CY7C024/024A/0241
CY7C025/0251
Semaphores are accessed by asserting SEM LOW. The SEM
pin functions as a chip select for the semaphore latches (CE
must remain HIGH during SEM LOW). A0–2 represents the
semaphore address. OE and R/W are used in the same manner
as a normal memory access. When writing or reading a
semaphore, the other address pins have no effect.
When writing to the semaphore, only I/O
0
is used. If a zero is
written to the left port of an available semaphore, a one appears
at the same semaphore address on the right port. That
semaphore can now only be modified by the side showing zero
(the left port in this case). If the left port now relinquishes control
by writing a one to the semaphore, the semaphore is set to one
Table 1. Non-Contending Read/Write
Inputs
CE
H
X
L
L
L
L
L
L
X
H
X
H
X
L
L
X
X
R/W
X
X
L
L
L
H
H
H
X
H
H
OE
X
X
X
X
X
L
L
L
H
L
L
X
X
X
X
UB
X
H
L
H
L
L
H
L
X
X
H
X
H
L
X
LB
X
H
H
L
L
H
L
L
X
X
H
X
H
X
L
SEM
H
H
H
H
H
H
H
H
X
L
L
L
L
L
L
High Z
High Z
High Z
Data In
Data In
High Z
Data Out
Data Out
High Z
Data Out
Data Out
Data In
Data In
for both sides. However, if the right port had requested the
semaphore (written a zero) while the left port had control, the
right port immediately owns the semaphore as soon as the left
port released it.
shows sample semaphore operations.
When reading a semaphore, all sixteen/eighteen data lines
output the semaphore value. The read value is latched in an
output register to prevent the semaphore from changing state
during a write from the other port. If both ports attempt to access
the semaphore within t
SPS
of each other, the semaphore is
definitely obtained by one side or the other, but there is no
guarantee which side controls the semaphore
Outputs
I/O
0
–I/O
7[3]
I/O
8
–I/O
15[4]
High Z
High Z
Data In
High Z
Data In
Data Out
High Z
Data Out
High Z
Data Out
Data Out
Data In
Data In
Operation
Deselected: Power Down
Deselected: Power Down
Write to Upper Byte Only
Write to Lower Byte Only
Write to Both Bytes
Read Upper Byte Only
Read Lower Byte Only
Read Both Bytes
Outputs Disabled
Read Data in Semaphore Flag
Read Data in Semaphore Flag
Write D
IN0
into Semaphore Flag
Write D
IN0
into Semaphore Flag
Not Allowed
Not Allowed
Table 2. Interrupt Operation Example (Assumes BUSY
L
=BUSY
R
=HIGH)
Function
Set Right INT
R
Flag
Reset Right INT
R
Flag
Set Left INT
L
Flag
Reset Left INT
L
Flag
Left Port
R/W
L
L
X
X
X
CE
L
L
X
X
L
OE
L
X
X
X
L
A
0L–11L
(1)FFF
X
X
(1)FFE
INT
L
X
X
L
H
R/W
R
X
X
L
X
CE
R
X
L
L
X
Right Port
OE
R
X
L
X
X
A
0R–11R
X
(1)FFF
(1)FFE
X
INT
R
L
H
X
X
Document #: 38-06035 Rev. *D
Page 5 of 21
CYPRESS [ CYPRESS SEMICONDUCTOR ]
