Datasheet

Memory Expansion and Chip-Select
MC68HC812A4 Data Sheet, Rev. 7
84 Freescale Semiconductor
8.3 Chip-Select Stretch
Each chip-select can be chosen to stretch bus cycles associated with it. Stretch can be zero, one, two, or
three whole cycles added which allows interfacing to external devices which cannot meet full bus speed
timing. Figure 8-3, Figure 8-4, Figure 8-5, and Figure 8-6 show the waveforms for zero to three cycles of
stretch.
Figure 8-3. Chip-Select with No Stretch
Figure 8-4. Chip-Select with 1-Cycle Stretch
Figure 8-5. Chip-Select with 2-Cycle Stretch
Figure 8-6. Chip-Select with 3-Cycle Stretch
UNSTRETCHED BUS CYCLE
CS
INTERNAL
E-CLOCK
ECLK PIN
CS
STRETCHED
INTERNAL
E-CLOCK
ECLK PIN
STRETCHED BY 1 CYCLE
CS
STRETCHED
INTERNAL
STRETCHED BY 2 CYCLES
E-CLOCK
ECLK PIN
STRETCHED BY 3 CYCLES
CS
STRETCHED
INTERNAL
E-CLOCK
ECLK PIN