ARM Technical Support Knowledge Articles

How should power-on reset be applied to the ARM7TDMI?

Applies to: ARM7TDMI


It is good practice to reset a static device immediately on power-up, to remove any undefined conditions within the device which may otherwise combine to cause a DC path and thereby increase current consumption. Most systems are reset by using a simple RC circuit on the reset pin to remove the undefined states within devices whilst clocking the device.

Note that nRESET must be held asserted for a minimum of two MCLK cycles to fully reset the core. It is necessary to reset the Embedded ICE Logic and the TAP controller as well, regardless of whether debug features are used or not. This is done by taking nTRST LOW for at least Tbsr.

You must also ensure that nWAIT is held high during the reset sequence.

During reset, the signals nMREQ and SEQ show internal cycles. After nRESET has been removed (i.e. taken HIGH), the ARM core does 2 further internal cycles before the first instruction is fetched from the reset vector (from 0x00). It then takes in total 3 MCLK cycles to advance this instruction through the fetch-decode-execute stages of the ARM instruction pipeline before this first instruction is executed.

See also:

Article last edited on: 2008-09-09 15:47:36

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