MC9S08QG8 Datasheet, PDF (221/300 Pages) Freescale Semiconductor, Inc – 8-BIT HCS08 Central Processor Unit

Serial Peripheral Interface (S08SPIV3)
Table 15-7. SPIS Register Field Descriptions (continued)
Field
5
SPTEF
4
MODF
Description
SPI Transmit Buffer Empty Flag — This bit is set when there is room in the transmit data buffer. It is cleared by
reading SPIS with SPTEF set, followed by writing a data value to the transmit buffer at SPID. SPIS must be read
with SPTEF = 1 before writing data to SPID or the SPID write will be ignored. SPTEF generates an SPTEF CPU
interrupt request if the SPTIE bit in the SPIC1 is also set. SPTEF is automatically set when a data byte transfers
from the transmit buffer into the transmit shift register. For an idle SPI (no data in the transmit buffer or the shift
register and no transfer in progress), data written to SPID is transferred to the shifter almost immediately so
SPTEF is set within two bus cycles allowing a second 8-bit data value to be queued into the transmit buffer. After
completion of the transfer of the value in the shift register, the queued value from the transmit buffer will
automatically move to the shifter and SPTEF will be set to indicate there is room for new data in the transmit
buffer. If no new data is waiting in the transmit buffer, SPTEF simply remains set and no data moves from the
buffer to the shifter.
0 SPI transmit buffer not empty
1 SPI transmit buffer empty
Master Mode Fault Flag — MODF is set if the SPI is configured as a master and the slave select input goes
low, indicating some other SPI device is also configured as a master. The SS pin acts as a mode fault error input
only when MSTR = 1, MODFEN = 1, and SSOE = 0; otherwise, MODF will never be set. MODF is cleared by
reading MODF while it is 1, then writing to SPI control register 1 (SPIC1).
0 No mode fault error
1 Mode fault error detected
15.3.5 SPI Data Register (SPID)
R
W
Reset
7
Bit 7
0
6
5
4
3
2
6
5
4
3
2
0
0
0
0
0
Figure 15-9. SPI Data Register (SPID)
1
0
1
Bit 0
0
0
Reads of this register return the data read from the receive data buffer. Writes to this register write data to
the transmit data buffer. When the SPI is configured as a master, writing data to the transmit data buffer
initiates an SPI transfer.
Data should not be written to the transmit data buffer unless the SPI transmit buffer empty flag (SPTEF)
is set, indicating there is room in the transmit buffer to queue a new transmit byte.
Data may be read from SPID any time after SPRF is set and before another transfer is finished. Failure to
read the data out of the receive data buffer before a new transfer ends causes a receive overrun condition
and the data from the new transfer is lost.
15.4 Functional Description
An SPI transfer is initiated by checking for the SPI transmit buffer empty flag (SPTEF = 1) and then
writing a byte of data to the SPI data register (SPID) in the master SPI device. When the SPI shift register
is available, this byte of data is moved from the transmit data buffer to the shifter, SPTEF is set to indicate
there is room in the buffer to queue another transmit character if desired, and the SPI serial transfer starts.
MC9S08QG8 and MC9S08QG4 Data Sheet, Rev. 1.01
Freescale Semiconductor
221