MC705P6ACPE Datasheet, PDF (42/98 Pages) Freescale Semiconductor, Inc – M68HC05 Microcontrollers

Serial Input/Output Port (SIOP)
7.2 SIOP Signal Format
The SIOP subsystem is software configurable for master or slave operation. No external mode selection
inputs are available (for instance, slave select pin).
7.2.1 Serial Clock (SCK)
The state of the SCK output normally remains a logic 1 during idle periods between data transfers. The
first falling edge of SCK signals the beginning of a data transfer. At this time, the first bit of received data
may be presented at the SDI pin and the first bit of transmitted data is presented at the SDO pin (see
Figure 7-2). Data is captured at the SDI pin on the rising edge of SCK. The transfer is terminated upon
the eighth rising edge of SCK.
The master and slave modes of operation differ only by the sourcing of SCK. In master mode, SCK is
driven from an internal source within the MCU. In slave mode, SCK is driven from a source external to the
MCU. The SCK frequency is dependent upon the SPR0 and SPR1 bits located in the mask option
register. Refer to 11.2 Mask Option Register for a description of available SCK frequencies.
BIT 0
BIT 1
BIT 2
BIT 3
BIT 4
BIT 5
BIT 6
BIT 7
SDO
SCK
100 ns
100 ns
SDI
BIT 0
BIT 1
BIT 2
BIT 3
BIT 4
BIT 5
BIT 6
BIT 7
Figure 7-2. SIOP Timing Diagram
7.2.2 Serial Data Input (SDI)
The SDI pin becomes an input as soon as the SIOP subsystem is enabled. New data may be presented
to the SDI pin on the falling edge of SCK.However, valid data must be present at least 100 nanoseconds
before the rising edge of SCK and remain valid for 100 nanoseconds after the rising edge of SCK. See
Figure 7-2.
7.2.3 Serial Data Output (SDO)
The SDO pin becomes an output as soon as the SIOP subsystem is enabled. Prior to enabling the SIOP,
PB5 can be initialized to determine the beginning state. While the SIOP is enabled, PB5 cannot be used
as a standard output since that pin is connected to the last stage of the SIOP serial shift register. Mask
option register bit LSBF permits data to be transmitted in either the MSB first format or the LSB first format.
Refer to 11.2 Mask Option Register for MOR LSBF programming information.
On the first falling edge of SCK, the first data bit will be shifted out to the SDO pin. The remaining data
bits will be shifted out to the SDO pin on subsequent falling edges of SCK. The SDO pin will present valid
data at least 100 nanoseconds before the rising edge of the SCK and remain valid for 100 nanoseconds
after the rising edge of SCK. See Figure 7-2.
MC68HC705P6A Advance Information Data Sheet, Rev. 2.1
42
Freescale Semiconductor