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8.1. About the ETM interface

The ARM920T trace interface port enables simple connection of an ETM9 to an ARM920T Rev 1. This interface does not exist on ARM920T Rev 0. The ARM9 Embedded Trace Macrocell (ETM9) provides instruction and data trace for the ARM9TDMI family of processors.

The interface is made up as follows:

  • ETMPWRDOWN input to the ARM920T

  • ETMCLOCK output to the ETM9

  • ETMnWAIT output to the ETM9

  • ETM<signal> outputs to the ETM9.

When ETMPWRDOWN is HIGH, the ETMCLOCK output and the ETM<signal> outputs are held stable. When ETMPWRDOWN is LOW, the ETMCLOCK and ETM<signal> outputs are enabled. This enables system power to be reduced when the ETM9 is not used. When the ETM9 is incorporated within a system, the ARM debug tools control ETMPWRDOWN, automatically setting the signal LOW at the start of a debug session. If the ETM9 is not incorporated within a system, then ETMPWRDOWN must be tied HIGH.

The ETMCLOCK output to the ETM9 is used by the ETM9 to sample the ETM<signal> outputs on the rising edge of ETMCLOCK, when ETMnWAIT is HIGH. ETMnWAIT is the nWAIT input signal to the ARM9TDMI, so this allows cycle-accurate tracing using ETMCLOCK. The ETMCLOCK signal is never stretched.

The ETM<signal> outputs are registered so that they can be sampled on the rising edge of ETMCLOCK.

The ETM<signal> timing is shown in Timing definitions for the ARM920T Trace Interface Port and signal descriptions in ARM920T Trace Interface Port signals.

The ETM9 (Rev0/0a) Technical Reference Manual contains details of how to integrate an ETM9 with an ARM920T Rev 1, including the pin correlation.

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