Nuts & Volts - June 2007

BY G.Y. XU

rate is 9600 bits per second, data is eight bits in one transaction, there is no error checking, and one start bit and one stop bit are attached to each transaction. So, it actually transmits 10 bits instead of eight bits for a byte of data.

To program the ATtiny11, you first assemble the source code using Atmel’s free assembler:

AVRASM -I UARTAVR.ASM UARTAVR.LST UARTAVR.HEX

A Simple Hardware and Software Implementation

The Atmel ATtiny11 is an inexpensive MCU (only 52 cents each or $38/100). It’s an eight pin chip, without a built-in serial port. However, using the circuit shown in Figure 2 and the simple assembly language program in Listing 1, we will show that a working serial port had been created for it.

This circuit is simple enough to build it on a breadboard. The LED is not required; it only serves as a reminder if you forget to plug in the power supply and the circuit is not working. Furthermore, you can even dispense with the 1 MHz crystal (XTAL) and use the chip’s internal 1 MHz RC-oscillator instead. Either way, the demo still works.

Refer to both Figure 2 and Listing 1 and notice that ATtiny11 pin 7 (PB2) is configured as RX, and pin 6 (PB1) is TX. The Dallas Semiconductor’s DS275 is a voltage level shifter which shifts the TTL level to an RS-232 level, and vice versa.

J1 is a female DB- 9 connector. Only three wires are necessary to connect to the PC serial port and match its DB- 9 male connector. You can use three longer wires starting from the breadboard and directly plug each end of wire into the female connector’s corresponding pin holes. This facilitates the connection between the DB- 9 and breadboard.

Now let’s look at the software program in Listing 1. It contains four sections. The first section starting at RESET is the main program. The remaining sections are three subroutines; their detailed executions will be analyzed later, but note that each subroutine corresponds to an essential part of the UART: bit rate generation, transmission, and reception. So, we have a software UART here.

Then you use an ATtiny11 programmer (such as the one published in the February ‘06 issue of Nuts & Volts). You can download all UARTAVR files from the Nuts & Volts website ( www.nuts volts.com). Notice that the size of this demo program is quite small — it only takes 94 bytes of program memory.

By the way, if you don’t have an ATtiny11 or its programmer, but you have an AT90S1200 (which is a 20-pin Flash MCU and doesn’t have a built-in serial port), you can still use the same program source code. The only change you need to make is in Figure 2; change component U2 to AT90S1200 and the corresponding pin numbers.

Any Windows PC has a terminal emulation program called Hyper Terminal, which is the one we will use to interact

with the demo circuit. From the Windows Start button, go ahead to HyperTerminal and configure it as connected to: COM1(or COM2), 9600 bps, 8N1. When the HyperTerminal window along with a “-“ cursor appears, it’s ready to go.

Power-up the demo circuit. You’ll see an “X” appear on the screen. Typing any character from the keyboard will be echoed on the screen (that’s what the program is expected to do). At this point, the software serial port is functioning okay.

Timing is Everything

First and foremost, the UART program must have a way to generate bit rates. This is done by calling the DLY49us routine, which delays 49 microseconds (µs).

Why go with a 1 MHz crystal? Because at a 1 MHz frequency, one clock cycle = 1 µs, and for all AVR MCUs, most of their instructions take just one clock cycle (or 1 µs) to execute. This makes the timing