Altera SerialLite II IP Core - Manual (Page 45)

3–22 Chapter 3: Parameter Settings
Link Layer Configuration
SerialLite II MegaCore Function January 2014 Altera Corporation
User Guide
The receiver Atlantic FIFO buffers include an end-of-packet based data available
feature which can be turned on by asserting the ctl_rxrdp_eopdav/ctl_rxhpp_eopdav
signals. The end-of-packet feature determines whether the dav remains high: if the
signal is asserted, and there is an end-of- packet beneath the FTL threshed, the dav
signal remains high until the end-of-packet is read out of the FIFO buffer. Otherwise,
if the signal is not asserted, the dav signal only remains high when the fill level of the
buffer is higher than the FTL value.
ctl_rxhpp_fth and ctl_rxrdp_fth are the threshold levels for the high priority and
regular data ports on the receiver Atlantic FIFO buffers. When the data fill level is
higher than the threshold level set by ctl_rxhpp_fth or ctl_rxrdp_fth, or dav = 1, it
means that there a large amount of data ready to be fetched at the FIFO buffer. You
must set these threshold levels based on your design requirements, and ensure that
the FIFO buffer does not underflow. You may also set the threshold levels to segment
size of a priority packet; or to the lowest level so that you can fetch data as soon as it is
stored in the FIFO buffer.
You can set ctl_rxhpp_ftl to 1 element unit so that it fetches the data from the RX
FIFO buffer as soon as there is data available. If you want to store some data before
fetching it, you can raise the threshold level.
The FIFO buffer threshold high (ctl_txrdp_fth/ctl_txhpp_fth) value for transmitter
variations controls when the txrdp_dav/txhpp_dav signals are asserted and
deasserted for the write side of the FIFO buffer, respectively. The txrdp_dav signal
indicates when there is room available to write new data into the FIFO buffer, and is
asserted when the fill level of the FIFO is less than the FTH setting, and deasserted
when the fill level of the FIFO is greater than the FTH.
For example, if FTH is five, and the fill level is four, the txrdp_dav/txhpp_dav signal is
high, indicating that the user can write data into the FIFO. If the fill level for this
example is six, the txrdp_dav/txhpp_dav signal is low, indicating that the user should
stop writing data into the FIFO.
ctl_txhpp_fth and ctl_txrdp_fth are the threshold levels for the high priority and
regular data ports on the transmitter Atlantic FIFO buffers. When the data fill level at
the FIFO buffer is lower than the threshold level set by ctl_txhpp_fth or
ctl_txrdp_fth
, or dav = 1, it means that there are plenty of spaces available for data to
write into the buffer. You must set these threshold levels high so that the user logic
knows whenever the FIFO buffer has available spaces for data buffering and to ensure
that overflow does not occur. However, these threshold settings should not exceed the
FIFO depth.
For example, if the transmitter buffer size is 4,096 bytes, and the transmitter FIFO
depth is 2,048 element units, you should set the level of ctl_txhpp_fth to 250 element
units.
TSIZE = 2, and one FIFO element = 2 bytes
Maximum TX FIFO level (TX 8 lane) = 2,048/8 = 256 element units
1 You can set any value below 256 element units for ctl_txhpp_fth; Altera
recommends a level of 250 element units or 8'hFA.