At 10:40 PM 12/14/96 EST, you wrote:
>I need to operate a 1 milliamp panel meter using a 16C54.
>The meter will be used to display measured periods, which
>I have set up the timer to determine. The problem I am
>having concerns driving the analog meter from 0 to 1 ma.
>My first approach was to build a R-2R ladder using port_b
>pins (yes, all 8 of them). The problem with this is that
>the meter was loading down the ladder (the meter in series
>with 5 Kohm), so I addded a unity-gain op-amp. But now I need
>a rail-to-rail op-amp to drive the meter to its full
>range (from 0 to 5 volts).
>
>I started looking at other ways to accomplish the above meter
>drive. I looked at commercial DAC chips, but all that I have found
>need split supplies or 10 volts or more (I am driving the PIC
>with 5 volts - no other supply is available). Is there a DAC
>chip on the market which operates on single 5-volt supply?
>
Please try the Motorola MC144110/MC144111
Single supply 4.5 - 15 Volt, 6bit DA, 6ch/4ch
for more info please check their web http://www.motorola.com
I am not sure the URL is correct.

>I have also looked at using a PWM approach. The internal
>timer is running at a frequency of 125 Khz, so I could set up
>a PWM output on one of the pins. My questions are:
>
>1) What filtering do I need on the output?  I only need around
>10 Hz response on the meter, so I guess a simple RC would
>work (right???)
Yes, the RC filter is OK, but if the PWM frequency is only 10Hz
the response would be very slow, I was using about 110Hz in my
Ham Radio Keyer design to indicate the Morse code speed, it works
fine, but in my design the PWM is only 4bit :) I made the diagram
and source code available in my home page, but the code is acturely
for MC68HC705K1.

73(Best Regards)

Jerry Meng, BA1FB

mailto:ba1fb@amsat.org
http://www.srsnet.com/~ba1fb