peripheral memory block (either I/O or memory ad-
dress space). The base address of the peripheral
memory block is programmable. PCS3–PCS0 are held
High during a bus hold condition. They are also held
High during reset.
MCS2–MCS0
(MCS2/PIO24, MCS1/PIO15, MCS0/PIO14)
Midrange Memory Chip Selects (output,
synchronous, internal pullup)
These pins indicate to the system that a memory ac-
cess is in progress to the corresponding region of the
midrange memory block. The base address and size of
the midrange memory block are programmable.
MCS2–MCS0 are held High during a bus hold condi-
tion. In addition, they have weak internal pullup resis-
tors that are active during reset. Unlike the UCS and
LCS chip selects, the MCS outputs assert with the mul-
tiplexed AD address bus.
PCS4 is not available on the Am186ER and Am188ER
microcontrollers.
Unlike the UCS/LCS chip selects, the PCS outputs as-
sert 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 covered by peripheral chip selects in the
80C186/80C188 microcontrollers.
NMI
PCS5/A1/PIO3
Nonmaskable Interrupt (input, synchronous, edge-
sensitive)
Peripheral Chip Select 5 (output, synchronous)
Latched Address Bit 1 (output, synchronous)
This pin indicates to the microcontroller that an inter-
rupt request has occurred. The NMI signal is the high-
est priority hardware interrupt and, unlike the INT4–
INT0 pins, cannot be masked. The microcontroller al-
ways transfers program execution to the location spec-
ified by the nonmaskable interrupt vector in the
microcontroller interrupt vector table when NMI is as-
serted.
PCS5—This pin indicates to the system that a memory
access is in progress to the sixth region of the periph-
eral memory block (either I/O or memory address
space). The base address of the peripheral memory
block is programmable. PCS5 is held High during a bus
hold condition. It is also held High 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 covered by peripheral chip selects in the 80C186
and 80C188 microcontrollers.
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 reg-
isters. 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 in-
terrupts are reenabled by software in the NMI interrupt
service routine, via the STI instruction for example, an
NMI currently in service will not have any effect on the
priority resolution of maskable interrupt requests. For
this reason, it is strongly advised that the interrupt ser-
vice routine for NMI does not enable the maskable in-
terrupts.
A1—When the EX bit in the MCS and PCS auxiliary
register is 0, this pin supplies an internally latched ad-
dress bit 1 to the system. During a bus hold condition,
A1 retains its previously latched value.
PCS6/A2/PIO2
Peripheral Chip Select 6 (output, synchronous)
Latched Address Bit 2 (output, synchronous)
PCS6—This pin indicates to the system that a memory
access is in progress to the seventh region of the pe-
ripheral memory block (either I/O or memory address
space). The base address of the peripheral memory
block is programmable. PCS6 is held High during a bus
hold condition or reset.
An NMI transition from Low to High is latched and syn-
chronized internally, and it initiates the interrupt at the
next instruction boundary. To guarantee that the inter-
rupt is recognized, the NMI pin must be asserted for at
least one CLKOUTA period. Because NMI is rising
edge sensitive, holding the pin High during reset has no
effect on program execution.
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 covered by peripheral chip selects in earlier gen-
erations of the Am186/Am188 microcontrollers.
PCS3–PCS0
(PCS3/PIO19, PCS2/PIO18,
PCS1/PIO17, PCS0/PIO16)
A2—When the EX bit in the MCS and PCS auxiliary
register is 0, this pin supplies an internally latched ad-
dress bit 2 to the system. During a bus hold condition,
A2 retains its previously latched value.
Peripheral Chip Selects (output, synchronous)
These pins indicate to the system that a memory ac-
cess is in progress to the corresponding region of the
34
Am186TMER and Am188TMER Microcontrollers Data Sheet
AMD [ AMD ]
