On Thu, Apr 7, 2011 at 11:21 AM, Øyvind Harboe <oyvind.har...@zylin.com> wrote:
> On Thu, Apr 7, 2011 at 11:14 AM, Drasko DRASKOVIC
> <drasko.drasko...@gmail.com> wrote:
>> Hi all,
>> I have one general question regarding OpenOCD scans.
>>
>> What happens when we set some field.in_value = NULL, and in reality
>> there are some bits that re shifted out on the scan chain ? Are they
>> just ignored ? Does that pose the problem ?
>
> This tells the JTAG drivers to throw away the bits that are clocked in
> (read from the TAP). The target hardware/TAP does not see a difference.
>
>> To give a concrete example, in the mips_ejtag.c we have something like :
>>
>> /* fastdata 1-bit register */
>> fields[0].num_bits = 1;
>> fields[0].out_value = &spracc;
>> fields[0].in_value = NULL;
>>
>> /* processor access data register 32 bit */
>> fields[1].num_bits = 32;
>> fields[1].out_value = t;
>>
>> if (write_t)
>> {
>> fields[1].in_value = NULL;
>> buf_set_u32(t, 0, 32, *data);
>> }
>> else
>> {
>> fields[1].in_value = (uint8_t *) data;
>> }
>>
>> So, no values are received (because fields[0].in_value = NULL and
>> fields[1].in_value = NULL), but in reality MIPS EJTAG shifts out 33
>> bits (one SPrAcc and 32 bits of DATA) upon this operation.
>>
>> My question is, in general, could not accepting these bits lead to
>> some problems or collisions on data lines (or USB bus) ?
>
> The bits are simply ignored rather than stored in memory. The
> protocol or target hardware is not affected as such.
Thanks Øyvind,
this is what I thougth...
Is there a special reason why not ignoring one bit, like :
fields[0].in_value = &spracc_in instead fields[0].in_value = NULL in
previos example gives :
downloaded 361540 bytes in 182.220215s (1.938 KiB/s) in the place of
previous downloaded 361540 bytes in 182.220215s (1.938 KiB/s) when
fields[0].in_value was ignored (NULL) ?
BTW. I use Amontec D2XX because otherwise libftdi dies when I change
from fields[0].in_value = NULL to fields[0].in_value = &spracc.
BR,
Drasko
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