CYD04S72V
CYD09S72V
CYD18S72V
Document Number : 38-06069 Rev. *M Page 9 of 30
Counter Increment Operation
Once the address counter register is initially loaded with an
external address, the counter can internally increment the
address value, potentially addressing the entire memory array.
Only the unmasked bits of the counter register are incremented.
The corresponding bit in the mask register must be a “1” for a
counter bit to change. The counter register is incremented by 1
if the least significant bit is unmasked, and by 2 if it is masked. If
all unmasked bits are “1,” the next increment will wrap the
counter back to the initially loaded value. If an Increment results
in all the unmasked bits of the counter being “1s,” a counter
interrupt flag (CNTINT
) is asserted. The next Increment will
return the counter register to its initial value, which was stored in
the mirror register. The counter address can instead be forced to
loop to 00000 by externally connecting CNTINT
to CNTRST.
[29]
An increment that results in one or more of the unmasked bits of
the counter being “0” will de-assert the counter interrupt flag. The
example in Figure 2 shows the counter mask register loaded with
a mask value of 0003Fh unmasking the first 6 bits with bit “0” as
the LSB and bit “16” as the MSB. The maximum value the mask
register can be loaded with is 1FFFFh. Setting the mask register
to this value allows the counter to access the entire memory
space. The address counter is then loaded with an initial value
of 8h. The base address bits (in this case, the 6th address
through the 16th address) are loaded with an address value but
do not increment once the counter is configured for increment
operation. The counter address will start at address 8h. The
counter will increment its internal address value till it reaches the
mask register value of 3Fh. The counter wraps around the
memory block to location 8h at the next count. CNTINT
is issued
when the counter reaches its maximum value.
Counter Hold Operation
The value of all three registers can be constantly maintained
unchanged for an unlimited number of clock cycles. Such
operation is useful in applications where wait states are needed,
or when address is available a few cycles ahead of data in a
shared bus interface.
Counter Interrupt
The counter interrupt (CNTINT) is asserted LOW when an
increment operation results in the unmasked portion of the
counter register being all “1s.” It is deasserted HIGH when an
Increment operation results in any other value. It is also
de-asserted by Counter Reset, Counter Load, Mask Reset and
Mask Load operations, and by MRST.
Counter Readback Operation
The internal value of the counter register can be read out on the
address lines. Readback is pipelined; the address will be valid
t
CA2
after the next rising edge of the port’s clock. If address
readback occurs while the port is enabled (CE0
LOW and CE1
HIGH), the data lines (DQs) will be three-stated. Figure 1 shows
a block diagram of the operation.
Retransmit
Retransmit is a feature that allows the Read of a block of memory
more than once without the need to reload the initial address.
This eliminates the need for external logic to store and route
data. It also reduces the complexity of the system design and
saves board space. An internal “mirror register” is used to store
the initially loaded address counter value. When the counter
unmasked portion reaches its maximum value set by the mask
register, it wraps back to the initial value stored in this “mirror
register.” If the counter is continuously configured in increment
mode, it increments again to its maximum value and wraps back
to the value initially stored into the “mirror register.” Thus, the
repeated access of the same data is allowed without the need
for any external logic.
Mask Reset Operation
The mask register is reset to all “1s,” which unmasks every bit of
the counter. Master reset (MRST
) also resets the mask register
to all “1s.”
Mask Load Operation
The mask register is loaded with the address value presented at
the address lines. Not all values permit correct increment
operations. Permitted values are of the form 2
n
–1 or 2
n
–2. From
the most significant bit to the least significant bit, permitted
values have zero or more “0s,” one or more “1s,” or one “0.” Thus
1FFFF, 003FE, and 00001 are permitted values, but 1F0FF,
003FC, and 00000 are not.
Mask Readback Operation
The internal value of the mask register can be read out on the
address lines. Readback is pipelined; the address will be valid
t
CM2
after the next rising edge of the port’s clock. If mask
readback occurs while the port is enabled (CE0
LOW and CE1
HIGH), the data lines (DQs) will be three-stated. Figure 1 shows
a block diagram of the operation.
Counting by Two
When the least significant bit of the mask register is “0,” the
counter increments by two. This may be used to connect the x72
devices as a 144-bit single port SRAM in which the counter of
one port counts even addresses and the counter of the other port
counts odd addresses. This even-odd address scheme stores
one half of the 144-bit data in even memory locations, and the
other half in odd memory locations.
Note
29. CNTINT
and CNTRST specs are guaranteed by design to operate properly at speed grade operating frequency when tied together.
