AN2408 Freescale Semiconductor / Motorola, AN2408 Datasheet - Page 19

• Manufacturer Part Number: AN2408
• Description: Examples of HCS12 External Bus Design A Companion Note to AN2287/D
• Manufacturer: Freescale Semiconductor / Motorola

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The PEAR register is known as the port E assignment register. It is used to control the setting of the
external bus dedicated to port E. It controls which of the external bus control signals are driven from port
E. Any signals that are not required for a particular interface may be used as general-purpose I/O. Notice
that setting the NOECLK bit turns off the external ECLK. All other signals are enabled by setting the
appropriate bit. In the above assignment the ECLK, LSTRB, and R/W signals are activated.
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The MODE register controls the MCU external bus capability, internal visibility, and port emulation
settings. The MODC, MODB, and MODA bits represent the mode setting detailed in the device user guide
and AN2287. IVIS enables all internal data accesses to be echoed externally for data acquisition or logic
analysis, depending on security settings.The EMK and EME bits enable port K and port E register
accesses to be echoed externally for port replacement.
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The EBICTL register is used to turn the ECLK output into a free-running clock source. In most external
environments the ESTR bit should remain clear. It may however, be useful for emulation systems, or
systems without external devices that require a clock source.
Freescale Semiconductor
/* Expansion Bus settings */
Regs.pear.byte = 0x0C; // %00001100
Regs.mode.byte = 0xE3; // %11100011
Regs.ebictl.byte = 0x00; // %0000001
Examples of HCS12 External Bus Design, Rev. 2
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|||||||+-Reserved
||||||+--Reserved
|||||+---RDWE----Read / Write (R/W) pin Enabled
||||+----LSTRE---Low Strobe (LSTRB) pin Enabled
|||+-----NECLK---External E clock is enabled
||+------PIPOE---IPIP0,1 indicate the instruction queue
|+-------Reserved
+--------NOACCE--No CPU free cycle visibility.
||||||||
|||||||+-EME-----PORTE and DDRE are removed from memory
||||||+--EMK-----PORTK and DDRK are removed from memory
|||||+---Reserved
||||+----IVIS----No visibility of internal ops on ext. bus
|||+-----Reserved
||+------MODA--\
|+-------MODB----Normal Expanded Wide
+--------MODC--/
Example #7 — Word-Wide External Device Interface – 100BASE-T Ethernet
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|||||||+-ESTR----E stretches for external access.
||||||+--Reserved
|||||+---Reserved
||||+----Reserved
|||+-----Reserved
||+------Reserved
|+-------Reserved
+--------Reserved
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