On 9/8/2016 3:35 PM, Charles Steinkuehler wrote:
> On 9/8/2016 12:41 PM, William Hermans wrote:
>>
>> It has long been programing technique to use DATAOUT |= BITx to set a
>> register
>> bit DATAOUT &= (~BITx) to clear a register bit, or something like if(DATAOUT
>> &
>> BITx){} or if((DATAOUT &BITx) == 0 or 1) to read a bit from a register.
>
> Yes, it has. But those short-hand snippits of C code are hiding an
> implicit read-modify-write cycle:
>
> DATAOUT |= BITx
>
> ...turns into:
>
> Read : <temp> = DATAOUT
> Modify: <temp> = <temp> | BITx
> Write : DATAOUT = <temp>
>
> ...where <temp> is probably a CPU register (unless your C compiler is
> _really_ bad!). But if you use the set/clear registers it's just:
>
> Write : SETDATAOUT = BITx
Oh...while we're on the subject, there's one more significant reason
the set/clear registers are helpful even if you're not worried about
multi-threaded operation: They're *FAST*
The ARM core on the BBB is running at 1 GHz, but the I/O is only
running at about 100 MHz. So that simple DATAOUT |= BITx stalls the
CPU until the system can provide the current value of DATAOUT to the
processor core, which likely takes a couple hundred nanoseconds (based
on the time it takes for the PRU to read a GPIO register).
The write, however, is very fast (for both the set/clear case and for
writing directly to DATAOUT). Writes are posted and the ARM has weak
memory ordering, so as long as you're not saturating the on-chip
interconnect fabric, the write will appear to complete instantly, the
ARM core will carry on executing instructions, and about 100 nS later
the GPIO register will update with the new value once the write
transaction has worked it's way through the on-chip interconnect fabric.
--
Charles Steinkuehler
char...@steinkuehler.net
--
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