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Serial Boot Facility (SBF)
Freescale Semiconductor
12-5
5. After the SPI memory accepts however many shift clock edges are necessary to respond to the
READ command, it turns on its previously tri-stated output and begins driving the msb of the byte
at address 0.
Bits [7:4] of this byte must be 0000, so that the required 1-to-0 transition can be detected on
SBF_DI to synchronize the SBF state machine. If bits [7:4] of this byte are not 0000, bits[3:0] are
ignored, another byte is clocked out of the SPI memory (SBF_DO remains at logic 0), and the SBF
state machine again tests for a 1-to-0 transition followed by four consecutive zero bits.
6. After the necessary 1-to-0 transition and reception of a byte with bits [7:4] equal to 0000, the SBF
pauses and bits [3:0] of the received byte select a new shift clock divider according to
7. The weak internal pull-up on SBF_DI is disabled.
8. The shift clock begins toggling at the new frequency, resuming the READ command already in
progress.
NOTE
Shift clock frequency adjustment follows a power-on/hard reset only. After
the new divisor is known, it is stored in the sticky SBFCR[BLDIV] field and
used for subsequent soft resets. This speeds reboot for systems that do not
benefit from the optional FAST_READ on soft reset feature (e.g., the SPI
memory does not support FAST_READ, or the input reference clock does
not exceed the maximum allowable frequency for the READ command).
12.4.2
Reset Configuration and Optional Boot Load
After the steps in
Section 12.4.1, “Serial Initialization and Shift Clock Frequency Adjustment
”, are
executed, the following is performed to load configuration data and optional boot code.
1. Next, the SBF shifts two bytes (16 bits) out of the SPI memory that indicate how many longwords,
if any, are to be read during the optional boot load sequence. These bytes are software-visible in
the SBFSR[BLL] field.
2. The read operation continues with four longwords (128 bits) of reset configuration data (one
longword in the 256-pin devices), formatted in the order presented in
Configuration (BOOTMOD[1:0] = 11)
3. At this point, the SBF determines whether or not to read boot code. If SBFSR[BLL] is non-zero,
BLL plus one longwords (4
(BLL + 1) bytes) are consecutively loaded into the SRAM.
NOTE
Although the SBF permits up to 65,536 longwords (262,144 bytes) to be
loaded, the maximum practical number that can be read is limited by the size
of the device’s internal SRAM (8192 longwords (32,768 bytes) for this
device).
Содержание MCF54455
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