AI Engine to PL AXI4-Stream interfaces support a subset of
the AXI4-Stream protocol (https://developer.arm.com/documentation/ihi0051/a/Introduction/About-the-AXI4-Stream-protocol).
Internally, AI Engine to PL AXI4-Stream interfaces are 64-bit
channels physically. And 128-bit occupies two adjacent physical channels. This
imposes additional requirements on sending data timely between AI Engine and PL via
AXI4-Stream interfaces. This section focuses on
TLAST and TKEEP requirements of the interfaces
to send data without stall.
TLAST is required for a 64-bit stream between the AI Engine and PL if single 32-bit words are sent.
AI Engine to PL 32-bit stream interfaces
are automatically internally up-sized to 64-bit interfaces by the AI Engine compiler. When sending 32-bit stream
data (to or from the PL from the AI Engine),
single 32-bit words without TLAST are held in the interface until a
second 32-bit word arrives to complete a 64-bit up-sizing. The solution is to assert
TLAST for the single 32-bit data. The data will be pushed into
AI Engine without stall.
When using 64-bit and 128-bit interfaces, it is valid to send data without
TLAST. Then, even number of data are sent without stall.
TLAST can be used depending on the need. When
TLAST is asserted, TKEEP can be used together
with TLAST to send arbitrate number of 32-bit data for 64-bit and
128-bit interfaces. TKEEP must be set correctly, either -1 (all
bits are 1) or partial 32-bit words are enabled, for example 0x0F. The lower parts
of the data should be asserted if only partial data is valid.
TLAST and
TKEEP usage to send an odd number of 32-bit words without stall
for AI Engine to PL AXI4-Stream interfaces:
| Interface Bit Width | TLAST | TKEEP | Note |
|---|---|---|---|
| 32-bit | 1 | -1 | 32-bit word to send without stall. |
| 64-bit | 1 | 0x0F | Lowest 32-bit word to send without stall. |
| 128-bit | 1 | 0x000F | Lowest 32-bit word to send without stall. |
| 128-bit | 1 | 0x00FF | Lowest two 32-bit word to send without stall. |
| 128-bit | 1 | 0x0FFF | Lowest three 32-bit word to send without stall. |
TLAST=1 & TKEEP=0xF0 for
sending only the highest 32-bit word is not valid.TLAST, even with 32-bit AI Engine to PL interface.
To make sure the last odd number is pushed out to PL, assert TLAST inside AI Engine kernel
by:writeincr(out,value,true);//"true" to assert TLAST