![](http://datasheet.mmic.net.cn/290000/XPC801ZP25_datasheet_16187868/XPC801ZP25_239.png)
External Bus Interface
13-2
MPC801 USER’S MANUAL
MOTOROLA
13
13.2 TRANSFER SIGNALS
The bus transfers information between the MPC801 and the external memory or peripheral
device. External devices can accept or provide 8,16, and 32 bits in parallel and must follow
the handshake protocol described later in this section. The maximum number of bits
accepted or provided during a bus transfer is defined as the port width.
The MPC801 contains an address bus that specifies the transfer’s address and a data bus
that transfers the data. Control signals indicate the beginning and type of the cycle, as well
as the address space and size of the transfer. The selected device then controls the length
of the cycle with the signal(s) used to terminate the cycle. A strobe signal for the address
bus indicates the validity of the address and provides timing information for the data. The
MPC801 bus is synchronous, but the bus and control input signals must be timed to setup
and hold times relative to the rising edge of the clock. In this situation, bus cycles can be
completed in two clock cycles.
Furthermore, for all inputs, the MPC801 latches the level of the input during a sample
window around the rising edge of the clock signal. This window is illustrated in Figure 13-1,
where tsu and tho are the input setup and hold times, respectively. To ensure that an input
signal is recognized on a specific falling edge of the clock, the input must be stable during
the sample window. If an input makes a transition during the window time period, the level
recognized by the MPC801 is not predictable. However, the MPC801 always resolves the
latched level to either a logic high or low before using it. In addition to meeting input setup
and hold times for deterministic operation, all input signals must obey the protocols
described in this section.
Figure 13-1. Input Sample Window
CLOCK
SIGNAL
tho
tsu
SAMPLE
WINDOW