3.4.1. IEEE 1149.1 compatibility

The TDO output from the ETM changes on the rising edge of TCK, when TCKEN is HIGH. For ARM7TDMI and ARM720T systems, you must add an external falling edge D-type flip-flop, as shown in Figure 3.9, to ensure full compatibility with IEEE 1149.1.

You do not need to do anything for ARM7TDMI-S based systems because the synchronization and adaptive clocking logic ensures that this register is not required.

Figure 3.9. TDO output retiming circuit for IEEE 1149.1 compatibility

Most run control hardware, such as Multi-ICE, does not require this D-type.


You must take care if you add this register and use the ARMTDO input to the ETM. This input must change around the rising edge of TCK. If it does not, it is incorrectly delayed by one cycle.

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