CY7C474 データシートの表示（PDF） - Cypress Semiconductor

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CY7C474

8K x 9 FIFO, 16K x 9 FIFO 32K x 9 FIFO with Programmable Flags

Cypress Semiconductor 

CY7C470

CY7C472

CY7C474

Switching Waveforms (Continued)

Full Flag and Write Data Flow-Through Mode

R

tWAF

tPW

W

tEFD

tEFD

E/F

DATA IN

DATA OUT

tA

DATA VALID

tSD

tHD

DATA VALID

7C470–18

Architecture

The CY7C470, CY7C472, and CY7C474 FIFOs consist of an

array of 8,192, 16,384, and 32,768 words of 9 bits each, re-

spectively. The control consists of a read pointer, a write point-

er, a retransmit pointer, control signals (i.e., write, read, mark,

retransmit, and master reset), and flags (i.e., Empty/Full, Half

Full, and Programmable Almost Full/Empty).

Resetting the FIFO

Upon power-up, the FIFO must be reset with a Master Reset

(MR) cycle. This causes the FIFO to enter the empty condition signi-

fied by the Empty flag (E/F) and Almost Full/Empty flag (PAFE) being

LOW, and Half Full flag (HF) being HIGH. The read pointer, write

pointer, and retransmit pointer are reset to zero. For a valid reset,

Read (R) and Write (W) must be HIGH tRPW/tWPW before the falling

edge and tRMR after the rising edge of MR.

Writing Data to the FIFO

Data can be written to the FIFO when it is not FULL[11]. A falling

edge of W initiates a write cycle. Data appearing at the inputs (D0–D8)

tSD before and tHD after the rising edge of W will be stored sequen-

tially in the FIFO.

Reading Data from the FIFO

Data can be read from the FIFO when it is not empty[12]. A

falling edge of R initiates a read cycle. Data outputs (Q0–Q8) are in a

high-impedance condition when the FIFO is empty and between read

operations (R HIGH). The falling edge of R during the last read cycle

before the empty condition triggers a high-to-low transition of E/F, pro-

hibiting any further read operations until tRFF after a valid write.

Retransmit

The retransmit feature is beneficial when transferring packets

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

the receiver and resent if necessary. Retransmission can start

from anywhere in the FIFO and be repeated without limitation.

The retransmit methodology is as follows: mark the current

value of the read pointer, after an error in subsequent read

operations return to that location and resume reading. This

effectively resends all of the data from the mark point. When

MARK is LOW, the current value of the read pointer is stored. This

operation marks the beginning of the packet to be resent. When RT

is LOW, the read pointer is updated with the mark location. During

each subsequent read cycle, data is read and the read pointer incre-

mented.

Care must be taken when using the retransmit feature. Use the

mark function such that the write pointer does not pass the

mark pointer, because further write operations will overwrite

data.

Programmable Almost Full/Empty Flag

The CY7C470/2/4 offer a variable offset for the Almost Empty

and the Almost Full condition. The offset is loaded into the

programmable flag register (PFR) during a master reset cycle.

While MR is LOW, the PFR can be loaded from Q8–Q0 by pulsing

R LOW or from D8–D0 by pulsing W LOW. The offset options are

listed in Table 2. See Table 1 for a description of the six FIFO states.

If the PFR is not loaded during master reset (R and W HIGH) the

default offset will be 256 words from Full and Empty.

Notes:

11. When the FIFO is less than half full, the flags make a LOW-to-HIGH transition on the rising edge of W and make the HIGH-to-LOW transition on the falling edge

of R. If the FIFO is more than half full, the flags make the LOW-to-HIGH transition on the rising edge of R and HIGH-to-LOW transition on the falling edge of W.

12. Full and empty states can be decoded from the Half-Full (HF) and Empty/Full (E/F) flags.

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