AM1806 datasheet(66/245 Pages) TI

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AM1806

SPRS658F – FEBRUARY 2010 – REVISED MARCH 2014

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6.4

Reset

6.4.1

Power-On Reset (POR)

A power-on reset (POR) is required to place the device in a known good state after power-up. Power-On

Reset is initiated by bringing RESET and TRST low at the same time. POR sets all of the device internal

logic to its default state. All pins are tri-stated with the exception of RESETOUT which remains active

through the reset sequence, and RTCK/GP8[0]. If an emulator is driving TCK into the device during reset,

then RTCK/GP8[0] will drive out RTCK. If TCK is not being driven into the device during reset, then

RTCK/GP8[0] will drive low. RESETOUT in an output for use by other controllers in the system that

indicates the device is currently in reset.

While both TRST and RESET need to be asserted upon power up, only RESET needs to be released for

the device to boot properly. TRST may be asserted indefinitely for normal operation, keeping the JTAG

port interface and device's emulation logic in the reset state.

TRST only needs to be released when it is necessary to use a JTAG controller to debug the device or

exercise the device's boundary scan functionality. Note: TRST is synchronous and must be clocked by

TCK; otherwise, the boundary scan logic may not respond as expected after TRST is asserted.

RESET must be released only in order for boundary-scan JTAG to read the variant field of IDCODE

correctly. Other boundary-scan instructions work correctly independent of current state of RESET. For

maximum reliability, the device includes an internal pulldown on the TRST pin to ensure that TRST will

always be asserted upon power up and the device's internal emulation logic will always be properly

initialized.

JTAG controllers from Texas Instruments actively drive TRST high. However, some third-party JTAG

controllers may not drive TRST high but expect the use of a pullup resistor on TRST. When using this type

of JTAG controller, assert TRST to intialize the device after powerup and externally drive TRST high

before attempting any emulation or boundary scan operations.

RTCK/GP8[0] is maintained active through a POR.

A summary of the effects of Power-On Reset is given below:

•

All internal logic (including emulation logic and the PLL logic) is reset to its default state

•

Internal memory is not maintained through a POR

•

RESETOUT goes active

•

All device pins go to a high-impedance state

•

The RTC peripheral is not reset during a POR. A software sequence is required to reset the RTC

CAUTION: A watchdog reset triggers a POR.

6.4.2

Warm Reset

A warm reset provides a limited reset to the device. Warm Reset is initiated by bringing only RESET low

(TRST is maintained high through a warm reset). Warm reset sets certain portions of the device to their

default state while leaving others unaltered. All pins are tri-stated with the exception of RESETOUT which

remains active through the reset sequence, and RTCK/GP8[0]. If an emulator is driving TCK into the

device during reset, then RTCK/GP8[0] will drive out RTCK. If TCK is not being driven into the device

during reset, then RTCK/GP8[0] will drive low. RESETOUT is an output for use by other controllers in the

system that indicates the device is currently in reset.

During an emulation, the emulator will maintain TRST high and hence only warm reset (not POR) is

available during emulation debug and development.

RTCK/GP8[0] is maintained active through a warm reset.

A summary of the effects of Warm Reset is given below:

•

All internal logic (except for the emulation logic and the PLL logic) is reset to its default state

Peripheral Information and Electrical Specifications

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