MC9S12HZ256 Datasheet, PDF (49/692 Pages) Freescale Semiconductor, Inc – HCS12 Microcontrollers

Chapter 1 MC9S12HZ256 Device Overview
The port E bit 4 pin is initially configured as ECLK output with stretch. The E clock output function
depends upon the settings of the NECLK bit in the PEAR register, the IVIS bit in the MODE register and
the ESTR bit in the EBICTL register. The E clock is available for use in external select decode logic or as
a constant speed clock for use in the external application system.
1.7.1.3 Normal Expanded Narrow Mode
This mode is used for lower cost production systems that use 8-bit wide external EPROMs or RAMs. Such
systems take extra bus cycles to access 16-bit locations but this may be preferred over the extra cost of
additional external memory devices.
Ports A and B are configured as a 16-bit address bus and port A is multiplexed with data. Internal visibility
is not available in this mode because the internal cycles would need to be split into two 8-bit cycles.
Because the PEAR register can only be written one time in this mode, use care to set all bits to the desired
states during the single allowed write.
The PE3/LSTRB pin is always a general-purpose I/O pin in normal expanded narrow mode. Although it
is possible to write the LSTRE bit in PEAR to “1” in this mode, the state of LSTRE is overridden and
port E bit 3 cannot be reconfigured as the LSTRB output.
It is possible to enable the pipe status signals on port E bits 6 and 5 by setting the PIPOE bit in PEAR, but
it would be unusual to do so in this mode. LSTRB would also be needed to fully understand system
activity. Development systems where pipe status signals are monitored would typically use special
expanded wide mode or occasionally special expanded narrow mode.
The PE4/ECLK pin is initially configured as ECLK output with stretch. The E clock output function
depends upon the settings of the NECLK bit in the PEAR register, the IVIS bit in the MODE register and
the ESTR bit in the EBICTL register. In normal expanded narrow mode, the E clock is available for use
in external select decode logic or as a constant speed clock for use in the external application system.
The PE2/R/W pin is initially configured as a general-purpose input with a pull-up but this pin can be
reconfigured as the R/W bus control signal by writing “1” to the RDWE bit in PEAR. If the expanded
narrow system includes external devices that can be written such as RAM, the RDWE bit would need to
be set before any attempt to write to an external location. If there are no writable resources in the external
system, PE2 can remain a general-purpose I/O pin.
1.7.1.4 Internal Visibility
Internal visibility is available when the MCU is operating in expanded wide modes or special narrow
mode. It is not available in single-chip, peripheral or normal expanded narrow modes. Internal visibility is
enabled by setting the IVIS bit in the MODE register.
If an internal access is made while E, R/W, and LSTRB are configured as bus control outputs and internal
visibility is off (IVIS = 0), E will remain low for the cycle, R/W will remain high, and address, data and
the LSTRB pins will remain at their previous state.
When internal visibility is enabled (IVIS = 1), certain internal cycles will be blocked from going external.
During cycles when the BDM is selected, R/W will remain high, data will maintain its previous state, and
address and LSTRB pins will be updated with the internal value. During CPU no access cycles when the
MC9S12HZ256 Data Sheet, Rev. 2.04
Freescale Semiconductor
49