9.4.2. Target interface logic levels

RealView ICE 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 (pin 1 on the JTAG header connector) feeds the reference voltage to the RealView ICE run control unit. This voltage, clipped at approximately 3.2V, 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 1.55V. The relationships of Voh and Vi(th) to VTref are shown in Figure 9.9.

Figure 9.9. Target interface logic levels

RealView ICE can adapt interface levels down to VTref less than 1V. If, however, VTref becomes less than approximately 0.85V, RealView ICE interprets this condition as Target Not Present, and the software reports this as an error condition.

The nTRST output from RealView ICE is effectively driven as an active low signal, so it is actively pulled to 0V but relies on a 4.7kΩ pull-up resistor to end the reset state. This is because it is common to wire-OR this signal with another source of nTRST, such as power-on reset in the target system.

The nSRST output from RealView ICE is a similarly-driven active low signal, and must be pulled-up with a resistor in the target system. Because this signal is also an input to the RealView ICE run control unit, there is a 4.7kΩ internal pull-up resistor. This is to avoid spurious lows on the input when nSRST is not connected to the target system.

See RealView ICE reset signals for information on the pull-up/pull-down resistances in the RealView ICE run control unit.

The input and output characteristics of the RealView ICE run control unit are compatible with logic levels from TTL-compatible, or CMOS logic in target systems. For information when assessing compatibility with other logic systems:

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