EE 476: Laboratory 4

Serially controlled, sinewave generator.

You will produce a sinewave generator which is controlled from an RS-232 serial line connected to a PC window (e.g. hyperterm). The sinewave generator frequency will be set by numbers typed on the PC keyboard. The code will display the frequency in the terminal window on the PC.

You will need to build a simple digital-to-analog converter as shown below.

You may want to make a table of sinewave values in flash memory and read them back at a variable rate to make different frequencies. I suggest using timer 1a to generate the timebase for samples. The following code fragment (in a .cseg) produces such a table.

 ;***************************
   ;table of values for a sinwave with peaks of 0xff and 0x00,
   ;and a period of 16 samples 

   sintbl: .db 0x80, 0xb1, 0xda, 0xf6, 0xff, 0xf6, 0xda, 0xb1 
           .db 0x80, 0x4f, 0x26, 0x0a, 0x00, 0x0a, 0x26, 0x4f
 ;*************************** 

Later in the program, you can use an LPM command indexed with the Z register to read out the table.

See the Atmel application notes for 16x16 bit fixed point divide code. There is also a 24x16 bit divide code written locally.

There are at least two ways of handling serial i/o:

• Polled transmit and receive functions. This scheme blocks the cpu while executing, but is often easier to setup. A demo program shows fairly high-level string send and receive functions. Another demo program shows one way of parsing numbers from from an input string.
• Interrupt driven transmit and receive functions. This scheme allows the fast cpu to perfrom other functions while waiting for the slow serial interface. A demo program shows basic UART transmit and receive interrupt routines for the AT90S4414 mcu. Note that the 'transmit buffer empty" interrupt is set when the mcu is reset. This means that you must initialize the message pointer before you enable interrupts, or you will get garbage sent to the terminal at each reset.

The serial port (connector J131) should be connected to COM2 of the PC. You should use a simple terminal program, such as Hyperterminal, on the PC connected to COM2. The terminal program should be set to 9600 baud, no parity, one stop bit, and no flow control. You can use either demo program as a test that the RS232 connnections and terminal program on the PC are correct.

The UART receive is on PortD0, which should be connected to J132, pin1 (the left pin when the buttons are near you). The UART transmit is on PortD1, which should be connected to J132, pin2. These connections assume a RS232 cable to the PC with straight-through connection.

Write a program and build a circuit which will:

• Generate sinewaves with 16 samples (or more) per cycle and a frequency range from 500 to 5000 Hz (or more). The sinewaves should be built from 8-bit samples and converted to a voltage with a DAC.
• Accept the ASCII command 'g' typed from a terminal window on the PC as a command to start sending an 8-bit sinewave output to a DAC and to print the output frequency in the form Freq= xxxx Hz(cr)(lf). The (CR)(LF) represents carriage-return and line-feed characters. The xxxx represents a four digit ascii number.
• Accept the ASCII command 's' typed from a terminal window on the PC as a command to stop the sinewave output and to prompt the user for a new frequency. The prompt will be Freq>. The program will then accept exactly four characters and convert the four characters to a new frequency to be generated.

When you demonstrate the program to a staff member, you should exercise the keyboard commands typed from the PC and show that the generated frequency is correct.

Your written lab report should include:

• The scheme you used to decode commands.
• The scheme you used to convert the ASCII input to a frequency.
• A heavily commented listing of your code.
• A schematic of your circuit.
• A waveform captured from the oscilloscope showing the waveform you generated.