mcf51ac256a Freescale Semiconductor, Inc, mcf51ac256a Datasheet - Page 454

• Manufacturer Part Number: mcf51ac256a
• Description: Mcf51ac Flexis
• Manufacturer: Freescale Semiconductor, Inc

16-Bit Serial Peripheral Interface (SPI16)
20.3.7
The SPI Control Register 3 introduces a 64bit FIFO function on both transmit and receive buffers to be
utilised on the SPI. Utilising this FIFO feature allows the SPI to provide high speed transfers of large
amounts of data without consuming large amounts of the CPU bandwidth.
Enabling this FIFO function will effect the behaviour of some of the Read/Write Buffer flags in the SPIxS
register namely:
The SPRF of the SPIxS register will be set when the Receive FIFO is filled and will interrupt the CPU if
the SPIE in the SPIxC1 register is set.
The SPTEF of the SPIxS register will be set when the Transmit FIFO is empty, and will interrupt the CPU
if the SPITIE bit is set in the SPIxC1 register. See SPIxC1 and SPIxS registers.
FIFO mode is enabled by setting the FIFOMODE bit, and provides the SPI with an 8-byte receive FIFO
and an 8-byte transmit FIFO to reduce the amount of CPU interrupts for high speed/high volume data
transfers.
Two interrupt enable bits TNEARIEN and RNFULLIEN provide CPU interrupts based on the
“watermark” feature of the TNEARF and RNFULLF flags of the SPIxS register.
Note: This register has sixread/write control bits. Bits 7 thro’ 6are not implemented and always read 0. Writes have no meaning
20-14
TNEAREF
RNFULLF
Reset
Reset
MARK
MARK
Field
or effect. Write to this register happens only when FIFOMODE bit is 1.
5
4
W
W
R
R
SPI Control Register 3 (SPIxC3) — Enable FIFO Feature
Bit 7
Transmit FIFO Nearly Empty Water Mark - This bit selects the mark after which TNEAREF flag is asserted.
0 TNEAREF is set when Transmit FIFO has16bits or less.
1 TNEAREF is set when Transmit FIFO has 32bits or less.
Receive FIFO Nearly Full Water Mark - This bit selects the mark for which RNFULLF flag is asserted
0 RNFULLF is set when Receive FIFO has 48bits or more
1 RNFULLF is set when Receive FIFO has 32bits or more.
0
0
7
7
MCF51AC256 ColdFire Integrated Microcontroller Reference Manual, Rev. 5
= Unimplemented or Reserved
6
0
0
6
6
Figure 20-12. SPI Match Register Low (SPIxML)
Table 20-9. SPIxC3 Register Field Descriptions
Figure 20-13. SPI Status Register (SPIxC3)
TNEAREF
MARK
5
0
0
5
5
RNFULL
MARK
4
0
0
4
4
Description
INTCLR
3
3
0
3
0
TNEARIEN
2
0
0
2
2
RNFULLIEN
Freescale Semiconductor
1
0
0
1
1
FIFOMODE
Bit 0
0
0
0
0