P R E L I M I N A R Y
the midrange memory block are programmable. MCS2
is configured for 8-bit or 16-bit bus size via the auxiliary
configuration register.
NMI
Nonmaskable Interrupt (input, synchronous,
edge-sensitive)
MCS2 is three-stated and held resistively High during a
bus hold condition. In addition, it has a weak internal
pullup resistor that is active during reset.
This pin indicates to the microcontroller that an
interrupt request has occurred. The NMI signal is the
highest priority hardware interrupt and, unlike the
INT6–INT0 pins, cannot be masked. The
microcontroller always transfers program execution to
the location specified by the nonmaskable interrupt
vector in the microcontroller interrupt vector table when
NMI is asserted.
If MCS0 is programmed to be active for the entire
middle chip-select range, then this signal is available
as a PIO or a DRAM control. If this pin is not
programmed as a PIO or DRAM control and if MCS0 is
programmed for the whole middle chip-select range,
this signal operates normally.
Although NMI is the highest priority interrupt source, it
does not participate in the priority resolution process of
the maskable interrupts. There is no bit associated with
NMI in the interrupt in-service or interrupt request
registers. This means that a new NMI request can
interrupt an executing NMI interrupt service routine. As
with all hardware interrupts, the IF (interrupt flag) is
cleared when the processor takes the interrupt,
disabling the maskable interrupt sources. However, if
maskable interrupts are re-enabled by software in the
NMI interrupt service routine, via the STI instruction for
example, the fact that an NMI is currently in service
does not have any effect on the priority resolution of
maskable interrupt requests. For this reason, it is
strongly advised that the interrupt service routine for
NMI should not enable the maskable interrupts.
LCAS—When either bank of DRAM is activated, the
LCAS functionality is enabled. The LCAS activates
when the DRAM access is for the AD7–AD0 byte.
LCAS also activates at the start of a DRAM refresh
access.
LCAS is three-stated and held resistively High during a
bus hold condition. In addition, LCAS has a weak
internal pullup resistor that is active during reset.
MCS3/RAS1/PIO25
Midrange Memory Chip Select 3
(output, synchronous, internal pullup)
Row Address Strobe 1 (output, synchronous)
MCS3—This pin indicates to the system that a memory
access is in progress to the fourth region of the
midrange memory block. The base address and size of
the mid-range memory block are programmable.
MCS3 is configured for 8-bit or 16-bit bus size by the
auxiliary configuration register.
An NMI transition from Low to High is latched and
synchronized internally, and it initiates the interrupt at
the next instruction boundary. To guarantee that the
interrupt is recognized, the NMI pin must be asserted
for at least one CLKOUTA period.
MCS3 is three-stated and held resistively High during a
bus hold condition. In addition, this pin has a weak
internal pullup resistor that is active during reset.
PCS1/PIO17, PCS0/PIO16
Peripheral Chip Selects (output, synchronous)
These pins indicate to the system that a memory
access is in progress to the corresponding region of the
peripheral memory block (either I/O or memory
address space). The base address of the peripheral
memory block is programmable.
If MCS0 is programmed for the entire middle chip-
select range, then this signal is available as a PIO or a
DRAM control. If MCS3 is not programmed as a PIO or
DRAM control and if MCS0 is programmed for the
entire middle chip-select range, this signal operates
normally.
The PCS chip selects can overlap either block of
DRAM. The PCS chip selects must have the same or
greater number of wait states as the bank of DRAM
they overlap. The PCS signals take precedence over
DRAM accesses when DRAM and memory-mapped
peripherals overlap.
RAS1—This pin is the row address strobe for the upper
DRAM block. The selection of RAS1 or UCS
functionality, along with their configurations, are set
using the UMCS register. When RAS1 is activated, the
code activating RAS1 must not reside in the UCS
memory block. When RAS1 is activated, UCS is
automatically deactivated and remains negated.
PCS1–PCS0 are three-stated and held resistively High
during a bus hold condition. In addition, PCS1–PCS0
each have a weak internal pullup resistor that is active
during reset.
RAS1 is three-stated and held resistively High during a
bus hold condition. In addition, RAS1 has a weak
internal pullup resistor that is active during reset.
Unlike the UCS and LCS chip selects, the PCS outputs
assert with the multiplexed AD address bus. Note also
that each peripheral chip select asserts over a 256-
byte address range, which is twice the address range
26
Am186ED/EDLV Microcontrollers
AMD [ AMD ]
