Target interface logic levels
DSTREAM is designed to interface with a wide range of target system logic levels. It does this by adapting its output drive and input threshold to a reference voltage supplied by the target system.
VTref feeds the reference voltage to the DSTREAM unit. This voltage is clipped internally at approximately 3.4V, and is used as the output high voltage (Voh) for logic 1s (ones) on TCK, TDI, and TMS. 0V is used as the output low voltage for logic 0s (zeroes). The input logic threshold voltage (Vi(th)) for the TDO, RTCK, and nSRST inputs is 50% of the Voh level, and so is clipped to approximately 1.7V. The relationships of Voh and Vi(th) to VTref are shown in the following figure:
Figure 1-8 Target interface logic levels
DSTREAM can adapt interface levels down to VTref of 1.2V.
By default, the nTRST and nSRST signals are pulled-up by 4.7K resistors within DSTREAM and driven (strong) low during resets. This allows the reset signals to be driven by other open-drain devices or switches on the target board. The polarity and high/low drive strengths can be configured within the software.
The input and output characteristics of the DSTREAM unit are compatible with logic levels from TTL-compatible, or CMOS logic in target systems. When assessing compatibility with other logic systems, the output impedance of all signals is approximately 50Î©.