GR740-UM-DS, Nov 2017, Version 1.7
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www.cobham.com/gaisler
GR740
4.3
Reset scheme
The device has an on-chip reset generator that creates a reset signal that is fed to the rest of the system.
This is asynchronously asserted when the external SYS_RESETN input is asserted and synchronously
deasserted a few cycles after the SYS_RESETN input has been deasserted.
The reset generation also considers the locking status of the PLLs, and will not deassert reset until the
PLL:s have achieved lock. In the event PLL lock is lost, the system will again go into reset. Only the
lock signals of PLLs that are used (not in bypass, or deselected by MEM_CLKSEL) are considered. If
external PLLs are also used on the board, a separate input SYS_EXTLOCK is available to allow also
including the lock status of these PLLs in the reset generation.
Where this default behavior is unwanted, the PLL_IGNLOCK bootstrap signal, when tied HIGH, will
cause the lock statuses of the internal PLLs to be ignored (treated as always in lock) in the reset gener-
ation. The SYS_EXTLOCK signal is never ignored. Since all the lock signals are available on pack-
age pins, custom lock handling can be implemented on board level.
The bootstrap signal sampling, the general purpose register bank, and the PLL reconfiguration mod-
ule have separate reset generation that is only reset when the master resetn signal is asserted and will
not be affected by PLL lock status.
The JTAG_TRST input asynchronously resets the JTAG TAP in the device. This can be asserted at
any time while the device is running without affecting device function provided that a JTAG debug
access into the system is not currently in progress. The JTAG_TRST input must be asserted on power-
up to ensure that the TAP instruction register can not power-up set to a test command. If JTAG is
unused, JTAG_TRST should be tied low on the board.
Other peripherals, such as Ethernet, SpaceWire and PCI are all reset via internal signals generated
from the SYS_RESETN input and PLL lock signals, as described above. To ensure proper reset of all
the clock domains in the device, care must be taken to ensure that all external clocks for interfaces that
will be used are active and toggling before the interface is enabled and ungated in the clock gating
unit.