4–2
Clocks, Cache, and External Interface
29 September 1997 – Subject To Change
Introduction to the External Interface
4.1 Introduction to the External Interface
A 21164PC-based system can be divided into three major sections:
•
21164PC microprocessor
•
External Bcache
•
System interface logic
The 21164PC external interface is optimized for uniprocessor-based systems and
mandates few design rules. The interface includes a 128-bit bidirectional data bus, a
36-bit bidirectional address bus, and several control signals.
Read latencies and data repetition rates of the external Bcache can be programmed
by means of register bits. The Bcache clock frequency for private read and write
operations is independent of the system interface clock frequency and makes for a
more flexible design.
The cache system supports a 64-byte block size to the external Bcache.
Figure 4–1 shows a simplified view of the external interface. The function and pur-
pose of each signal is described in Chapter 3.
4.1.1 System Interface
This section describes the system or external bus interface. The system interface is
made up of bidirectional address and command buses, a data bus that is shared with
the Bcache interface, and several control signals.
The system interface is under the control of the bus interface unit (BIU) in the CBU.
The system interface is a 128-bit bidirectional data bus.
The cycle time of the system interface is programmable to speeds of 4 to 15 times the
CPU cycle time (sysclk ratio). All system interface signals are driven or sampled by
the 21164PC on the rising edge of signal sys_clk_out1_h. In this chapter, this edge
is sometimes referred to as “sysclk.” Precisely when interface signals rise and fall
does not matter as long as they meet the setup and hold times specified in Chapter 9.