28.16.2.1. Command Fields Definition
Whenever the dispatcher core receives a descriptor, it decodes the descriptor and dispatches the read command to the Read Master IP to perform read data transfer. The following sections describe each of the field's definition of the read command.
Address
The read address fields correspond to the source address for each data transfer. If a standard descriptor is used and an attempt to write a 64-bit address is made, the upper 32 bits are lost and can cause the hardware to alias a lower address space. 64-bit addressing requires the use of the extended descriptor format.
Length
The length field is used to specify the number of bytes to transfer per descriptor.
Transmit Channel
Specify the channel number during Avalon® -MM to Avalon® -ST transfers. Applicable only when Channel Enable parameter is enabled.
Generate Start-Of-Packet (SOP)
To transmit a start-of-packet on the first beat of an Avalon® -MM to Avalon® -ST transfer. Applicable only when Packet Support Enable parameter is enabled.
Generate End-Of-Packet (EOP)
To transmit an end-of-packet on last beat of an Avalon® -MM to Avalon® -ST transfer. Applicable only when Packet Support Enable parameter is enabled.
Stop
To stop the Read Master IP from issuing the next read transaction when the Stop Dispatcher control bit is set in the Dispatcher core.
Reset
To soft reset the Read Master IP when the Reset Dispatcher control bit is set in the Dispatcher core.
Burst Count
The programmable read burst counts are only available when using the extended descriptor format and with Programmable Burst Enable parameter enabled. The programmable burst count is optional and can be disabled in the read hosts. Because the programmable burst count is an 8-bit field, the maximum that you can program burst counts of 1 to 128, with a power of 2.
The maximum programmable burst count is 128, even when you instantiate the mSGDMA Read Master IP with Maximum Burst Count parameter to different selections up to 1024. Programing to 0 or burst count greater than the parameter value of Maximum Burst Count, gets the maximum burst count from the Maximum Burst Count parameter selected during instantiation time. Refer to the Maximum Burst Count parameter in the parameter table.
Stride Addressing
The read stride fields are optional and only available when using the extended descriptor format and with Stride Addressing Enable parameter enabled. The stride value determines how the read host increment the address when accessing memory. The stride value is in terms of words, so the address incrementing is dependent on the host data width.
When stride is enabled, the host defaults to sequential accesses, which is the equivalent to a stride distance of one. A stride of zero instructs the host to continuously access the same address. A stride of two instructs the host to skip every other word in a sequential transfer. You can use this feature to perform interleaved data accesses, or to perform a frame buffer row and column transpose.
Transmit Error
This field is commonly used for transmitting error information downstream to streaming components, such as an Ethernet MAC. This field control the error bits on the streaming output of the read host. This field is applicable only when Error Enable parameter is enabled.
Early Done Enable
This field is to hide the latency between read data request. When this field is set, the read host does not wait for pending reads to return before requesting for another data. Typically, this bit is set for all descriptors except the last one. This bit is most effective for hiding high read latency. For example, it reads from SDRAM, PCIe, and SRIO. This field cannot be used for unaligned data or when packet support is enabled.