At 10:31 AM 24/04/96 -0400, you wrote:
>On Wed, 24 Apr 1996, Prashant Bhandary wrote:
>
>> Have you looked at the surface mount version of 16c84? Not much can be
>> gained by grinding that critter down! I've had this idea for a tiiiny
>> circuit but the challenge of taking on SMT and the lack of a cheap,
>> small SMD H bridge and crystal have stopped me so far.
>
>Use a ceramic resonator.  You can use 2 pins from 16c84 as a 20 milliamp H
>bridge.  What would you use for a tiiiny source of power and tiiiny
>motors?
>

OK, remember you asked for this :-)

A new standard, DCC, has been established for model trains. The signal is
sent through the tracks. Normally the train has connections going directly
from the tracks to the motor and you control speed and direction by
controlling the voltage on the rails. But in this case, a decoder sits
in between and decodes the packet and drives the motor appropriately.
Now the decoder should fit inside a train and the scale of the train
defines the magnitude of the problem. I model N scale which is 1:160
and believe me when I say tiiiny I mean every i in that word.

As you can guess the power supply is connected to the tracks and can be as big
as you want it. The motor is already in the train. What remains is this
small space at your disposal to squeeze a PIC and an H bridge driver. The
16C84 is ideal for this job. It even has an EEPROM which is one of the
requirements. The SMT 4Mhz part is avilable in single unit quantities locally.
So is an SMT H bridge. The other SMT parts would be a few caps and resistors
and a bridge rectifier.

The ceramic resonator seems to be a good solution. The frequency would be 4 MHZ
as I need all the speed I can get. I will look around for an SMT version. The
PIC as an H bridge driver is not an option as the motor runs at a few hundred
milliamps(paralleling the outputs isn't enough either). I have found this SMT
H bridge  a little big and is a dual. I forget the part no. and I believe it
is a 24 pin gul wing SMT. I would like to see a single H bridge which is a
little smaller and possibly cheaper.

Regards

Prashant

P.S. I am posting this to the list too to get a few hints on SMT PIC apps.
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