C8051F040/1/2/3/4/5/6/7
194 Rev. 1.5
16.5.3. Split Mode with Bank Select
When EMI0CF.[3:2] are set to ‘10’, the XRAM memory map is split into two areas, on-chip space and off-
chip space.
• Effective addresses below the 4k boundary will access on-chip XRAM space.
• Effective addresses above the 4k boundary will access off-chip space.
• 8-bit MOVX operations use the contents of EMI0CN to determine whether the memory access is on-
chip or off-chip. The upper 8-bits of the Address Bus A[15:8] are determined by EMI0CN, and the lower
8-bits of the Address Bus A[7:0] are determined by R0 or R1. All 16-bits of the Address Bus A[15:0] are
driven in “Bank Select” mode.
• 16-bit MOVX operations use the contents of DPTR to determine whether the memory access is on-
chip or off-chip, and the full 16-bits of the Address Bus A[15:0] are driven during the off-chip transac-
tion.
16.5.4. External Only
When EMI0CF[3:2] are set to ‘11’, all MOVX operations are directed to off-chip space. On-chip XRAM is
not visible to the CPU. This mode is useful for accessing off-chip memory located between 0x0000 and the
4k boundary.
• 8-bit MOVX operations ignore the contents of EMI0CN. The upper Address bits A[15:8] are not driven
(identical behavior to an off-chip access in “Split Mode without Bank Select” described above). This
allows the user to manipulate the upper address bits at will by setting the Port state directly. The lower
8-bits of the effective address A[7:0] are determined by the contents of R0 or R1.
• 16-bit MOVX operations use the contents of DPTR to determine the effective address A[15:0]. The full
16-bits of the Address Bus A[15:0] are driven during the off-chip transaction.
16.6. Timing
The timing parameters of the External Memory Interface can be configured to enable connection to
devices having different setup and hold time requirements. The Address Setup time, Address Hold time, /
RD and
/WR strobe widths, and in multiplexed mode, the width of the ALE pulse are all programmable in units of
SYSCLK periods through EMI0TC, shown in SFR Definition 16.3, and EMI0CF[1:0].
The timing for an off-chip MOVX instruction can be calculated by adding 4 SYSCLK cycles to the timing
parameters defined by the EMI0TC register. Assuming non-multiplexed operation, the minimum execution
time for an off-chip XRAM operation is 5 SYSCLK cycles (1 SYSCLK for /RD or /WR pulse + 4 SYSCLKs).
For multiplexed operations, the Address Latch Enable signal will require a minimum of 2 additional SYS-
CLK cycles. Therefore, the minimum execution time of an off-chip XRAM operation in multiplexed mode is
7 SYSCLK cycles (2 SYSCLKs for /ALE, 1 for /RD or /WR + 4 SYSCLKs). The programmable setup and
hold times default to the maximum delay settings after a reset.
Table 16.1 lists the AC parameters for the External Memory Interface, and Figure 16.4 through Figure 16.9
show the timing diagrams for the different External Memory Interface modes and MOVX operations.
