From: Harold Hallikainen <harold@mai.hallikainen.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Wed, 29 Apr 2020 22:35:18 -0700
Subject: Re: [PIC] Jump Tables and Memory Access on PIC18F47K40
Thread-Topic: [PIC] Jump Tables and Memory Access on PIC18F47K40
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You only need to mess with PCLAT, PCLATH, and PCLATU if you are directly
manipulating PCL. The PC can deal with all of memory fine on its own. But
when you write to it (typically for a table read), you can only write the
lowe 8 bits, so the higher bits are pulled from the PCLAT. So, previously,
your table was in low enough memory that PCLATU was not needed (0x00
loaded at reset was fine).

Harold


> Hi Nicola,
>
>  =C2=A0 That was the piece I was missing, you nailed it! I was pretty sur=
e it
> had something to do with PCLAT, and even looked at it in the data sheet
> but somehow missed the "upper" option. What confuses me now is how was
> my program able to run correctly all this time without it? Until now, my
> program was living in flash below the 64K boundary, but I would think
> that crossing over that point would only require the Most Significant
> Bit of the address. So I do not know how it managed to jump correctly
> without that part of the address being set all this time.
>  =C2=A0 Anyway, I have fixed it everywhere I use it and the program is
> working as expected. Wish I would have asked for help days ago!
>
> Thanks!
> Rob
>
>
> On 4/28/2020 4:29 PM, Nicola Perotto wrote:
>> Hi Rob,
>>
>>
>> On 29/04/2020 00:53, Rob wrote:
>>> Hello Group,
>>>
>>>    =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=
=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 ;values =
are in the
>>> range of 0 to xx for
>>> jump table offsets
>>>    =C2=A0=C2=A0=C2=A0 movlw=C2=A0=C2=A0=C2=A0 high(KeyModeJumpTable)=C2=
=A0=C2=A0=C2=A0 ;load high address
>>> byte in PCLATH
>>>    =C2=A0=C2=A0=C2=A0 movwf=C2=A0=C2=A0=C2=A0 PCLATH
>>>    =C2=A0=C2=A0=C2=A0 rlncf=C2=A0=C2=A0=C2=A0 vKeycode,W=C2=A0=C2=A0=C2=
=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 ;multiply
>>> by 2
>>>    =C2=A0=C2=A0=C2=A0 movwf=C2=A0=C2=A0=C2=A0 vTempX=C2=A0=C2=A0=C2=A0 =
=C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0=
 ;temp
>>> storage
>>>    =C2=A0=C2=A0=C2=A0 movlw=C2=A0=C2=A0=C2=A0 KeyModeJumpTable
>>>    =C2=A0=C2=A0=C2=A0 addwf=C2=A0=C2=A0=C2=A0 vTempX,w=C2=A0=C2=A0=C2=
=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 ;TableIndex
>>> from 0 to number of
>>> items -1
>>>    =C2=A0=C2=A0=C2=A0 btfsc=C2=A0=C2=A0=C2=A0 STATUS,C=C2=A0=C2=A0=C2=
=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 ;check Carry
>>> flag
>>>    =C2=A0=C2=A0=C2=A0 incf=C2=A0=C2=A0=C2=A0 PCLATH,F=C2=A0=C2=A0=C2=A0=
 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 ;gotcha!,
>>> compensate for carry
>>>    =C2=A0=C2=A0=C2=A0 movwf=C2=A0=C2=A0=C2=A0 PCL=C2=A0=C2=A0=C2=A0 =C2=
=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =
=C2=A0=C2=A0=C2=A0 ;now
>>> jump safely
>> you need to use also PCLATU
>>   =C2=A0=C2=A0=C2=A0 movlw=C2=A0 Upper(KeyModeJumpTable)
>>   =C2=A0=C2=A0=C2=A0 movwf=C2=A0=C2=A0=C2=A0 PCLATU
>>
>>   =C2=A0=C2=A0=C2=A0 Nic
>>
>
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