gil@jtsv16.COM (gil) (06/27/90)
Greetings Networld,
[ I posted this earlier in the week but I don't think it left this
computer's memory. Sorry for the inconvenience if you've already seen
this, but here goes ... ]
Following are two requests:
1. ADDING LED BACKLIT LCD DISPLAY TO CMOS PROCESSOR
---------------------------------------------------
I have a requirement to add an LCD display with LED backlighting to a
CMOS processor (68HC705). The display itself was a piece of cake ...
since it draws only ~2mA I used a direct connection to the I/O ports.
The LED backlighting, however, seems slightly more tricky (to me,
anyway). When the LEDs are on the unit draws 180mA which I think is
too much for the I/O pins on the 'HC705.
I think I can get around this by using a transistor but with my
limited electronics background I'm not sure about this. Could some
kind sci.electrician please direct me on this?
2. MATRIX KEYPAD FUNNIES
------------------------
I added a matrix keypad to the above processor ('HC705) again using
parallel ports directly, as follows:
P0 >------------------------------+
|
P1 >------------------------+ |
| |
P2 >------------------+ | |
| | |
P3 >------------+ | | |
| | | | R1
P4 <------------O-----O-----O-----O--/\/\/\----+
| | | | R2 |
P5 <------------O-----O-----O-----O--/\/\/\----+
| | | | R3 |
P6 <------------O-----O-----O-----O--/\/\/\----+
| | | | R4 |
P7 <------------O-----O-----O-----O--/\/\/\----+
|
_ (gnd)
where >'s are the port outputs, <'s are the port inputs, O's are the
keypad switches and /\/\/\ are resistors.
Everthing seems OK expect that the P7 line seems to not fuction
correctly unless I have my 'scope probe attached to it. Sounds to me
like some sort of capacitance problem although with my limited
experience I am not sure. Again, could some net.soul set me straight
about this?
Thanks in advance,
Gil
+-----------------------------+-------------------------------------------+
| Gil Hauer | |
| Tech Noir Inc. | gil@jtsv16.jts.com |
| Toronto +1 416 653-8276 | or: uunet!jtsv16!gil |
+-----------------------------+-------------------------------------------+