
Serial Host Interface Programming Model
DSP56364 24-Bit Digital Signal Processor Users Manual, Rev. 2
7-12 Freescale Semiconductor
It is recommended that an SHI individual reset be generated (HEN cleared) before changing HCKFR.
HCKFR is cleared during hardware reset and software reset.
7.4.6.5 HCSR Reserved Bits—Bits 23, 18 and 16
These bits in HCSR are reserved and unused. They are read as 0s and should be written with 0s for future
compatibility.
7.4.6.6 HCSR FIFO-Enable Control (HFIFO)—Bit 5
The read/write control bit HCSR FIFO-enable control (HFIFO) selects the size of the receive FIFO. When
HFIFO is cleared, the FIFO has a single level. When HFIFO is set, the FIFO has 10 levels. It is
recommended that an SHI individual reset be generated (HEN cleared) before changing HFIFO. HFIFO
is cleared during hardware reset and software reset.
7.4.6.7 HCSR Master Mode (HMST)—Bit 6
The read/write control bit HCSR Master (HMST) determines the operating mode of the SHI. If HMST is
set, the interface operates in the Master mode. If HMST is cleared, the interface operates in the Slave
mode. The SHI supports a single-master configuration, in both I
2
C and SPI modes. When configured as
an SPI Master, the SHI drives the SCK line and controls the direction of the data lines MOSI and MISO.
The SS line must be held deasserted in the SPI Master mode; if the SS line is asserted when the SHI is in
SPI Master mode, a bus error will be generated (the HCSR HBER bit will be set—see Section 7.4.6.18,
"Host Bus Error (HBER)—Bit 21"). When configured as an I
2
C Master, the SHI controls the I
2
C bus by
generating start events, clock pulses, and stop events for transmission and reception of serial data. It is
recommended that an SHI individual reset be generated (HEN cleared) before changing HMST. HMST is
cleared during hardware reset and software reset.
7.4.6.8 HCSR Host-Request Enable (HRQE[1:0])—Bits 8–7
The read/write Host-Request Enable control bits (HRQE[1:0]) are used to enable the operation of the
HREQ
pin. When HRQE[1:0] are cleared, the HREQ pin is disabled and held in the high impedance state.
If either HRQE0 or HRQE1 are set and the SHI is operating in a Master mode, the HREQ
pin becomes an
input that controls SCK: deasserting HREQ
will suspend SCK. If either HRQE0 or HRQE1 are set and the
SHI is operating in a Slave mode, HREQ becomes an output and its operation is defined in Table 7-5.
HRQE[1:0] should be modified only when the SHI is idle (HBUSY = 0). HRQE[1:0] are cleared during
hardware reset and software reset.
Table 7-5 HREQ Function In SHI Slave Modes
HRQE1 HRQE0 HREQ Pin Operation
0 0 High impedance
0 1 Asserted if IOSR is ready to receive a new word
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