June 2000, Issue 119

The Chips are Alive with the Sound of Music
Imitatiing the Dead Melody IC

TOS

In the synthesizer industry, TOS means top octave synthesizer, which was developed in the ’70s for electronic organs. A 1- or 2-MHz input would be internally divided to produce 12 equally spaced frequency outputs. The 12 semitones comprise the musical scale, an octave. An octave is the interval between two notes where one frequency is twice or half of the other. When an octave is produced, dividing a note’s frequency by two derives the remaining notes in lower octaves. Hence, notes are based on the single clock’s input frequency.

Because TOSs are hardware devices, no overhead was necessary. Doing this with a processor requires hardware PWMs or an execution speed that supports software PWM. Hardware PWMs are not available on these low-end micros. Execution speed of both of these is 1 µs using their internal 4-MHz oscillators. Software divide-by-N counters can be coded using an interrupt routine. Look at Figure 2 to see how this is accomplished.

At least two divide-by-N counters are part of the timer overflow interrupt, there is an accurate time base tick. Upon N counts or ticks, an action is performed. Here, two actions are necessary. The first, frequency creation, is done using an 8-bit count and by complementing an output bit each time the divide-by-N reaches zero. Two transitions of this bit equal one cycle of output frequency (period = two divide-by-N counts).

The second action concerns duration, which requires a 16-bit count. This is the duration of the fastest note, in this case I’m using the eighth note (one beat in x/8 time). The longest path through this routine must include reloading the "N" each time a divide-by-N counter reaches zero for a single note (frequency counter) and a 23-µs duration (beat counter). Although I could reload timer0 with any number greater than the maximum interrupt time, I used 64 µs to give the rest of the code enough time to execute between interrupts.