The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 13
The DAC is configured in its flow-through and voltage-switching modes. In flow-through
mode, the analog output continuously reflects the data inputs. The chip has several control
signals for latching inputs and outputs, but these aren’t needed by our circuit.
In voltage-switching mode, the analog output is a voltage proportional to the value of the
byte formed by D0-D7. An LM385 2.5-volt reference is applied across current output
terminals IOUT1 and IOUT2, and the output appears at VREF. (This configuration is the inverse
of the device’s current-switching mode, where VREF is an input and IOUT1 and IOUT2 are
outputs, as their names suggest.)
Op amp U2A buffers the output, and U2B is a low-pass filter that helps to smooth VOUT.
A BASIC Program
Listing 13-4 causes a sine wave to appear at VOUT. The sine wave represents the value of
the trigonometric sine function for an angle that varies continuously from 0 to 360 degrees,
or 0 to 6.28 (2*PI) radians. Lines 90-110 are a loop that selects 256 equally-spaced points
along one cycle of the sine wave, calculates the sine for each, and stores the values in RAM.
The program uses BASIC-52’s SIN operator in calculating the values. Sine values normally
vary from +1 to -1, but line 100 adjusts the values so that they vary from 0 to 255, which is
the range of inputs accepted by the 8-bit DAC. Using these values, 0 is the negative peak,
255 is the positive peak, and the zero crossing occurs midway between points 127 and 128.
To generate the sine wave, Lines 150-180 are a loop that reads each value in sequence from
RAM and writes it to an output port at E400h. After writing a complete cycle, the program
loops back and begins another. The sine wave repeats endlessly, until the user presses
CONTROL+C.
Listing 13-4 creates a perfectly good sine wave, but at a very low frequency. Using
12-Megahertz crystal to clock the 8052, the frequency is only about 0.7 Hertz, or 1.5 seconds
per cycle.
Adding Assembly Language
To speed things up, Listing 13-5 is an assembly-language routine that performs the functions
of lines 150-180 in Listing 13-4. As in the original program, Listing 13-5 copies values in
sequence from RAM to E400h, repeating the sequence after 256 writes. The routine
illustrates a couple of major differences between BASIC and assembly-language programming.
One is that assembly language has no built-in FOR, DO, or WHILE loops. Instead, you
have to create loop structures from the instructions available. Listing 13-5 creates a 256-step
228 The Microcontroller Idea Book