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Unformatted text preview: t keep signals stable
registers require enable signals which now need ports as well µ
C device A
device B Q EN
EN device A 7 Encode n single-bit device ports using log n bits of a
controller port Conflict on device signals (e.g., one signal affects both) D $ enabled decoder: one-hot, input-only device ports
registered decoder: input-only (but not one-hot) device ports
multiplexor: output-only device ports µC log(n)
bits D µC n one-hot
Q device B log(n)
enable Q EN n register
select lines n signals registered decoder enabled decoder data n signals µC
select bits multiplexor 4 8 % .
0 8 % .
0 Partition the address space
Assign memory-mapped locations
Software Address bus selects device
Data bus contains data loads read from the device
stores write to the device Can exploit unused bits for device input-only ports # 7 $ $ Problem of port shortage so common port expansion
Easily connect to the microprocessor
Timing on ports may be slightly different
May not support interrupts & Exploit specialized functions (e.g., UART, timers)
Attempt to connect directly to a device port without adding inte...
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- Fall '09