Pin Descriptions
MODA and MODB (MODA/LIR and MODB/VSTBY)
MC68HC11E Family
—
Rev. 4
Technical Data
MOTOROLA
Pin Descriptions
37
There should be a single pullup resistor near the MCU interrupt input pin
(typically 4.7 k
). There must also be an interlock mechanism at each
interrupt source so that the source holds the interrupt line low until the
MCU recognizes and acknowledges the interrupt request. If one or more
interrupt sources are still pending after the MCU services a request, the
interrupt line will still be held low and the MCU will be interrupted again
as soon as the interrupt mask bit in the MCU is cleared (normally upon
return from an interrupt). Refer to
Section 5. Resets and Interrupts
.
V
PPE
is the input for the 12-volt nominal programming voltage required
for EPROM/OTPROM programming. On devices without
EPROM/OTPROM, this pin is only an XIRQ input.
2.9 MODA and MODB (MODA/LIR and MODB/V
STBY
)
During reset, MODA and MODB select one of the four operating modes:
Single-chip mode
Expanded mode
Test mode
Bootstrap mode
Refer to
Section 4. Operating Modes and On-Chip Memory
.
After the operating mode has been selected, the load instruction register
(LIR) pin provides an open-drain output to indicate that execution of an
instruction has begun. A series of E-clock cycles occurs during
execution of each instruction. The LIR signal goes low during the first
E-clock cycle of each instruction (opcode fetch). This output is provided
for assistance in program debugging.
The V
STBY
pin is used to input random-access memory (RAM) standby
power. When the voltage on this pin is more than one MOS threshold
(about 0.7 volts) above the V
DD
voltage, the internal RAM and part of the
reset logic are powered from this signal rather than the V
DD
input. This
allows RAM contents to be retained without V
DD
power applied to the
MCU. Reset must be driven low before V
DD
is removed and must remain
low until V
DD
has been restored to a valid level.