ASX340AT_16 Datasheet, PDF (23/54 Pages) ON Semiconductor – 1/4‐inch Color CMOS NTSC/PAL Digital Image SOC

Host Command Process Flow
ASX340AT
Host could insert an
optional delay here
No
No
Issue
Command
Wait for a
response?
No
Read Command
register
Host could insert an
optional delay here
Yes
Read Command
register
Doorbell
bit clear ?
Yes
Command has
parameters ?
Yes
Write parameters
to
Parameter Pool
Write command
to
Command register
No
At this point
Command Register
contains response code
Doorbell bit
clear?
Yes
Command
has response
No
parameters ?
Yes
Read response
parameters from
Parameter Pool
Done
Figure 21. Interface Structure
COMMAND FLOW
The host issues a command by writing (through a
two-wire interface bus) to the command register. All
commands are encoded with bit 15 set, which automatically
generates the host command (doorbell) interrupt to the
microprocessor.
Assuming initial conditions, the host first writes the
command parameters (if any) to the parameters pool (in the
command handler’s logical page), then writes the command
to command register. The firmware interrupt handler then
signals the Command Handler task to process the command.
If the host wishes to determine the outcome of the
command, it must poll the command register waiting for the
doorbell bit to be cleared. This indicates that the firmware
completed processing the command. When the doorbell bit
is cleared, the contents of the command register indicate the
command’s result status. If the command generated
response parameters, the host can now retrieve these from
the parameters pool.
NOTES: The host must not write to the parameters pool,
nor issue another command, until the previous
command completes. This is true even if the
host does not care about the result of the
previous command. Therefore, the host must
always poll the command register to determine
the state of the doorbell bit, and ensure the bit is
cleared before issuing a command.
For a complete command list and further information
consult the Host Command Interface Specification.
An example of how (using DevWare) a command may be
initiated in the form of a “Preset” follows.
www.onsemi.com
23