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General Description
Pin Descriptions
M68HC11E Family — Rev. 5
Data Sheet
MOTOROLA
General Description
27
The VSTBY 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 VDD voltage, the internal RAM and part of the reset logic are powered
from this signal rather than the VDD input. This allows RAM contents to be retained
without VDD power applied to the MCU. Reset must be driven low before VDD is
removed and must remain low until VDD has been restored to a valid level.
1.4.7.1 VRL and VRH
These two inputs provide the reference voltages for the analog-to-digital (A/D)
converter circuitry:
VRL is the low reference, typically 0 Vdc.
VRH is the high reference.
For proper A/D converter operation:
VRH should be at least 3 Vdc greater than VRL.
VRL and VRH should be between VSS and VDD.
1.4.8 STRA/AS
The strobe A (STRA) and address strobe (AS) pin performs either of two separate
functions, depending on the operating mode:
In single-chip mode, STRA performs an input handshake (strobe input)
function.
In the expanded multiplexed mode, AS provides an address strobe function.
AS can be used to demultiplex the address and data signals at port C. Refer to
1.4.9 STRB/R/W
The strobe B (STRB) and read/write (R/W) pin act as either an output strobe or as
a data bus direction indicator, depending on the operating mode.
In single-chip operating mode, STRB acts as a programmable strobe for
In expanded multiplexed operating mode, R/W is used to indicate the direction of
transfers on the external data bus. A low on the R/W pin indicates data is being
written to the external data bus. A high on this pin indicates that a read cycle is in
progress. R/W stays low during consecutive data bus write cycles, such as a
double-byte store. It is possible for data to be driven out of port C, if internal read
visibility (IRV) is enabled and an internal address is read, even though R/W is in a
Memory for more information about IRVNE (internal read visibility not E).