Page 104 - Embedded Microprocessor Systems Real World Design

Partial Address Decoding
Say a microprocessor with a 20-bit address bus (1Mbit space) needs an 8K x 8
EPROM at location FEOOO. Decoding the entire range of addresses would require
that 6 bits (A14 through A19) be decoded. If only A16 through A19 are decoded
using a four-input NAND gate, the EPROM will be addressed anytime the proces-
sor accesses anything in the range FOOOO through FFFFF. This works as long as
nothing else needs to go in that range. The EPROM may be accessed in the address
space starting at F0000, F2000, F4000, and so on.

Linear Address Decoding

Assume that a microprocessor with a l&bit address bus (64K space) needs an 8K
EPROM and a 2K RAM. The EPROM goes at location 0. To decode this, connect
A15 from the processor to the EPROM -CE input. Connect A14 through an inverter
to the RAM -CE input. Now the EPROM is accessed anytime that A15 is 0, which
is anywhere in the lower 32K. The RAM is selected any time that A14 is 1, which
occurs from 4000 to 7FFF and from COOO to FFFF. The first range also enables the
EPROM, causing a bus conflict. However, if the software addresses the EPROM from
0000 to lFFF and the RAM from COOO to C7FF, the EPROM will be deselected and
no bus conflict occurs. This principle can be expanded to as many decodes as there
are available address lines.

Buffer Always Enabled

When using data bus buffers, it is not always necessary to enable and disable the
buffer’s tristate outputs. Instead, the buffer can be enabled all the time, usually by
grounding the enable pin, and the direction can be controlled. In this scheme, the
processor side of the buffer normally is the input and the peripheral side normally
is the output. The direction is reversed only when the processor reads from the
peripheral. When using this technique, the buffer must switch directions to drive
the processor data bus only when the processor is not driving it and must switch
back only after the peripheral has stopped driving it. Otherwise, you will get bus
contention.

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EMC Considerations

Most embedded systems end up in products that require certification to EMC stan-
dards. In the United States, the Federal Communications Commission (FCC) has
limits on how much RF energy a product can emit. The European Community also
has standards for EMC compatibility, and they include susceptibility to external RF

86 Embedded Micr@ocessm Systems