CY7C4255V, CY7C4265V

CY7C4275V, CY7C4285V

Document Number: 38-06012 Rev. *D

Page 7 of 24

Programmable Almost Empty/Almost Full Flag

The CY7C4255/65/75/85V features programmable Almost

Empty and Almost Full Flags. Each flag can be programmed

(described in section Programming on page 6) a specific

distance from the corresponding boundary flags (Empty or Full).

When the FIFO contains the number of words or fewer for which

the flags have been programmed, the PAF or PAE is asserted,

signifying that the FIFO is either Almost Full or Almost Empty.

See Table 3 for a description of programmable flags.

When the SMODE pin is tied LOW, the PAF flag signal transition

is caused by the rising edge of the write clock and the PAE flag

transition is caused by the rising edge of the read clock.

Retransmit

The retransmit feature is beneficial when transferring packets of

data. It enables the receipt of data to be acknowledged by the

receiver and retransmitted if necessary.

The Retransmit (RT) input is active in the standalone and width

expansion modes. The retransmit feature is intended for use

when a number of writes equal to or less than the depth of the

FIFO have occurred and at least one word has been read since

the last RS cycle. A HIGH pulse on RT resets the internal read

pointer to the first physical location of the FIFO. WCLK and

after the retransmit pulse. With every valid read cycle after

retransmit, previously accessed data is read and the read pointer

is incremented until it is equal to the write pointer. Flags are

governed by the relative locations of the read and write pointers

and are updated during a retransmit cycle. Data written to the

FIFO after activation of RT are transmitted also. The full depth of

the FIFO can be repeatedly retransmitted.

Width Expansion Configuration

The CY7C4255/65/75/85V can be expanded in width to provide

word widths greater than 18 in increments of 18. During width

expansion mode, all control line inputs are common and all flags

are available. Empty (Full) flags must be created by ANDing the

Empty (Full) flags of every FIFO. The PAE and PAF flags can be

detected from any one device. This technique avoids reading

data from, or writing data to the FIFO that is “staggered” by one

clock cycle due to the variations in skew between RCLK and

WCLK. Figure 2 on page 8 demonstrates a 36-word width by

using two CY7C4255/65/75/85Vs.

Table 3. Flag Truth Table

Number of Words in FIFO

FF PAF HF PAE EF

7C4255V – 8 K × 18

7C4265V – 16 K × 18

7C4275V – 32 K × 18

7C4285V – 64 K × 18

0

000

H

H

H

L

L

1 to n[2]

1 to n[2]

1 to n[2]

1 to n[2]

HH

H

L

H

(n+1) to 4096

(n+1) to 8192

(n+1) to 16384

(n+1) to 32768

H

H

H

H

H

4097 to (8192–(m+1)) 8193 to (16384 –(m+1)) 16385 to (32768–(m+1)) 32769 to (65536 –(m+1))

H

H

L

H

H

(8192–m)[3] to 8192

(16384–m)[3] to 16384

(32768–m)[3] to 32767

(65536–m)[3] to 65535

HL

LH

H

8192

16384

32768

65536

L

L

L

H

H

Notes

2. n = Empty Offset (Default Values: CY7C4255/65/75/85V n = 127).

3. m = Full Offset (Default Values: CY7C4255/65/75/85V n = 127).

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