gnupic: Re: [gnupic] about data tables and shared variables
Subject:
Re: [gnupic] about data tables and shared variables
From:
Rob Pearce ####@####.####
Date:
30 Aug 2008 21:30:15 -0000
Message-Id: <200808302229.17415.rob@bdt-home.demon.co.uk>
On Saturday 30 August 2008, Lope Vega wrote:
> One of them is about data tables. I'm trying to drive 3 7-segment displays
> which I've got multiplexed. Since I don't really yet understand table
> reads/writes, and haven't seen an example which adequates to what I'm
> doing, I thought on using `computed gotos':
I notice somebody has already pointed out the need to set PCLATH. Nobody seems
to have mentioned the word/byte addressing thing. Suffice to say that if
you're using an 18F family device then each entry of your retlw table
occupies two addresses, so you probably want to shift W left before adding it
to PCL.
Alternatively you could use table reads. It would look something like this:
read_table:
movwf TBLPTRL
movlw high(table)
movwf TBLPTRH
clrf TBLPTRU
tblrd *
movf TABLAT,w
return
datapage code 0x200 ; Force the table to a page boundary
db b'01000000', b'01111001' ; 0 and 1
db b'00100100', b'00110000' ; 2 and 3
.... etc
Note that I've put two values on each line of the data. Unfortunately it
appears that if you define a single byte in the code space then gpasm
allocates a whole word for it. What you want is to allocate a byte per value,
so you need to persuade gpasm to do that by supplying two values per line.
Hope that helps,
Rob
