
Basic Booting Process
16-18
ADSP-BF59x Blackfin Processor Hardware Reference
When the
BFLAG_FILL
flag is set there is no payload. The argument con-
tains the 32-bit fill value, which is most likely a zero.
If the
BFLAG_FIRST
flag is set, the argument contains the relative
next-DXE pointer for multi-DXE applications. For single-DXE applica-
tions the field points to the next free boot source address after the current
DXE’s boot stream.
Boot Host Wait (HWAIT) Feedback Strobe
The
HWAIT
feedback strobe is a handshake signal that is used to hold off
the host device from sending further data while the boot kernel is busy.
On ADSP-BF59x processors, this feature is implemented by a GPIO that
is toggled by the boot kernel as required. The
PG4
GPIO is used for this
purpose.
The signal polarity of the
HWAIT
strobe is programmable by an external
resistor in the 10 k
Ω
range.
A pull-up resistor instructs the
HWAIT
signal to be active high. In this case
the host is permitted to send header and footer data when
HWAIT
is low,
but should pause while
HWAIT
is high. This is the mode used in SPI slave
boot on other Blackfin products.
Similarly, a pull-down resistor programs active-low behavior.
Note that the
HWAIT
signal is implemented slightly differently than
on ADSP-BF53x Blackfin processors. In the ADSP-BF59x
proces-
sors, the meaning of the pulling resistor is inverted and
HWAIT
is
asserted by default during reset.
The boot kernel first senses the polarity on the respective
HWAIT
pin. Then
it enables the output driver but keeps the signal in its asserted state. The
signal is not released until the boot kernel is ready for data, or when a
receive DMA is started. As soon as the DMA completes,
HWAIT
becomes
active again.
Содержание ADSP-BF59x Blackfin
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