6.4.2. Target interface logic levels

Multi-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 Multi-ICE interface 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 6.9.

Figure 6.9. Target interface voltage levels

The adaptive interface levels work down to VTref less than 1V. If, however, VTref becomes less than approximately 0.85V, Multi-ICE interprets this condition as Target Not Present, and the software reports this as an error condition.

The nTRST output from Multi-ICE is effectively driven open collector, so it is actively pulled to 0V but relies on a pull-up resistor within the target system 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 Multi-ICE is similarly driven open collector, and must be pulled-up with a resistor in the target system. As this signal is also an input to the Multi-ICE interface unit, there is a large-value internal pull-up resistor (51kO to Voh). This is to avoid spurious lows on the input when nSRST is not connected to the target system.

The input and output characteristics of the Multi-ICE interface unit are compatible with logic levels from TTL-compatible, or CMOS logic in target systems. For information when assessing compatibility with other logic systems, the output impedance of all signals is approximately 100O.

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