control register (UMCS, LMCS, MMCS, PACS, and
MPCS) contains a single-bit field that determines
whether the external ready signal is required or ig-
nored. The internal memory ignores the external ready
signal.
CHIP-SELECT UNIT
The Am186ER and Am188ER microcontrollers contain
logic that provides programmable chip-select genera-
tion for both memories and peripherals. The logic can
be programmed to provide external ready and wait-
state generation and latched address bits A1 and A2.
The chip-select lines are active for all memory and I/O
cycles in their programmed areas, whether they are
generated by the CPU or by the integrated DMA unit.
The number of wait states to be inserted for each ac-
cess to an external peripheral or memory region is pro-
grammable. The chip-select control registers for UCS,
LCS, MCS3–MCS0, PCS6, and PCS5 contain a two-bit
field that determines the number of wait states from
zero to three to be inserted. PCS3–PCS0 use three bits
to provide additional values of 5, 7, 9, and 15 wait
states. The chip-select control register for internal
memory always specifies no wait states.
Chip-Select Timing
The timing for the UCS and LCS outputs is modified
from the original Am186 microcontroller. These outputs
now assert in conjunction with the nonmultiplexed ad-
dress bus for normal memory timing. To enable these
outputs to be available earlier in the bus cycle, the num-
ber of programmable memory size selections has been
reduced.
When external ready is required, internally pro-
grammed wait states will always complete before exter-
nal ready can terminate or extend a bus cycle. For
example, if the internal wait states are set to insert two
wait states, the processor samples the external ready
pin during the first wait cycle. If external ready is as-
serted at that time, the access completes after six cy-
cles (four cycles plus two wait states). If external ready
is not asserted during the first wait state, the access is
extended until ready is asserted, which is followed by
one more wait state followed by t4.
Ready and Wait-State Programming
The Am186ER and Am188ER microcontrollers can be
programmed to sense a ready signal for each of the ex-
ternal peripheral or memory chip-select lines. The ex-
ternal ready signal can be either the ARDY or SRDY
signal as shown in Figure 11. For diagrams of the syn-
chronous ready waveforms and asynchronous ready
waveforms, refer to page 97. Each external chip-select
ARDY
D
Q
Bus Ready
CLKOUTA
Rising Edge
D
Q
Falling Edge
SRDY
D
Q
Falling Edge
Figure 11. ARDY and SRDY Synchronization Logic Diagram
Am186TMER and Am188TMER Microcontrollers Data Sheet
49
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