>/ <drasko.draskovic at gmail.com
<https://lists.berlios.de/mailman/listinfo/openocd-development>> 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) ?
both 361540 bytes in 182.220215s ??? please correct.
If you do not need to read TDO please use fields[0].in_value = NULL , ever !
If you use fields[0].in_value = &spracc_in instead fields[0].in_value = NULL, the communication USB to Amontec JTAGkey dongles could
be slower, because the TDO must be returned back to host computer, and more USB access will be generated.
Laurent Gauch
http://www.amontec.com
http://www.amontec.com/jtagkey.shtml
Amontec JTAGkey-2 : Hi-Speed USB JTAG Cable
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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