GR740-UM-DS, Nov 2017, Version 1.7
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GR740
15.5.2 Implemented PCI responses
The PCI target can terminate a PCI access with the following responses.
•
Retry:
This response indicates the PCI target is busy by either fetching data for the AMBA AHB
bus on a PCI read or emptying the write FIFO for a PCI write. A new PCI read access will always
be terminated with a retry at least one time before the PCI target is ready to deliver data.
•
Disconnect with data:
Terminate the transaction and transfer data in the current data phase. This
occurs when the PCI master request more data and the next FIFO is not yet available or for a PCI
burst access with the Memory Read command.
•
Disconnect without data:
Terminate the transaction without transferring data in the current data
phase. This occurs if the CBE change within a PCI burst write.
•
Target Abort:
Indicates that the current access caused an internal error and the target is unable to
finish the access. This occurs when the core receives a AMBA AHB error during a read opera-
tion.
15.5.3 Supported byte-enables (CBE)
The PCI target only supports aligned 8-, 16-, and 32-bit accesses.
The supported combinations of CBE are: 0000, 1110, 1101, 1011, 0111, 1100, 0011.
All other combinations of CBE are interpret as a 32-bit access (CBE = 0000) except for writes with
CBE set to 1111, which is treated as a no-operation (no write will be performed).
15.5.4 PCI to AHB translation
Each PCI BAR has translation register (mapping register) to translate the PCI access to an AMBA
AHB address area. These mapping registers are accessible via the core specific Extended PCI Config-
uration Space. The number of implemented bits in these registers correspond to the size of (and num-
ber of implemented bits in) the BARs registers.
15.5.5 PCI system host signal
When the PCI system host signal is asserted the PCI target responds to configuration cycles when no
IDSEL signal is asserted (none of AD[31:11] are asserted). This is done for the PCI master, in a sys-
tem host position, to be able to configure its own PCI target.
15.5.6 Error handling
The PCI target terminates the access with target-abort when the PCI target requests data from the
AHB bus which results in an error response on the AHB bus. Because writes to the PCI target is
posted, no error is reported on write AHB errors.
When a PCI master is terminated with a retry response it is mandatory for that master to retry the
access until the access is completed or terminated with target-abort. If the master never retries the
access, the PCI target interface would be locked on this access and never accept any new access. To
recover from this situation, the PCI target has a option to discard an access if it is not retried within
2
15
clock cycles. This discard time out can be enabled via the “AHB IO base address and PCI bus
config” register located in the core specific Extended PCI Configuration Space.
15.6
DMA Controller
The DMA engine is descriptor based and uses two levels of descriptors.