Communication

The ATmega168 has tons of hardware built into it. Let’s unleash the serial communication. Everyone has programmed the ‘Hello World’ program. You’ve got your micro breadboarded and running at 16MHz. You’ve got WinAVR up and running. We’ve already demonstrated LED control. Now it’s time to pass some serial data back and forth.

I am not a huge coder. I just want my printf() statement to do what it is supposed to do. I don’t use a hardware debugger, I debug by printf statements. Sure, there are limitations with this, but for 90% of the applications out there, debugging with printf statements works just fine.

First, a quick history of RS232. What is RS232? It’s just a name for a standard that has propagated from generation to generation of computers. The first computers had serial ports that used RS232, and even current computers have serial ports (or at least USB ports that act like RS232 ports). Back in the day, serial information needed to be passed from devices like printers, joysticks, scanners, etc to the computer. The simplest way to do this was to pass a series of 1s and 0s to the computer. Both the computer and the device agreed on a speed of information – ‘bits per second’. A computer would pass image data to a printer at 9600 bits per second and the printer would listen for this stream of 1s and 0s expecting a new bit every 1/9600 = 104us (104 micro-seconds, 0.000104 seconds). As long as the computer output bits at the pre-determined speed, the printer could listen.

Zoom forward to today. Electronics have changed a bit. Before they were relatively high power, high voltage devices. The standard that is ‘RS232’ dictates that a bit ranges from -12V to +12V. Modern electronics do not operate at such high positive and negative voltages. In fact, our ATmega168 runs 0V to 5V. So how do we get our 5V micro to talk the RS232 +/-12V voltages? This problem has been solved by the IC manufacturers of the world. They have made an IC that is generically known as the MAX232 (very close to RS232, no?).

The MAX232 is an IC originally designed by a company called Maxim IC that converts the +/-12V signals of RS232 down to the 0/5V signals that our ATmega168 can understand. It also boosts the voltage of our ATmega168 to the needed +/-12V of the RS232 protocol so that a computer can understand our ATmega168 and vice versa. To get our ATmega168 IC sending serial characters to a computer, we have to send these serial signals through a MAX232 circuit so that the computer receives +/-12V RS232 signals. Don’t worry if you’re working with a chip labeled ‘ICL232’ or ‘ST232’ – these are just generics of the MAX232. Everyone says ‘MAX232’ just like they say ‘Kleenex’ or Coke. The ICs all function the same and nearly all have the same pinout.

The MAX232 circuit that we will be breadboarding looks like this:

MAX232 Circuit – Eagle schematic / PDF

This MAX232 IC uses three 0.1uF capacitors (C5, C6, C7) to operate (go read about ‘charge pumps‘). You must have these installed. The forth (C8) is what is called a ‘decoupling cap’. As the MAX232 IC switches various signals (from +/-12V to 0/5V) it uses bits of current. Because it needs these bits of current in bursts, it can disrupt your 5V supply. The C8 0.1uF capacitor helps ‘decouple’ or remove the ill effects of this IC (switching back and forth) from your power supply. This decoupling cap should be placed near the VCC and GND pins of the IC. This helps remove noise from your power system. Will your breadboard work without decoupling caps? Sure it will! Go without! But the day will come when something stops working and you’re not sure why. Could it be my code? Do I have a short somewhere? A disconnect? Or perhaps I don’t have enough decoupling caps?

A decoupling cap is meant to provide a quick burst of energy if the power supply dips down – sort of like a UPS system for your IC. The further the decoupling cap is from the IC, the less ability it has to provide that quick burst (long wires have intrinsic capacitance of their own). It’s always good engineering practice to have at least one 0.1uF cap near any IC. Placing them within 0.5″ of the VCC and GND pins is good. Placing them all the way across your breadboard won’t do harm, they just won’t provide as much help.

JP2 is a DB9 connector. It’s called a ‘DB9’ connector because it contains 9 pins and is used universally for serial connections. You’ll need a male to female serial cable to connect your breadboard’s DB9 connector to the computer. The ‘male’ end of the cable has the metal pins, the ‘female’ end has the black colored plastic that receives the pins. If you look very close at a DB9 connector in real life, you can just make out some small numbers next to the holes.

So what all does this do? The ATmega168 is going to send 5V signals to the MAX232 IC. The MAX232 IC in turn will convert those 5V signals to +/-12V RS232 signals that the computer can understand through the DB9 port on the back of the computer. Admittedly this can be a bit ugly to setup at first. Will you believe me that once setup, this will be your life-line to sanity? The serial connection is everything! You’ll need one on almost every application you do.

Breadboard with MAX232 and large loop-back jumper installed

Once you have everything wired, you’ll need to open up a terminal program. If you’re playing under Windows, you can open up the included ‘Hyperterminal’ program normally located under Programs->Accessories->Communications.  Linux and Apple people, you probably know how to get a terminal program running (sorry I can’t be more help!).

All terminal programs have the same basic function: to do serial. All you need to specify is a few simple rules to get your micro playing successfully with your computer. Let’s just get through the Hyperterminal screens:

Call it whatever you want

More than likely, the serial port on your computer is COM1

You want 9600bps 8-N-1 without flow control

The main settings are 9600bps and 8-N-1. This means that the micro and the computer agree to talk at a rate of 9600bits per second (bps) and that each byte will have 8 data bits, with no parity bit, and only 1 stop bit. This ‘8-N-1’ is very common and basic. If you like pain, go read about parity, 1.5 stop bits, and 5 data bits. No one really uses it in the breadboarding world.

Mkay, you’ve got hyperterminal open and kicking. You’ve got your MAX232 (or equivalent circuit) built up and powered on. Before you connect it to your micro, you should test that the MAX232 circuit works. The easiest way to test a MAX232 circuit is to tie TX and RX together. It’s called a ‘loop-back’ (the big yellow wire pictured above). Pretty self explanatory, but just follow along:

When you press the ‘A’ key on the computer in the hyperterminal window, a series of 1s and 0s get generated and pumped out the serial port on the back of your computer (8 bits: ‘01000001’ to be specific = 65d = 41h – see for more info). These 0s and 1s hit the MAX232 on your breadboard which dutifully changes these RS232 signals to TTL signals. The 0s and 1s get asserted on the R1OUT pin. Because you’ve tied the TX and RX pins together (R1OUT should be shorted to T1IN) these 0s and 1s get sent right back out the MAX232 and down the DB9 serial cable. Upon hitting the computer, the computer ‘sees’ these 1s and 0s and says ‘oh! there is a device passing me the ASCII character A’. The computer then displays the character ‘A’ in the hyperterminal screen. This is the essence of a loop-back test. If everything is kosher, you should be able to jam away on the keyboard and see those letters echoed back to the terminal window. Pull the jumper out and the characters should stop echoing. Got it? Use it! In the future, when you need to test a serial interface, short TX and RX together to make sure things are working correctly.

All right, you’ve got the MAX232 working correctly. Now connect the TX and RX pins of the ATmega168 to the MAX232 circuit.

ATmega8 with power supply and MAX232 circuit. Eagle schematic / PDF

You may have noticed C9 magically appeared next to the ATmega168 in the schematic above. This is a 0.1uF decoupling capacitor for the ATmega168. A 0.1uF capacitor places near the ATmega168’s VCC and GND pins will help reduce power supply noise being injected into the ATmega168. Again, your board will more than likely run without decoupling caps but I just want to instill in you a habit of using 0.1uF like candy.

TX and RX connections between MAX232 and ATmega8

Savvy travelers will note (upside down) the picture MAX232 IC is actually a SP3232(EBCP). What is this ‘SP3232’? It’s a the Sipex generic of the MAX232. Notice the ‘3’ in front of the 232? The original MAX232 ICs were designed to interface 5V logic to RS232. Because circuits started to run on lower voltages (3.3V for instance) the IC manufacturers had to redesign the MAX232 ICs to be more efficient so that they could take this lower voltage and boost it up to 12V for RS232. Hence the 3V designation ‘SP3232’. This IC can input 3V TLL signals and successfully convert them to RS232. We are operating our breadboard at 5V but we are able to run our MAX232 from 3V up to 5V without problems.

Trivia: In the picture above, which IC is the older sibling? These ICs have simple date codes: 0641 and 0625 means both ICs were manufactured in 2006 in the 41st and 25th weeks of the year.

You should now have the hardware in place to allow you to do printf statements. Let’s mess with some code! [Source]

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