GR740-UM-DS, Nov 2017, Version 1.7
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GR740
then processor 0 will also enter power-down mode. If the device has debug mode enabled via the
DSU_EN signal (=HIGH) then the processors will enter debug mode instead of power-down mode.
For more information see chapter about the clock gating unit, section 25.
4.10
Debug AHB bus clocking
All members of the Debug AHB bus will be gated off when the DSU_EN signal is low.
4.11
Notes on Ethernet interface clock and mode switch
The Ethernet interface transmit clocks (ETH0_TXCLK and ETH1_TXCLK) are used internally in the
device to clock registers that selects between 10/100 and 1000 Mbit (Gigabit) mode for the respective
Ethernet controller. The default PHY (Ethernet transceiver) behaviour is to enter 10/100 Mbit mode
after reset. When a mode switch is made to 1000 Mbit mode, a signal will change internally in the
Ethernet controller. For this signal change to propagate through the register that selects between 10/
100 and 1000 Mbit mode the corresponding TXCLK must be present. Ethernet PHYs may disable the
TXCLK when entering 1000 Mbit mode and this may cause the internal register value in the GR740
to remain at the 10/100 Mbit value after the PHY has entered 1000 Mbit operation. When this happens
the system will not be able to transmit Ethernet traffic.
When the Ethernet debug communication link (EDCL) is enabled then the Ethernet controller will
automatically try to configure the PHY after reset. If the device is connected to a Gigabit network then
care must be taken to ensure that the TXCLK is available after the PHY has switched to 1000 Mbit
mode.
If the device will only be used on a 10/100 Mbit network then the TXCLK inputs can be connected
directly to the PHY 10/100 transmit clock. If the device will only be used on a 1000 Mbit network
then the Gigabit transmit clock can be connected to the TXCLK input.
If the device should adapt to both 10/100 Mbit networks and 1000 Mbit networks then the TXCLK
input(s) should be connected to the PHY 10/100 transmit clock(s). Software then needs to perform a
special sequence when the PHY has determined that it is connected to a Gigabit network: If the soft-
ware driver finds that the device is connected to a Gigabit network then software needs to force the
PHY into 10/100 Mbit mode in order to enable the TXCLK. Software can then re-enable 1000 Mbit
operation. Note that this allows the system to adapt to both 10/100 networks and 1000 Mbit networks.
With this configuration, the EDCL will still be unavailable after reset when connected to a Gigabit
network.