5962-9161706QZC(2007) 查看數據表（PDF） - Atmel Corporation
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5962-9161706QZC Datasheet PDF : 27 Pages
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mally used in accessing a standard static RAM. Each of the flags has a unique address accessed by
either side through address pins A0-A2. None of the other address pins has any effect when accessing
the semaphores. Only data pin D0 is used when writing to a semaphore. If a low level is written to an
unused semaphore location, the flag will be set to zero on that side and to one on the other side (see
Table 5). The semaphore can now only be modified by the side showing the zero. Once a one is written
to this location from the same side, the flag will be set to one for both sides (unless a request is pending
from the other side) and the semaphore can then be written to by either side.
The effect the side writing a zero to a semaphore location has of locking out the other side is the
reason for the use of semaphore logic in interprocessor communication. (A thorough discussion
of the use of this feature follows below). A zero written to the semaphore location from the
locked-out side will be stored in the semaphore request latch for that side until the semaphore is
relinquished by the side having control. When a semaphore flag is read its value is distributed to
all data bits so that a flag set at one reads as one in all data bits and a flag set at zero reads as
all zeros. The read value is latched into the output register of one side when its semaphore
select (SEM) and output enable (OE) signals go active. This prevents the semaphore changing state in
the middle of a read cycle as a result of a write issued by the other side. Because of this latch, a repeated
read of a semaphore flag in a test loop must cause either signal (SEM or OE) to go inactive, otherwise
the output will never change.
The semaphore must use a WRITE/READ sequence in order to ensure that no system level
conflict will occur. A processor requests access to shared resources by attempting to write a
zero to a semaphore location. If the semaphore is already in use, the semaphore request latch
will contain a zero, yet the semaphore flag will appear as a one, and the processor will detect
this status in the subsequent read (see Table 5). For example, assume a processor writes a zero
to the left port at a free semaphore location. On a subsequent read, the processor will verify that
it has written successfully to that location and will assume control over the resource concerned.
If a processor on the right side then attempts to write a zero to the same semaphore flag it will
fail, as will be verified by a subsequent read returning a one from the semaphore location on the
right side has a READ/WRITE sequence been used instead, system conflict problems could
have occurred during the interval between the read and write cycles.
It must be noted that a failed semaphore request needs to be followed by either repeated reads
or by writing a one to the same location. The simple logic diagram for the semaphore flag in Fig-
ure 2 illustrates the reason for this quite clearly. Two semaphore request latches feed into a
semaphore flag. The first latch to send a zero to the semaphore flag will force its side of the
semaphore flag low and other side high. This status will be maintained until a one is written to
the same semaphore request latch. Should a zero be written to the other side’s semaphore
request latch in the meantime, the semaphore flag will flip over to this second side as soon as a
one is written to the first side’s request latch. The second side’s flag will now stay low until its
semaphore request latch is changed to a one. Thus, clearly, if a semaphore flag is requested
and the processor requesting it no longer requires access to the resource, the entire system can
hang up until a one is written to the semaphore request latch concerned.
Semaphore timing becomes critical when both sides request the same token by attempting to
write a zero to it at the same time. Semaphore logic is specially conceived to resolve this prob-
lem. The logic ensures that only one side will receive the token if simultaneous requests are
made. The first side to make a request will receive the token where request do not arrive at the
same time. Where they do arrive at the same time, the logic will assign the token arbitrarily to
one of the ports. It should be noted, however, that semaphores alone do not guarantee that
access to a resource is secure. As with any powerful programming technique, errors can be
introduced if semaphores are misused or misinterpreted. Code integrity is of the utmost perfor-
mance when semaphores are being used instead of slower, more restrictive hardware-intensive
systems.
6 M67025E
4146N–AERO–04/07
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