Axcelerator Family FPGAs
v2.8
2-81
FIFO Flag Logic
The FIFO is user configurable into various DEPTHs and
details.
Bits 11 to 5 are active for all modes.
As the data word size is reduced, more least-
significant bits are added to the address.
As the number of cascaded blocks increases, the
number of significant bits in the address increases.
For example, if four blocks are cascaded as a 1kx16 FIFO
with each block having a 1kx4 aspect ratio, bits 11 to 2 of
the address will be used to specify locations within each
RAM block, whereas bits 13 and 12 will be used to specify
the RAM block.
The AFULL and AEMPTY flag threshold values are
programmable. The threshold values are AFVAL and
AEVAL, respectively. Although the trigger threshold for
each flag is defined with eight bits, the effective number
of threshold bits in the comparison depends on the
configuration. The effective number of threshold bits
corresponds to the range of active bits in the FIFO
Note:
Inactive counter bits are set to zero.
Figure 2-62 FIFO Address Counters
Table 2-93 FIFO Flag Logic
Mode
Inactive AEVAL/AFVAL bits
Inactive DIFF bits (set to 0)
DIFF comparison to AFVAL/AEVAL
Non-cascade
[7:4]
[15:12]
DIFF[11:8] withAE/FVAL[3:0]
Cascade 2 blocks
[7:5]
[15:13]
DIFF[12:8] withAE/FVAL[4:0]
Cascade 4 blocks
[7:6]
[15:14]
DIFF[13:8] withAE/FVAL[5:0]
Cascade 8 blocks
[7]
[15]
DIFF[14:8] withAE/FVAL[6:0]
Cascade 16 blocks
None
DIFF[15:8] withAE/FVAL[7:0]
CNTR [12]
activate
FIFO Address Counters
>> REN [4:0], RAD [11:0]
>> WEN [4:0], WAD [11:0]
[12:W] [13:W]
[14:W]
[15:W]
128x36
1kx4
512x9
256x18
[11:5]
[11:4]
[11:3]
[11:2]
[11:1]
[11:0]
4kx1
2kx2
Variable Active Address Space
CNTR [15]
activate
CNTR [2]
activate
CNTR [3]
activate
CNTR [4]
activate
CNTR [11:5]
always active
CNTR [13]
activate
CNTR [14]
activate
CNTR [0]
activate
CNTR [1]
activate
Cas 16 blks
by 1
by 2
by 4
by 9
by 18
by 36
Cas 2 blks
Cas 4 blks
Cas 8 blks
Mode when
Active
Counter
Bits
R/W EN[3]
R/W ADD[0]
R/W ADD[1]
R/W ADD[2]
R/W ADD[3]
R/W ADD[7:5]
R/W ADD[11:8]
R/W EN[0]
R/W EN[1]
R/W EN[2]
R/W ADD[4]
FIFO Address
AEVAL/AFVAL[7]
not compared
AEVAL/AFVAL[3:0]
AEVAL/AFVAL[4]
AEVAL/AFVAL[5]
AEVAL/AFVAL[6]
CNTR [15:0]
Alignment of
Threshold bits