Device
PRM
PRCM
nRESETOUT
nRESETIN
Global reset source
nRESET_IN_OUT
VDDSHV6
1.8 or 3.3 V
Pullup
Reset
button
RESET to
peripherals on
board
Power, Reset, and Clock Management
•
Some PRCM and Control module registers are warm reset insensitive and maintain their value
throughout a warm reset
•
SYSBOOT pins are not latched with a warm reset. The device will boot with the SYSBOOT values
from the previous cold reset.
•
Most debug subsystem logic is not affected by warm reset. This allows you to maintain any debug
sessions throughout a warm reset event.
•
PLLs are not affected by warm reset
As an output, nRESETIN_OUT can be used to reset external devices. nRESET_OUT will drive low during
a cold reset or an internally generated warm reset. After completion of a cold or warm reset,
nRESETIN_OUT will continue to drive low for a period defined by PRM_RSTTIME.RSTTIME1. RSTTIME1
is a timer that counts down to zero at a rate equal to the high frequency system input clock CLK_M_OSC.
This allows external devices to be held in reset for some time after the AM335x comes out of reset.
Caution must be used when implementing the nRESETIN_OUT as an bi-directional reset signal. Because
of the short maximum time allowed using RSTTIME1, it does not supply an adequate debounce time for
an external push button circuit. The processor could potentially start running while external components
are still in reset. It is recommended that this signal be used as input only (do not connect to other devices
as a reset) to implement a push button reset circuit to the AM335x, or an output only to be able to reset
other devices after an AM335x reset completes.
8.1.7.4.1.1 Warm Reset Input/Reset Output (nRESETIN_OUT)
Any global reset source (internal or external) causes nRESETIN_OUT to be driven and maintained at the
boundary of the device for at least the amount of time configured in the PRCM.PRM_RSTTIME.
RSTTIME1 bit field. This ensures that the device and its related peripherals are reset together. The
nRESETIN_OUT output buffer is configured as an open-drain; consequently, an external pull-up resistor is
required.
After the de-assertion, the bi-directional pin nRESETIN_OUT is tri-stated to allow for assertion from off
chip source (externally).
Figure 8-20. External System Reset
Note: It is recommended to implement warm reset as an input only (for example, push button) or an output only (to
reset external peripherals), not both.
The device will have one pin nRESETIN_OUT which reflects chip reset status. This output will always
assert asynchronously when any chip watchdog timer reset occurs if any of the following reset events
occurs:
•
POR (only internal stretched portion of reset event after bootstrap is latched)
•
External Warm reset (nRESETIN_OUT pin, only internal stretched portion of reset event after
bootstrap is latched)
•
Emulation reset (Cold or warm from ICEPICK)
•
Reset requestor
•
SW cold/warm reset
538
Power, Reset, and Clock Management (PRCM)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated