Detailed Specifications
2- 96
R e v i sio n 1 8
FIFO Flag Logic
The FIFO is user configurable into various DEPTHs and WIDTHs.
Figure 2-62 shows the FIFO address
counter 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 address space (
Table 2-94).Note:
Inactive counter bits are set to zero.
Figure 2-62 FIFO Address Counters
Table 2-94 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