CYD01S36V
CYD02S36V/36VA/CYD04S36V
CYD09S36V/CYD18S36V
Document Number: 38-06076 Rev. *G
Page 7 of 28
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 wraps 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 returns 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.
[21]
An increment that
results in one or more of the unmasked bits of the counter being
“0” de-asserts the counter interrupt flag. The example in
Figure
3
on page 9
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 3FFFFh. 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 starts at address 8h. The counter increments 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 is 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) are three-stated.
Figure 2
on page 8 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 opera-
tions. 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
7FFFF, 003FE, and 00001 are permitted values, but 7F0FF,
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 is 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) are three-stated.
Figure 2
on page 8 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 x36
devices as a 72-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 72-bit data in even memory locations, and the
other half in odd memory locations.
Note
21. CNTINT and CNTRST specs are guaranteed by design to operate properly at speed grade operating frequency when tied together.
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