On Thu, Jan 31, 2002 at 11:14:27AM +1100, Sean Alcorn - Avion Sydney wrote:
> Hi Guys,
>
> I am just about to embark on my next project, and am in the planning stages.
>
> Whilst I understand the difference between BCD and straight binary, I am a
> little confused about what actually determines whether a value is stored as
> BCD or straight binary.

It's all a matter of perspective.

>
> For example, the command;
>
> RETLW    99
>
> Would return the value 99 as BCD in W right?

Presuming that you have your default radix set to hex W will be 0x99.
Now what it means is a matter of perspective. If your software "decides"
that it's BCD then is has a value of 99. However if it "decides" that it's
hex when it'll have a value of 153 decimal.

>
> But what about;
>
> RETLW    255

Too big if the default radix is hex. 0xFF if radix is decimal.

> or
> MOVLW   99

Same as the 99 above.

> or
> MOVLW    255

Same as the 255 above. Just to make sure I tested with gpasm. Here's
the List line:
------------

0000 3055      00001         movlw   255

------------

Note that only the 0x55 part got transferred into the instruction.
The 2 was truncated.

>
> Obviously the 255 gets stored as straight binary, but how is that 'decision'
> made? Is 99 the maximum for BCD?

99 is the maximum for 2 digit BCD, just as 99 is the maximum decimal for
2 digits. You can have multitype BCD for as many digits you need.

I'd like to help you. Managing BCD and converting to and from decimal
is a bit of work. For my digital thermostat project I had to read binary
Celsius, convert to binary Fahrenheit, convert that to BCD, and finally
display the result on a 2 digit 7 segment display. You can get the code
from my PIC page: http://www.finitesite.com/d3jsys

Hope this helps,

BAJ

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body