Pseudo Static RAM (PSRAM) Support
Reading and Writing the PCB
The Am186ER and Am188ER microcontrollers support
the use of PSRAM devices in low memory chip-select
(LCS) space only. When PSRAM mode is enabled, the
timing for the LCS signal is modified by the chip-select
control unit to provide a CS precharge period during
PSRAM accesses. The 50-MHz timing of the
Am186ER and Am188ER microcontrollers is appropri-
ate to allow 70-ns PSRAM to run with one wait state.
PSRAM mode is enabled through a bit in the Low Mem-
ory Chip-Select (LMCS) Register. The PSRAM feature
is disabled on CPU reset.
Code intended to execute on the Am188ER microcon-
troller should perform all writes to the PCB registers as
byte writes. These writes will transfer 16 bits of data to
the PCB Register even if an 8-bit register is named in
the instruction. For example, out dx, al results in
the ax value being written to the port address in dx.
Reads to the PCB should be done as word reads. Code
written in this manner will run correctly on the
Am188ER and Am186ER microcontrollers.
Unaligned reads and writes to the PCB result in unpre-
dictable behavior on both the Am186ER and Am188ER
microcontrollers.
In addition to the LCS timing changes for PSRAM pre-
charge, the PSRAM devices also require periodic re-
fresh of all internal row addresses to retain their data.
Although refresh of PSRAM can be accomplished sev-
eral ways, the Am186ER and Am188ER microcontrol-
lers implement auto refresh only.
For a complete description of all the registers in the
PCB, refer to the Am186ER and Am188ER Microcon-
trollers User’s Manual, order #21684.
CLOCK AND POWER MANAGEMENT
The Am186ER and Am188ER microcontrollers gener-
ate RFSH, a refresh signal, to the PSRAM devices
when PSRAM mode is enabled. No refresh address is
required by the PSRAM when using the auto refresh
mechanism. The RFSH signal is multiplexed with the
MCS3 signal pin. When PSRAM mode is enabled,
MCS3 is not available for use as a chip-select signal.
The clock and power management unit of the
Am186ER and Am188ER microcontrollers includes a
phase-locked loop (PLL) and a second programmable
system clock output (CLKOUTB).
Phase-Locked Loop (PLL)
In a traditional 80C186/80C188 design, the internal clock
frequency is half the frequency of the crystal. Because of
the internal PLL on the Am186ER and Am188ER micro-
controllers, the internal clock generated by both micro-
controllers can operate at up to four times the frequency
of the crystal. The Am186ER and Am188ER microcon-
trollers operate in the following modes:
The refresh control unit must be programmed before
accessing PSRAM in LCS space. The refresh counter
in the Clock Prescaler (CDRAM) Register must be con-
figured with the required refresh interval value. The re-
fresh counter reload value in the CDRAM Register
should not be set to less than 18 (12h) in order to pro-
vide time for processor cycles between refreshes. The
refresh address counter must be set to 000000h to pre-
vent the MCS3–MCS0 or PCS6–PCS0 chip selects
from asserting. UCS may randomly assert during a
PSRAM refresh.
n Divide by Two—Frequency of the system clock is
half the frequency of the crystal with PLL disabled.
n Times One—Frequency of the system clock will be
the same as the external crystal with PLL enabled.
n Times Four—Frequency of the system clock is four
LCS is held High and the A bus is not used during re-
fresh cycles. The LMCS Register must be configured to
external ready ignored (R2 = 1) with one wait state
(R1–R0 = 01b), and the PSRAM mode enable bit (SE)
must be set. The ending address of LCS space in the
LMCS Register must also be programmed.
times the frequency of the crystal with PLL enabled.
The default Times Four mode must be used for processor
frequencies above 40 MHz. The Divide by Two mode
should be used for frequencies below 16 MHz. The clock-
ing mode is selected using CLKSEL1 and CLKSEL2 on
reset. Table 8 provides the maximum and minimum fre-
quencies for X1, X2, and CLKOUTA according to clocking
mode.
PERIPHERAL CONTROL BLOCK (PCB)
The integrated peripherals of the Am186ER and
Am188ER microcontrollers are controlled by 16-bit
read/write registers. The peripheral registers are con-
tained within an internal 256-byte control block. The
registers are physically located in the peripheral de-
vices they control, but they are addressed as a single
256-byte block. Figure 9 on page 46 shows a map of
these registers.
Table 8. Maximum and Minimum Clock
Frequencies
X1/X2
Max
X1/X2 CLKOUTA CLKOUTA
Mode
Min
Max
Min
Divide by 2 40 MHz 30 MHz
20 MHz
40 MHz
50 MHz
15 MHz
16 MHz
16 MHz
Times 1
Times 4
40 MHz 16 MHz
12.5 MHz 4 MHz
44
Am186TMER and Am188TMER Microcontrollers Data Sheet
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