Nread response transaction – Altera RapidIO II MegaCore Function User Manual
Page 107
Chapter 4: Functional Description
4–65
Logical Layer Interfaces
August 2014
Altera Corporation
RapidIO II MegaCore Function
User Guide
NREAD Response Transaction
In the first clock cycle of the NREAD response on the Avalon-ST pass-through
interface, as shown in
, the IP core asserts gen_tx_ready to indicate it is
ready to sample data. In the same cycle, user logic asserts gen_tx_valid. Because both
gen_tx_ready
and gen_tx_valid are asserted, this clock cycle is an Avalon-ST ready
cycle. The user logic provides valid data on gen_tx_data for the IP core to sample, and
asserts gen_tx_startofpacket to indicate the current value of gen_tx_data is the
initial piece of the current packet (the start of packet). On gen_tx_packet_size, user
logic reports the full length of the packet is 0x28, which is decimal 40, because the
packet comprises eight bytes of header and 32 bytes of payload data.
The user logic provides the 32-byte payload and 8-byte header on the same bus,
gen_tx_data[127:0]
. Transferring these 40 bytes of information requires three clock
cycles. During all of these cycles, the IP core holds gen_tx_ready high and user logic
holds gen_tx_valid high, indicating the cycles are all Avalon-ST ready cycles. In the
second cycle, user logic holds gen_tx_startofpacket and gen_tx_endofpacket low,
because the information on gen_tx_data is neither start of packet nor end of packet
data. In the third clock cycle, user logic asserts gen_tx_endofpacket and sets
gen_tx_empty
to the value of 0x8 to indicate that eight bytes of the data in the current
clock cycle are invalid—in other words, only the initial eight (sixteen minus eight)
bytes of data available on gen_tx_data in the current clock cycle are valid.
prio[1:0]
[103:102]
2’b01
tt[1:0]
[101:100]
2’b01
Indicates 16-bit device IDs.
ftype[3:0]
[99:96]
4'b0010
The value of 2 indicates a Request Class packet.
destinationID[15:0]
[95:80]
16’hDDDD
sourceID[15:0]
[79:64]
16’hAAAA
ttype[3:0]
[63:60]
4’b0100
The value of 4 indicates an NREAD transaction.
size[3:0]
[59:56]
4'b1100
The size and wdptr values encode the maximum size
of the payload field. In this example, they decode to a
value of 32 bytes. For details, refer to Table 4-3 in Part
1: Input/Output Logical Specification of the RapidIO
Interconnect Specification, Revision 2.2.
transactionID[7:0]
[55:48]
8'hBB
address[28:0]
[47:19]
{28’h7654321,
1’b0}
wdptr
[18]
0
Refer to the comment for size.
xamsbs[1:0]
[17:16]
2’b00
Reserved[15:0]
[15:0]
16’h0000
Table 4–40. NREAD Request Receive Example: RapidIO Header Fields in gen_rx_hd_data Bus (Part 2 of 2)
Field
gen_rx_hd_data
Bits
Value
Comment