Rainbow Electronics DS89C450 - Manual (Page 32)

DS89C430/DS89C440/DS89C450 Ultra-High-Speed Flash Microcontrollers
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Note that there are a few exceptions for this mode of operation when PAGES1 and PAGES2 are set to 00b:
· PSEN is asserted for both a page hit and a page miss for a full clock cycle.
· The execution of external MOVX instruction causes a page miss.
· A page miss occurs when fetching the next external instruction following the execution of an external MOVX
instruction.
Figure 10 shows the external memory cycle for this bus structure. The first case illustrates a back-to-back
execution sequence for the one-cycle page mode (PAGES1 = PAGES0 = 0b). PSEN remains active during page hit
cycles, and page misses are forced during and after MOVX executions, independent of the most significant byte of
the subsequent addresses. The second case illustrates a MOVX execution sequence for two-cycle page mode
(PAGES1 = 0 and PAGES0 = 1).
PSEN is active for a full clock cycle in code fetches. Note that changing the most
significant byte of the data address causes the page misses in this sequence. The third case illustrates a MOVX
execution sequence for four-cycle page mode (PAGES1 = 1 and PAGES0 = 0). There is no page miss in this
execution cycle as the most significant byte of the data address is assumed to match the last program address.
The second page mode (page mode 2) external bus structure multiplexes the most significant address byte with
data on P2 and uses P0 for the least significant address byte. This bus structure is used to speed up external code
fetches only. External data memory access cycles are identical to the nonpage mode except for the different
signals on P0 and P2.
Figure 11 illustrates the memory cycle for external code fetches.
Figure 11. Page Mode 2, External Code Fetch Cycle (CD1:CD0 = 10)
Internal Memory Cycles
C2 C3 C4 C1 C2 C1 C2
XTAL1
ALE
Port 0
Port 2
PSEN
C1
Ext Code Fetches
Page Miss Page Hit Page Hit
Data Data
LSB Add LSB Add LSB Add
Data MSB Add
Stretch External Data Memory Cycle in Page Mode
The DS89C430 allows software to adjust the speed of external data memory access by stretching the memory bus
cycle in page mode operation just like nonpage mode operation. The following tables summarize the stretch values
and their effect on the external MOVX memory bus cycle and the control signals’ pulse width in terms of the
number of oscillator clocks. A stretch machine cycle always contains four system clocks, independent of the logic
value of the page mode select bits.