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Debug Port
12-28
Note
•
As shown in Overview of protocol operation on page 12-22, there is always a
turnaround between the end of the packet request from the host and the start of
the acknowledgement from the SW-DP target. The default turnaround is exactly
one serial clock cycle, but see the description of Trn in Key to illustrations of
operations on page 12-22 for more information.
There is a turnaround whenever there is a change in the direction of data transfer
over the serial SWD connection. If an operation that is described as immediate
involves a change in the data transfer direction then the operation must start
immediately after the turnaround.
•
All SWD transfers are made LSB-first. Therefore, the OK response of b001
appears on the wire as 1, followed by 0, followed by 0, as shown in Figure 12-8
on page 12-25 and Figure 12-9 on page 12-25.
If the host requested a write access it must start the write transfer immediately after
receiving the acknowledgement from the target. This behavior is the same whether the
write is to the DP or to an AP. However, the SW-DP can buffer AP writes, as described
in Access Port write buffering on page 12-32.
If the host requested a read access to the DP then the SW-DP sends the read data
immediately after the acknowledgement. Because there is no change in the data transfer
direction between the acknowledgement and the read data there is no turnaround
between these phases. This is shown in Figure 12-9 on page 12-25.
Read accesses to the AP are posted. This means that the result of the access is returned
on the next transfer. If the next access you have to make is not another AP read then you
must insert a read of the DP RDBUFF Register to obtain the posted result, see The Read
Buffer, RDBUFF on page 12-59.
When you must make a series of AP reads, you only have to insert one read of the
RDBUFF Register:
•
On the first AP read access, the read data returned is Undefined. You must discard
this result.
•
If you immediately make another AP read access this returns the result of the
previous AP read.
•
You can repeat this for any number of AP reads.
•
Issuing the last AP read packet request returns the last-but-one AP read result.
•
You must then read the DP RDBUFF Register to obtain the last AP read result.
Copyright © 2005, 2006 ARM Limited. All rights reserved.
ARM DDI 0337B
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