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3-4
functionality in this instance. See Table 3-1 on page 3-10.) The pin is
sampled within three crystal clock cycles after the rising edge of RE
S.
BHE/ADEN is three-stated during bus holds and ONCE mode.
See section 5.5.1 and section 5.5.2 for additional information on
enabling and disabling the AD bus during the address phase of a bus
cycle.
CLKOUTA Clock Output A (output, synchronous)
This pin supplies the internal clock to the system. Depending on the
value of the Power-Save Control (PDCON) Register, CLKOUTA
operates at either the CPU fundamental clock frequency (which varies
with the Divide By Two, Times One, and Times Four clocking modes),
the power-save frequency, or is three-stated. CLKOUTA remains active
during reset and bus hold conditions.
CLKOUTB Clock Output B (output, synchronous)
This pin supplies an additional clock to the system. Depending on the
value of the Power-Save Control (PDCON) Register, CLKOUTB
operates at either the CPU fundamental clock frequency (which varies
with the Divide By Two, Times One, and Times Four clocking modes),
the power-save frequency, or is three-stated. CLKOUTB remains active
during reset and bus hold conditions.
DEN
Data Enable (output, three-state, synchronous)
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.
DRQ1–DRQ0 DMA Requests (input, synchronous, level-sensitive)
These pins indicate to the microcontroller that an external device is
ready for DMA channel 1 or channel 0 to perform a transfer. DRQ1–
DRQ0 are level-triggered and internally synchronized.
The DRQ signals are not latched and must remain active until serviced.
DT/R
Data Transmit or Receive (output, three-state, synchronous)
This pin indicates which direction data should flow through an external
data-bus transceiver. When DT/R
is asserted High, the microcontroller
transmits data. When this pin is deasserted Low, the microcontroller
receives data. DT/R
is three-stated during a bus hold or reset condition.
GND Ground
These pins connect the system ground to the microcontroller.
HLDA Bus Hold Acknowledge (output, synchronous)
When an external bus master requests control of the local bus (by
asserting HOLD), the microcontroller completes the bus cycle in
progress and then relinquishes control of the bus to the external bus
master by asserting HLDA and three-stating DEN
, RD, WR, S2–S0,
AD15–AD0, S6, A19–A0, BHE
, WHB, WLB, and DT/R, and then driving
the chip selects UCS
, LCS, MCS3–MCS0, PCS6–PCS5, and PCS3–
PCS
0 High.