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N2HET: Update the bit field "Next program address"

oliver.gruendonner
Intellectual 830 points

Hi.

I have three questions regarding the TMS570LS31x devices, module N2HET. I want to update the "Next program address" bit field of the Program Field of a BR instruction, in case the N2HET is turned on, i.e. HETGCR.TO = 1. (Just as a note: I think the question is the same for all N2HET instructions.)

In the E2E discussion "Stop CNT count on NHET" (http://e2e.ti.com/support/microcontrollers/hercules/f/312/p/240786/845681.aspx) is stated several times, that the Program field of N2HET instructions should not be updated by a CPU write access in case the N2HET is ON (i.e. HETGCR.TO = 1).

In this E2E discussion "Stop CNT count on NHET" has Anthony posted (Posted by Anthony F. Seely on Jan 28 2013 10:02 AM) the following: "You can safely update the NEXT field of an instruction through the new 'REMP' destination that is available on the ADD, SUB, etc.. instruction. ". But I don't understand how this should work. Reason: If I read in spnu499b.pdf, chapter "20.5.3.4 ADC, ADD, AND, OR, SBB, SUB, XOR" the following pages, it seems to me, that it's only possible to update the least significant 9 bits of the Program Field:
*) Page 840, 2nd paragraph
*) Page 841, Table 20-54, line for REMP
*) Page 844, below the comment "WRITE REMOTE DESTINATION STAGE"
But the bit field "Next program address" is not part of the least significant 9 bits.

So the three questions are:
1. Is it possible to update the bit field "Next program address" with the ADC, ADD, AND, etc. instructions? If yes: Can you state a short example how this is possible?
2. If the answer of question 1 is no: IMHO the only possibility, to update the bit field "Next program address", would be with the following sequence:
   a. Turn the N2HET off (i.e. set HETGCR.TO = 0).
   b. Update the bit field "Next program address".
   c. Turn the N2HET on (i.e. set HETGCR.TO = 1).
3. Is there an other possibility to update the bit field "Next program address" in case the N2HET is turned on?

Regards,
Oliver.

  • 0
    TI__Guru 69070 points

    Oliver,

    Thanks for catching the error - I corrected the other post.   It is the REMOTE field that can be updated by the new instructions ADD, ADD, AND, etc. not the next program address field.

    To update the next program address field you do need to halt the N2HET as you mention in 2.

    The ability to modify the REMOTE field of an instruction was added to as a way to facilitate subroutine creation.   There isn't any stack on N2HET and the working register count is quite limited.  But lets say you have an alogorithm that is processing some array of measurements;  you can change the 'pointer' to this data array by modifying the REMOTE field of key instructions in the subroutine before branching to it. 

    To change the 'return address' you can't write to NEXT PROGRAM ADDRESS, because this is in the P field of an instruction.   However, you could place a "BR" instruction in your subroutine for the return instruction and use the MOV64 instruction to modify the 'Conditional Address" field of the BR instruction.  If you set the instruction for event=NOCOND then the branch will occur unconditionally and it will always use the Conditional Address.