C8051F02X Datasheet, PDF (148/272 Pages) Silicon Laboratories – 8K ISP FLASH MCU Family

C8051F020/1/2/3
16.4. Multiplexed and Non-multiplexed Selection
The External Memory Interface is capable of acting in a Multiplexed mode or a Non-multiplexed mode, depending
on the state of the EMD2 (EMI0CF.4) bit.
16.4.1. Multiplexed Configuration
In Multiplexed mode, the Data Bus and the lower 8-bits of the Address Bus share the same Port pins: AD[7:0]. In this
mode, an external latch (74HC373 or equivalent logic gate) is used to hold the lower 8-bits of the RAM address. The
external latch is controlled by the ALE (Address Latch Enable) signal, which is driven by the External Memory Inter-
face logic. An example of a Multiplexed Configuration is shown in Figure 16.3.
In Multiplexed mode, the external MOVX operation can be broken into two phases delineated by the state of the ALE
signal. During the first phase, ALE is high and the lower 8-bits of the Address Bus are presented to AD[7:0]. During
this phase, the address latch is configured such that the ‘Q’ outputs reflect the states of the ‘D’ inputs. When ALE
falls, signaling the beginning of the second phase, the address latch outputs remain fixed and are no longer dependent
on the latch inputs. Later in the second phase, the Data Bus controls the state of the AD[7:0] port at the time /RD or
/WR is asserted.
See Section “16.6.2. Multiplexed Mode” on page 156 for more information.
Figure 16.3. Multiplexed Configuration Example
A[15:8]
ADDRESS BUS
74HC373
E
ALE
G
AD[7:0] ADDRESS/DATA BUS D
Q
M
V_DD
I
8
F
/WR
/RD
A[15:8]
A[7:0]
64K X 8
SRAM
I/O[7:0]
CE
WE
OE
148
Rev. 1.4