CY7C43646
CY7C43666
CY7C43686
Document #: 38-06023 Rev. *C
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HIGH (see Table 3). FIFO reads on Port B and writes to Port
C are independent of any concurrent Port A operation.
The set-up and hold time constraints to the port clocks for the
port Chip Selects and Write/Read Selects are only for enabling
write and read operations and are not related to
high-impedance control of the data outputs. If a port enable is
LOW during a clock cycle, the port’s Chip Select and
Write/Read Select may change states during the set-up and
hold time window of the cycle.
When operating the FIFO in FWFT Mode with the Output
Ready flag LOW, the next word written is automatically sent to
the FIFO’s output register by the LOW-to-HIGH transition of
the port clock that sets the Output Ready flag HIGH, data
residing in the FIFO’s memory array is clocked to the output
register only when a read is selected using the port’s Chip
Select, Write/Read Select, Enable, and Mailbox Select.
When operating the FIFO in CY Standard Mode, regardless of
whether the Empty Flag is LOW or HIGH, data residing in the
FIFO’s memory array is clocked to the output register only
when a read is selected using the port’s Chip Select,
Write/Read Select, Enable, and Mailbox Select.
Synchronized FIFO Flags
Each FIFO is synchronized to its port clock through at least two
flip-flop stages. This is done to improve flag-signal reliability by
reducing the probability of the metastable events when CLKA,
CLKB, and CLKC operate asynchronously to one another.
EFA/ORA, AEA, FFA/IRA, and AFA are synchronized to
CLKA. EFB/ORB and AEB are synchronized to CLKB.
FFC/IRC and AFC are synchronized to CLKC. Table 5 and
Table 6 show the relationship of each port flag to FIFO1 and
FIFO2.
Empty/Output Ready Flags (EFA/ORA, EFB/ORB)
These are dual-purpose flags. In the FWFT Mode, the Output
Ready (ORA, ORB) function is selected. When the Output
Ready flag is HIGH, new data is present in the FIFO output
register. When the Output Ready flag is LOW, the previous
data word is present in the FIFO output register and attempted
FIFO reads are ignored.(See footnote #1)
In the CY Standard Mode, the Empty Flag (EFA, EFB) function
is selected. When the Empty flag is HIGH, data is available in
the FIFO’s RAM memory for reading to the output register.
When Empty flag is LOW, the previous data word is present in
the FIFO output register and attempted FIFO reads are
ignored.
The Empty/Output Ready flag of a FIFO is synchronized to the
port clock that reads data from its array. For both the FWFT
and CY Standard modes, the FIFO read pointer is incremented
each time a new word is clocked to its output register. The
state machine that controls an Output Ready flag monitors a
write pointer and read pointer comparator that indicates when
the FIFO SRAM status is empty, or empty+1.
In FWFT Mode, from the time a word is written to a FIFO, it
can be shifted to the FIFO output register in a minimum of
three cycles of the Output Ready flag synchronizing clock.
Therefore, an Output Ready flag is LOW if a word in memory
is the next data to be sent to the FIFO output register and three
cycles have not elapsed since the time the word was written.
The Output Ready flag of the FIFO remains LOW until the third
LOW-to-HIGH transition of the synchronizing clock occurs,
simultaneously forcing the Output Ready flag HIGH and
shifting the word to the FIFO output register.
In the CY Standard Mode, from the time a word is written to a
FIFO, the Empty Flag will indicate the presence of data
available for reading in a minimum of two cycles of the Empty
Flag synchronizing clock. Therefore, an Empty Flag is LOW if
a word in memory is the next data to be sent to the FIFO output
register and two cycles have not elapsed since the time the
word was written. The Empty Flag of the FIFO remains LOW
until the second LOW-to-HIGH transition of the synchronizing
clock occurs, forcing the Empty Flag HIGH; only then can data
be read.
A LOW-to-HIGH transition on an Empty/Output Ready flag
synchronizing clock begins the first synchronization cycle of a
write if the clock transition occurs at time tSKEW1 or greater
after the write. Otherwise, the subsequent clock cycle can be
the first synchronization cycle.
Full/Input Ready Flags (FFA/IRA, FFC/IRC)
This is a dual-purpose flag. In FWFT Mode, the Input Ready
(IRA and IRC) function is selected. In CY Standard Mode, the
Full Flag (FFA and FFC) function is selected. For both timing
modes, when the Full/Input Ready flag is HIGH, a memory
location is free in the SRAM to receive new data. No memory
locations are free when the Full/Input Ready flag is LOW and
attempted writes to the FIFO are ignored.
The Full/Input Ready flag of a FIFO is synchronized to the port
clock that writes data to its array. For both FWFT and CY
Standard modes, each time a word is written to a FIFO, its
write pointer is incremented. The state machine that controls
a Full/Input Ready flag monitors a write pointer and read
pointer comparator that indicates when the FIFO SRAM status
is full, or full–1. From the time a word is read from a FIFO, its
previous memory location is ready to be written to in a
minimum of two cycles of the Full/Input Ready flag synchro-
nizing clock. Therefore, an Full/Input Ready flag is LOW if less
than two cycles of the Full/Input Ready flag synchronizing
clock have elapsed since the next memory write location has
been read. The second LOW-to-HIGH transition on the
Full/Input Ready flag synchronizing clock after the read sets
the Full/Input Ready flag HIGH.
A LOW-to-HIGH transition on a Full/Input Ready flag synchro-
nizing clock begins the first synchronization cycle of a read if
the clock transition occurs at time tSKEW1 or greater after the
read. Otherwise, the subsequent clock cycle can be the first
synchronization cycle.
Almost Empty Flags (AEA, AEB)
The Almost Empty flag of a FIFO is synchronized to the port
clock that reads data from its array. The state machine that
controls an Almost Empty flag monitors a write pointer and
read pointer comparator that indicates when the FIFO SRAM
status is almost empty, or almost empty+1. The Almost Empty
state is defined by the contents of register X1 for AEB and
register X2 for AEA. These registers are loaded with preset
values during a FIFO reset, programmed from Port A, or
programmed serially (see Almost Empty flag and Almost Full
flag offset programming above). An Almost Empty flag is LOW
when its FIFO contains X or less words and is HIGH when its
FIFO contains (X+1) or more words. [2]
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