Posted: Fri May 05, 2006 12:56 am
Hi there Robert,
In my counter, i never intended to go through the trouble of
lighting the decimal segment even though i intended to use it with
a 10 second time base (as well as a 1 second time base) to
get readings with resolution of 0.1 Hz (20MHz was my goal using
a count of 200000000 and 'implied' dp to the left of the right-most
digit). For one application i wanted to look at the stability of
various kinds of oscillators.
But, if i was going to light the decimal point, i would imagine i would
have to multiplex that as an 8th segment, maybe using 3 bits of a
4 bit latch to hold the current position of the dp (zero through seven).
The latch outputs would then run to a 'one of eight' decoder
and the decoder outputs could be scanned with NAND or NOR gates,
the output of these gates driving the dp 'segment'. The scan pulse
comes from the scan generator that scans the digits and occurs
at the correct point in time because the two counts are equal only
when the scan count is the same as the dp count.
Alternately, we could use a comparator logic package to detect 'equals'
between the dp latch outputs and the scan count in binary. When the
two are equal a pulse outputs from the comparator.
Also, an neat alternative to using a dp latch is to use instead another
4-bit counter, where you can set the dp position by pulsing this counter.
That would be neat i think as you could move the dp up up up
and then it would reset back to the first digit after 8 pulses (maybe from
a toggle switch or from logic). If the counter was a presettable type
you could also load it from other logic or whatever.
The only trick is the dp has to be a pulse that occurs for the same
time the scan pulse occurs, and only for that digit.
There may be simpler ways too ... if you want we can give this
some more thought.
Let's see, the above would take the following logic packages:
Circuit #1
1. DP counter or latch
2. 1 of 8 decoder
3. one NOR gate package for placing 4 decimal positions, or two NOR
packs for placing the full 8 decimal positions.
Circuit #2
1. DP counter or latch
2. 4-bit logic comparator package
3. 1 of 8 decoder
Circuit #3
1. DP counter or latch
2. For placing the dp in up to 4 positions, program a 4 bit ROM to detect
equality between the scan count and the dp count and generate a 4 bit
dp scan pattern, or, for placing the dp in up to 8 different positions
program an 8 bit ROM to do same only generate an 8 bit dp scan pattern.
(The rom chip takes care of all the logic necessary to generate the
scan pattern to drive the decimal points, provided we have access to
the scan generator binary output. If we dont have access to this,
we can still do it but we'll require a larger ROM memory).
So circuit 1 takes either 3 or 4 packages, while circuit 2 always
takes 3 packages. Circuit 3 only takes 2 packages but requires
programming a rom chip (4 bit or 8 bit, with 256 memory cells),
and the rom program pattern is very easy to generate on a
computer.
Sound ok, or sound too complex for what it does?
Either way, sounds quite interesting to me now
In my counter, i never intended to go through the trouble of
lighting the decimal segment even though i intended to use it with
a 10 second time base (as well as a 1 second time base) to
get readings with resolution of 0.1 Hz (20MHz was my goal using
a count of 200000000 and 'implied' dp to the left of the right-most
digit). For one application i wanted to look at the stability of
various kinds of oscillators.
But, if i was going to light the decimal point, i would imagine i would
have to multiplex that as an 8th segment, maybe using 3 bits of a
4 bit latch to hold the current position of the dp (zero through seven).
The latch outputs would then run to a 'one of eight' decoder
and the decoder outputs could be scanned with NAND or NOR gates,
the output of these gates driving the dp 'segment'. The scan pulse
comes from the scan generator that scans the digits and occurs
at the correct point in time because the two counts are equal only
when the scan count is the same as the dp count.
Alternately, we could use a comparator logic package to detect 'equals'
between the dp latch outputs and the scan count in binary. When the
two are equal a pulse outputs from the comparator.
Also, an neat alternative to using a dp latch is to use instead another
4-bit counter, where you can set the dp position by pulsing this counter.
That would be neat i think as you could move the dp up up up
and then it would reset back to the first digit after 8 pulses (maybe from
a toggle switch or from logic). If the counter was a presettable type
you could also load it from other logic or whatever.
The only trick is the dp has to be a pulse that occurs for the same
time the scan pulse occurs, and only for that digit.
There may be simpler ways too ... if you want we can give this
some more thought.
Let's see, the above would take the following logic packages:
Circuit #1
1. DP counter or latch
2. 1 of 8 decoder
3. one NOR gate package for placing 4 decimal positions, or two NOR
packs for placing the full 8 decimal positions.
Circuit #2
1. DP counter or latch
2. 4-bit logic comparator package
3. 1 of 8 decoder
Circuit #3
1. DP counter or latch
2. For placing the dp in up to 4 positions, program a 4 bit ROM to detect
equality between the scan count and the dp count and generate a 4 bit
dp scan pattern, or, for placing the dp in up to 8 different positions
program an 8 bit ROM to do same only generate an 8 bit dp scan pattern.
(The rom chip takes care of all the logic necessary to generate the
scan pattern to drive the decimal points, provided we have access to
the scan generator binary output. If we dont have access to this,
we can still do it but we'll require a larger ROM memory).
So circuit 1 takes either 3 or 4 packages, while circuit 2 always
takes 3 packages. Circuit 3 only takes 2 packages but requires
programming a rom chip (4 bit or 8 bit, with 256 memory cells),
and the rom program pattern is very easy to generate on a
computer.
Sound ok, or sound too complex for what it does?
Either way, sounds quite interesting to me now