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74FR25900 View Datasheet(PDF) - Fairchild Semiconductor

Part Name
Description
Manufacturer
74FR25900 Datasheet PDF : 7 Pages
1 2 3 4 5 6 7
Functional Description
The 74FR25900 allows 9-bit data to be transferred from
any of three 9-bit I/O ports to either of the two remaining
I/O ports. The device employs latches in all paths for either
transparent or synchronous operation. Readback capability
from any port to itself is also possible.
Data transfer within the 74FR25900 is controlled through
use of the select (S0 and S1) and output-enable (OEA, OEB
and OEC) inputs as described in Table 1. Additional control
is available by use of the latch-enable inputs (LEAC, LECA,
LEBC, LECB) allowing either synchronous or transparent
transfers (see Table 2). Table 1 indicates several readback
conditions. By latching data on a given port and initiating
the readback control configuration, previous data may be
read for system verification or diagnostics. This mode may
be useful in implementing system diagnostics.
Data at the port to be readback must be latched prior to
enabling the outputs on that port. If this is not done, a
closed data loop will result causing possible data integrity
problems. Note that the A and B Ports allow readback with-
out affecting any other port. C Port, however, requires inter-
ruption of either A or B Ports to complete its readback path.
PINV controls inversion of the C8 bit. A LOW on PINV
allows C8 data to pass unaltered. A HIGH causes inversion
of the data. See Table 3. This feature allows forcing of par-
ity errors for use in system diagnostics. This is particularly
helpful in 486 processor designs as the 486 does not pro-
vide odd/even parity selection internally.
TABLE 1. Datapath Control
Inputs
S0 S1 OEA OEB OEC
Function
LXH
L
L Port A to Port C
LLH
H
H Port A to Port B
LO H
H
L Port A to B+C
HL
L
L
H Port B to Port A
HX H
L
L Port B to Port C
HO L
L
L Port B to A+C
XH L
L
H Port C to Port A
XH H
H
H Port C to Port B
XH L
H
H Port C to A+B
XX H
L
H Outputs Disabled
L
L
L
X
X (Readback to A)
(Note 1)
LH L
X
L (Readback to A or C)
(Note 1)
HL X
H
X (Readback to B)
(Note 1)
HH X
H
L (Readback to B or C)
(Note 1)
Note 1: Readback operation in latched mode only. Transparent operation
could result in unpredictable results.
TABLE 2. Latch-Enable Control
LExx
L
L
H
Input
L
H
X
Output
L
H
Q0
TABLE 3. PINV Control
PINV
L
L
H
H
C8
A8 or B8
L
L
H
H
L
H
H
L
Key:
L = LOW Voltage
H = HIGH Voltage Level
Q0 = Output state prior to LExx LOW-to-HIGH transition
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