TMS320VC5501 Datasheet, PDF (129/193 Pages) Texas Instruments – TMS320VC5501 Fixed-Point Digital Signal Processor

Functional Overview
3.17 Notice Concerning TCK
Under certain conditions, the emulation hardware may corrupt the emulation control state machine or may
cause it to lose synchronization with the emulator software. When emulation commands fail as a result of the
problem, Code Composer Studio Integrated Development Environment (IDE) may be unable to start or it
may report errors when interacting with the TMS320C55x DSP (for example, when halting the CPU,
reaching a breakpoint, etc.).
This phenomenon is observed when an erroneous clock edge is generated from the TCK signal inside the
C55x DSP. This can be caused by several factors, acting independently or cumulatively:
• TCK transition times (as measured between 2.4 V and 0.8 V) in excess of 3 ns.
• Operating the C55x DSP in a socket, which can aggravate noise or glitches on the TCK input.
• Poor signal integrity on the TCK line from reflections or other layout issues.
A TCK edge that can cause this problem might look similar to the one shown in Figure 3−53. A TCK edge that
does not cause the problem looks similar to the one shown in Figure 3−54. The key difference between the
two figures is that Figure 3−54 has a clean and sharp transition whereas Figure 3−53 has a “knee” in the
transition zone. Problematic TCK signals may not have a knee that is as pronounced as the one in
Figure 3−53. Due to the TCK signal amplification inside the chip, any perturbation of the signal can create
erroneous clock edges.
As a result of the faster edge transition, there is increased ringing in Figure 3−54. As long as the ringing does
not cross logic input thresholds (0.8 V for falling edges, and 2.4 V for rising edges), this ringing is acceptable.
When examining a TCK signal for this issue, either in board simulation or on an actual board, it is very important
to probe the TCK line as close to the DSP input pin as possible. In simulation, it should not be difficult to probe
right at the DSP input. For most physical boards, this means using the via for the TCK pad on the back side
of the board. Similarly, ground for the probe should come from one of the nearby ground pad vias to minimize
EMI noise picked up by the probe.
Code Composer Studio, TMS320C55x, and C55x are trademarks of Texas Instruments.
December 2002 − Revised November 2004
SPRS206H 129