68HC05CL16 datasheet(75/145 Pages) MOTOROLA

75 / 145 page

June 11, 1997

GENERAL RELEASE SPECIFICATION

MC68HC05CL48

SERIAL PERIPHERAL INTERFACE

MOTOROLA

REV 2.0

9-3

from the bus clock. The clock rate is selected by bits (SPR1,SPR0) in the control

system clock to an enabled Slave or Slaves. The clock is used by the Master to

latch incoming Slave data on the MISO pin and shift out data to the Slave device

on the MOSI pin. The Master and Slave must be operated in the same timing

mode. The type of clock and its’ relationship with the data is controlled by bits

CPOL and CPHA. Reference Figure 9-1.

9.1.4 SS SLAVE SELECT (INPUT)

9.1.4.1

Slave Mode

The slave select (SS input) is generated by the master (parallel port may be used)

and used to “enable one” of several slaves to accept and/or return data or “enable

several” slaves to accept data. To insure a data byte transfer, the SS signal must

be low prior to occurrence of SCK and must not become high until after the 8th

(last) SCK cycle. Figure 9-1 shows the clock (SCK) and data relationship.

Depending on the state of the CPHA control bit, the SS pin pulled low: (1) allows

the ﬁrst bit of data onto the MISO system line for transfer and (2) prevents the

Slave from reading or writing the data register. A further description of the affect of

the (SS) pin and (CPHA) control bit on the i/o data register is given in the

description of the (WCOL) status ﬂag. The (WCOL) ﬂag warns the Slave if it has

had a conﬂict between a transmission and a write of the data register. A high level

on SS forces MISO to the hi-Z state. Also, SCK and MOSI are ignored by the

disabled slave.

9.1.4.2

Master Mode

In this mode, Slave Select (SS) input is monitored to assure that it stays false

(high). If Slave Select becomes true, the device immediately exits the master

mode and becomes a slave (MSTR=0). Also, control bit (SPE) is forced to a zero

causing all SPI system pins to be inputs. An interrupt ﬂag (MODF) is set warning

the device that the above events have occurred. The signiﬁcance of this is that a

collision has occurred; that is, two devices have both become masters. This is

normally the result of software error, although some systems may allow the

default master to “knock all other masters off the bus” if an erroneous bus state is

detected. This is, of course, a catastrophic event and it is the responsibility of the

default master to completely “clean up” the system.

9.2

SPI REGISTERS

The register addresses only show the low order address bits i.e ABL(1:0). The

registers can be placed anywhere in the device memory map by generating an

appropriate ‘Module Select’ signal in the map logic.

68HC05CL16 Datasheet(PDF) 75 Page - Motorola, Inc

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