SMJ44C251B Datasheet, PDF (17/53 Pages) Texas Instruments – 262144 BY 4-BIT MULTIPORT VIDEO RAM

SMJ44C251B
262144 BY 4-BIT
MULTIPORT VIDEO RAM
SGMS058A – MARCH 1995 – REVISED JUNE 1995
serial-access operation
The serial-read and serial-write operations can be performed through the SAM port simultaneously and
asynchronously with DRAM operations except during transfer operations. The preceding transfer operation
determines the input or output state of the SAM port. If the preceding transfer operation is a read-transfer
operation, the SAM port is in the output mode. If the preceding transfer operation is a write- or
pseudo-write-transfer operation, the SAM port is in the input mode.
Serial data can be read out of or written into SAM by clocking SC starting at the tap point loaded by the preceding
transfer cycle, proceeding sequentially to the most significant bit (bit 511), then wrapping around to the least
significant bit (bit 0) (see Figure 10).
0
1
2
Tap
510 511
Figure 10. Serial Pointer Direction for Serial Read / Write
For split-register read-transfer operation, serial data can be read out from the active half of SAM by clocking
SC starting at the tap point loaded by the preceding split-register-transfer cycle, then proceeding sequentially
to the most significant bit of the half, bit 255 or bit 511. If there is a split-register-read transfer to the inactive half
during this period, the serial pointer points next to the tap-point location loaded by that split register (see
Figure 11, Case I). If there is no split-register read transfer to the inactive half during this period, the serial pointer
points next to bit 256 or bit 0, respectively (see Figure 11, Case II).
Case I
0
Tap
254 255
256
Tap
510 511
Case I I
0
Tap
254 255
256
Tap
Figure 11. Serial Pointer for Split-Register Read
510 511
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