Holtek IR remote control simultaneous button presses

Hi,

I am a student trying to design an IR remote transmitter circuit using
the Holtek HT12A encoder chip
(http://www.holtek.com/pdf/consumer/2_12e.PDF). The circuit I have
designed is simple enough with the switches on the remote unit pulling
the D8-D11 active-low inputs to ground in a particular combination,
and hence causing the corresponding data bits to be transmitted. The
inputs D8-D11 internally default to logic 1 using pull-up resistors.
Transmission is enabled when a logic low is detected at any of these
inputs.

My problem is that when 2 switches are pressed simultaneously an
erronous code can be transmitted. Eg.:
Switch 1 = 1110
Switch 2 = 1101
Switch 3 = 1100

If in the example above, switches 1 and 2 are pressed simultaneously
then the code for switch 3 is transmitted.

I have solved this problem using an 74HC148 priority encoder chip but
I have never seen this used in any other design - it also seems too
heavy on hardware if I wanted to start this design into production.

Does anyone out there know any simpler way to prevent simulataneous
key presses from occuring? An ideal response would be for all switches
to be left floating (or pulled up) if another switch press is
detected.


I hope all you geniuses out there can be of some help!

Thanks,

Paul