User`s manual

4: INTERFACING TO THE MOTOROLA MPC821 MICROPROCESSOR
APPLICATION NOTES (S19A-G-005-05) EPSON 5-27
4.3 S1D13504 Bus Interface
This section is summary of the bus interface modes available on the S1D13504, and offers some
detail on the General Purpose Bus mode used to implement the interface to the MPC821.
4.3.1 Bus Interface Modes
The S1D13504 implements a general-purpose 16-bit interface to the host microprocessor, which
may operate in one of several modes compatible with most of the popular embedded microprocessor
families. Four bus interface modes are supported:
• Hitachi SH-3.
• Motorola MC68000 (using Upper Data Strobe/Lower Data Strobe).
• Motorola MC68020/MC68030/MC683xx (using Data Strobe/DSACKx).
• Generic Bus (Chip Select, plus individual Read Enable/Write Enable for each byte).
Mode selections are made during reset by sampling the state of the memory data lines. Table 5-8 in
the “S1D13504 Hardware Functional Specification”, document number S19A-A-002-xx, details
the values needed the memory data lines, to select the desired mode.
After releasing reset, the bus interface signals assume their selected configuration. Table 5-9 in the
“S1D13504 Hardware Functional Specification” shows the function of each bus interface signal for
each of the interface modes.
Two other mode selections are also made at time of hardware reset, to control whether the bus inter-
face is big endian or little endian, and also to select the polarity of the READY signal. Some bus
interfaces require a particular setting for these parameters, but the ability to select them independent
of the bus interface timing offers tremendous flexibility in configuring the S1D13504 to support
other CPUs.
After reset, the Host Interface Disable bit in the Miscellaneous Disable Register (REG[1Bh]) will be
set to logic ‘1’, meaning that the S1D13504 will not respond to any host accesses until a write to
REG[1Bh] clears this bit to 0. When debugging a new hardware design, this can sometimes give the
appearance that the interface is not working, so it is important to remember to clear this bit before
proceeding with debugging.