Using the External Memory Interface for I/O expansion

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4

Using the External Memory Interface for I/O

expansion

The EMI can be used not only to interface external memories, but also to create additional

microcontroller ports. The EMI can be used to expand the number of inputs and/or outputs

lines when all GPIOs are used and you need more GPIOs. In this section we give a solution

using latches and buffers that allows you to expand the number of inputs and/or outputs

using the EMI interface of the STR710 microcontroller.

4.1

Figure 11 shows how the input/output expander is implemented using 74LCX573 (8-bit

latch) to create an 8-line output port and 74LCX244 (8-bit buffer) to create an 8-line input

port (see 74LCX573 and 74LCX244 datasheets). The latch inputs and the buffer outputs are

connected to the LSB data byte: D0 to D7.

Figure 11.

Eight-channel expander

To put a byte of data on the latch outputs, you must write the data in an address located in

address range corresponding to the CSn.x bank. For example, if the CSn.x used is CSn.0,

and the EMI address is not remapped at address 0 (EMI address range = 0x60000000 to

74V1G02

74V1G08

STR710

74LCX573

WRL

8 outputs

D[0..15]

CSn.x

RD

74LCX244

OE

1A-2A

16

D[0..7]

LE

D[0..7]

1G

2G

EMI

data bus

1Y-2Y

D[0..7]

Q[0..7]

8 inputs

16

1

11

19

1

8

8

8

8

3.3v

20

GND

VDD

10

GND

10

3.3v

20

VDD