GR740-UM-DS, Nov 2017, Version 1.7
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GR740
To enable a descriptor the enable (EN) bit should be set and after this is done, the descriptor should
not be touched until the enable bit has been cleared by the GRETH_GBIT. The rest of the fields in the
descriptor are explained later in this section.
14.3.2 Starting transmissions
Enabling a descriptor is not enough to start a transmission. A pointer to the memory area holding the
descriptors must first be set in the GRETH_GBIT. This is done in the transmitter descriptor pointer
register. The address must be aligned to a 1 kB boundary. Bits 31 to 10 hold the base address of
descriptor area while bits 9 to 3 form a pointer to an individual descriptor. The first descriptor should
be located at the base address and when it has been used by the GRETH_GBIT the pointer field is
incremented by 8 to point at the next descriptor. The pointer will automatically wrap back to zero
when the next 1 kB boundary has been reached (the descriptor at address offset 0x3F8 has been used).
The WR bit in the descriptors can be set to make the pointer wrap back to zero before the 1 kB bound-
ary.
The pointer field has also been made writable for maximum flexibility but care should be taken when
writing to the descriptor pointer register. It should never be touched when a transmission is active.
The final step to activate the transmission is to set the transmit enable bit in the control register. This
tells the GRETH_GBIT that there are more active descriptors in the descriptor table. This bit should
always be set when new descriptors are enabled, even if transmissions are already active. The descrip-
tors must always be enabled before the transmit enable bit is set.
14.3.3 Descriptor handling after transmission
When a transmission of a packet has finished, status is written to the first word in the corresponding
descriptor. The Underrun Error bit is set if the transmitter RAM was not able to provide data at a suf-
ficient rate. This indicates a synchronization problem most probably caused by a low clock rate on the
AHB clock. The whole packet is buffered in the transmitter RAM before transmission so underruns
cannot be caused by bus congestion. The Attempt Limit Error bit is set if more collisions occurred
than allowed. When running in 1000 Mbit mode the Late Collision bit indicates that a collision
occurred after the slottime boundary was passed.
The packet was successfully transmitted only if these three bits are zero. The other bits in the first
descriptor word are set to zero after transmission while the second word is left untouched.
The enable bit should be used as the indicator when a descriptor can be used again, which is when it
has been cleared by the GRETH_GBIT. There are three bits in the GRETH_GBIT status register that
13
Interrupt enable (IE) - Enable Interrupts. An interrupt will be generated when the packet from this
descriptor has been sent provided that the transmitter interrupt enable bit in the control register is set.
The interrupt is generated regardless if the packet was transmitted successfully or if it terminated
with an error.
12
Wrap (WR) - Set to one to make the descriptor pointer wrap to zero after this descriptor has been
used. If this bit is not set the pointer will increment by 8. The pointer automatically wraps to zero
when the 1 kB boundary of the descriptor table is reached.
11
Enable (EN) - Set to one to enable the descriptor. Should always be set last of all the descriptor
fields.
10: 0
LENGTH - The number of bytes to be transmitted.
Table 226.
Address offset 0x4 - GRETH_GBIT transmit descriptor word 1
31
0
ADDRESS
31: 0
Address (ADDRESS) - Pointer to the buffer area from where the packet data will be loaded.
Table 225.
Address offset 0x0 - GRETH_GBIT transmit descriptor word 0