Figure 1-8 Target Interface Logic Levels - ARM DS-5 Manual

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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:

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 50Ω.

Related references

1.3 ASIC guidelines on page 1-18.

1.4.1 PCB connections on page 1-20.

ARM 100956_0527_00_en

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1 ARM DSTREAM System Design Guidelines

Target VTref (V)

Figure 1-8 Target interface logic levels

1.4 PCB guidelines

Voutput high Level - Voh

Vinput threshold - Vi (th)

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