GR740-UM-DS, Nov 2017, Version 1.7
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GR740
3
Signals
3.1
Bootstrap signals
The power-up and initialisation state is affected by several external signals as shown in table 23. The
bootstrap signals taken via GPIO are saved when the on-chip system reset is released. This occurs
after deassertion of the SYS_RESETN input and lock of all active PLLs (see also reset description in
section 4). This means that if a peripheral, such as the Ethernet controller, is clock gated off and then
reset and enabled at a later time, the bootstrap signal value will be taken from the saved value present
in a general purpose register described in section 28. See also section 4.9 for further information on
the conditions for clock gating per peripheral.
Table 23.
Bootstrap signals
Bootstrap signal
Description
DSU_EN
Enables the Debug Support Unit (DSU) and other members connected to the Debug AHB bus. If
DSU_EN is HIGH the DSU and the Debug AHB bus will be clocked. If DSU_EN is LOW the
DSU and all members on the Debug AHB bus will be clock gated off.
A special case exists for the Ethernet controllers. These controller have master interfaces con-
nected to the Debug AHB bus and debug traffic can optionally be routed to this bus. If DSU_EN
is LOW then the Ethernet Debug Communications Link (EDCL) functionality will be disabled
and the Ethernet controllers will be clock gated off after reset. If DSU_EN is HIGH then the
Ethernet controller clocks will be enabled. With DSU_EN HIGH, the EDCL functionality will
be further configured by GPIO[7:0] as described further down in this table.
BREAK
Puts all processors in debug mode when asserted while DSU_EN is HIGH. When DSU_EN is
LOW, BREAK is assigned to the timer enable bit of the watchdog timer and also controls if the
first processor starts executing after reset.
PCIMODE_ENABLE
Enables PCI mode. If the bootstrap signal MEM_IFWIDTH is HIGH then PCIMODE_EN-
ABLE selects if the top-half of the SDRAM interface should be used for the PCI controller
(HIGH) or Ethernet port 1 (LOW).
MEM_IFWIDTH
Selects the width of SDRAM interface. If this signal is LOW then the external memory interface
uses 64 data bits with up to 32 check bits. If this signal is HIGH then the external memory inter-
face uses 32 data bits with up to 16 check bits and the top half of the SDRAM interface is used
for PCI or Ethernet port 1, as determined by the PCIMODE_ENABLE bootstrap signal.
MEM_CLKSEL
The value of this signal determines the clock source for the SDRAM memory. If this signal is
low then the memory clock and the system clock has the same source, otherwise the source for
the memory clock is the MEM_EXTCLOCK clock input.
GPIO[5:0]
Sets the least significant address nibble of the IP and MAC address for Ethernet Debug Commu-
nication Link (EDCL) 0 and 1. GPIO [1:0] is also connected to the SpaceWire TDP controller:
For the Ethernet controllers:
GPIO[1:0] sets the least significant bits of the nibble for EDCL 0 and EDCL1
GPIO[3:2] sets the top nibble bits for EDCL 0 and GPIO[5:4] set the top nibble bits for EDCL1.
It is possible to disable the EDCLs at reset with bootstrap signals. As mentioned, when DSU_EN
is LOW then the EDCLs will be disabled. EDCL 0 is also disabled if GPIO[3:0] is set to 0b1111
when Ethernet controller 0 leaves reset. EDCL 1 is disabled when GPIO[7:4] is set to 0b1111
when Ethernet controller 1 leaves reset. Note that this means that the disable condition for EDCL
1 makes use of the bootstrap signals GPIO[7:6] that are used to configure SpaceWire router dis-
tributed interrupts.
The connections to the SpaceWire TDP controller are as follows:
GPIO[0] is connected to the set elapsed time input, see section 31.3.11.
GPIO[1] is connected to the increment elapsed time input, see section 31.3.3.
Note: The TDP connections are only available in silicon revision 1.