reason, the A0 signal cannot be used in place of the
AD0 signal to determine refresh cycles. PSRAM re-
freshes also provide an additional RFSH signal (see
the MCS3/RFSH pin description on page 33).
ALE
Address Latch Enable (output, synchronous)
This pin indicates to the system that an address ap-
pears on the address and data bus (AD15–AD0 for
the Am186ER microcontroller or AO15–AO8 and
AD7–AD0 for the Am188ER microcontroller). The ad-
dress is guaranteed valid on the trailing edge of ALE.
This pin is three-stated during ONCE mode.
ADEN—If BHE/ADEN is held High or left floating dur-
ing power-on reset, the address portion of the AD bus
(AD15–AD0) is enabled or disabled during LCS and
UCS bus cycles based on the DA bit in the LMCS and
UMCS registers. If the DA bit is set, the memory ad-
dress is accessed on the A19–A0 pins. This mode of
operation reduces power consumption. For more infor-
mation, see the Bus Operation section on page 41.
There is a weak internal pullup resistor on BHE/ADEN
so no external pullup is required.
ARDY
Asynchronous Ready (input, asynchronous,
level-sensitive)
This pin indicates to the microcontroller that the ad-
dressed memory space or I/O device will complete a
data transfer. The ARDY pin accepts a rising edge that
is asynchronous to CLKOUTA and is active High. The
falling edge of ARDY must be synchronized to
CLKOUTA. To always assert the ready condition to the
microcontroller, tie ARDY High. If the system does not
use ARDY, tie the pin Low to yield control to SRDY.
If BHE/ADEN is held Low on power-on reset, the AD
bus drives both addresses and data. Changing the DA
bit of the LMCS and UMCS registers will have no effect.
(S6 and UZI also assume their normal functionality in
this instance. The PIO Mode and Direction registers
cannot reconfigure these pins as PIOs. See Table 3 on
page 36.) The pin is sampled within three crystal clock
cycles after the rising edge of RES. BHE/ADEN is
three-stated during bus holds and ONCE mode.
BHE/ADEN
(Am186™ER Microcontroller Only)
Bus High Enable (three-state, output, synchronous)
Address Enable (input, internal pullup)
Note: Once the above modes are set, they can be
changed only by resetting the processor.
BHE—During a memory access, this pin and the least-
significant address bit (AD0 or A0) indicate to the sys-
tem which bytes of the data bus (upper, lower, or both)
participate in a bus cycle. The BHE/ADEN and AD0
pins are encoded as shown in Table 2.
CLKOUTA
Clock Output A (output, synchronous)
This pin supplies the internal clock to the system. De-
pending on the value of the Power-Save Control Regis-
ter (PDCON), CLKOUTA operates at either the CPU
fundamental frequency (which varies with the divide by
two, times one, and times four clocking modes), the
power-save frequency, or is three-stated (see Figure 10
on page 48). CLKOUTA remains active during reset
and bus hold conditions.
BHE is asserted during t1 and remains asserted
through t3 and tW. BHE does not need to be latched.
BHE is three-stated during bus hold and reset condi-
tions.
On the Am186ER microcontroller, WLB and WHB im-
plement the functionality of BHE and AD0 for high and
low byte write enables.
CLKOUTB
Clock Output B (output, synchronous)
This pin supplies a clock to the system. Depending on
the value of the Power-Save Control Register (PD-
CON), CLKOUTB operates at either the CPU funda-
mental frequency (which varies with the divide by two,
times one, and times four clocking modes), the power-
save frequency, or is three-stated (see Figure 10 on
page 48). CLKOUTB remains active during reset and
bus hold conditions.
Table 2. Data Byte Encoding
BHE AD0 Type of Bus Cycle
0
0
1
1
0
1
0
1
Word Transfer
High Byte Transfer (Bits 15–8)
Low Byte Transfer (Bits 7–0)
Refresh
DEN/PIO5
Data Enable (output, three-state, synchronous)
BHE/ADEN also signals DRAM refresh cycles when
using the multiplexed address and data (AD) bus. A re-
fresh cycle is indicated when both BHE/ADEN and AD0
are High. During refresh cycles, the A bus and the AD
bus are not guaranteed to provide the same address
during the address phase of the AD bus cycle. For this
This pin supplies an output enable to an external data-
bus transceiver. DEN is asserted during memory, I/O,
and interrupt acknowledge cycles. DEN is deasserted
when DT/R changes state. DEN is three-stated during
a bus hold or reset condition.
Am186TMER and Am188TMER Microcontrollers Data Sheet
31
AMD [ AMD ]
