The Maplin Electronic Circuits Handbook, 2nd Edition

192 COMPUTER INTERFACING

of FFH. The following Z80 code could be used to generate a low-frequency

stepped ramp waveform:

LOOP1

LOOP2

LDA, FFH

:

Generate all 256 voltage levels

LDC,0

:

Starting at zero

OUT (FFH), C

:

Send it to the DAC

INCC

:

Count up in C

DEC A

:

Count down in A

CPO

:

Finished one cycle?

JR NZ,LOOP2 :No

JPLOOP1 :Yes

keep going!

- now start another!

In practice, appropriate delays can be added to produce any desired

output frequency up to a maximum which is ultimately determined by

the microprocessor's clock frequency.

In applications where an appreciable value of output current is required,

an additional output buffer may