CT2579 datasheet(13/41 Pages) AEROFLEX

13 / 41 page

APPLICATION NOTE #108

Released 9/98

completion of the read/write command. The read/write operation must be completed within

the remaining 8 µSec. If a read/write operation starts after the NCMDSTRB strobe has

begun, the NACK will not occur and thus hold off the subsystem for the duration of the DMA

cycle from internal RAM.

During NCMDSTRB, the Command Word is loaded into the Command/Status FIFO stack

and the NEMPTY line goes high. The user can utilize this signal as an indication that some

activity has occurred. The Command/Status FIFO stores up to 32 command words for the

subsystem to review. This allows the processor to only response to the 1553 unit when

something has occurred. Constant polling of the 1553 unit is not required. To reduce

processor intervention even further, the Command/Status FIFO will only store commands

that have associated data with it.

MIL-STD-1760 FEATURES

If the 1760 features are enabled (NENCHK line is held low), the checksum word is automatically

generated and transmitted as the last data word. The subsystem processor does not have to

calculate or load the checksum word into RAM. The SmaRT unit automatically does this in hardware

and transmits the correct checksum data word as the last word out. This reduces subsystem

processor overhead significantly.

SIGNALS THAT INDICATE CHECKSUM FAILURE

For 1760 applications, the STATUS line indicates if a message has failed checksum. The STATUS

line will stay high for a Transmit Command word because there is no associated data words received

for checksum validation. STATUS is sampled on the falling edge of NCMDSTRB. The STATUS line

can be tied to any of the Status Word Bit inputs to set those bits in the event of a checksum fail.

STATUS is an open output line that will set the selected Status Word Bits for the Status Word

response in the current message. This is one of the great features for this product. The subsystem

does not have to verify the checksum in software to detect the error. The SmaRT unit automatically

does this in hardware and the unit is able to flag the error and set the a Status Word Bit on the

CURRENT Status Word response . This minimizes processor overhead and reduces response time

in notifying the Bus Controller that an error has occurred.

For 1760 applications, the NVALCHK signal does not change from it's previous state since a Transmit

Command Word has no incoming data words. NVALCHK is a latched version of the STATUS signal

and is updated only on Receive Commands. It is valid on the falling edge of NCMDSTRB of a

Receive Command and remains stable until the next Receive Command message. A Transmit

Command message will not alter this signal because a Transmit Command does not require an

incoming checksum validation.

T0-15

NEMPTY

NVALCHK

CMD WD

PREVIOUS STATE

NCMDSTRB

8.5 µSec

* * *

CT2579 데이터시트(PDF) 13 Page - Aeroflex Circuit Technology

CT2579 데이터시트(HTML) 13 Page - Aeroflex Circuit Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

데이터시트 다운로드

링크 URL