lm8322 National Semiconductor Corporation, lm8322 Datasheet - Page 18

• Manufacturer Part Number: lm8322
• Description: Mobile I/o Companion Supporting Key-scan, I/o Expansion, Pwm, And Access.bus Host Interface
• Manufacturer: National Semiconductor Corporation

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14.0 PWM Output Generation
Three pulse-width modulated (PWM) outputs are provided
with advanced capabilities for ramp-up and ramp-down of the
PWM duty cycle and execution of simple to complex com-
mand sequences. These capabilities are supported by three
independent script-execution engines capable of au-
tonomous operation after setup and launch by the host.
Figure 12 shows the architecture of a script-execution engine.
The host has three commands for interfacing to the script ex-
ecution engine. The following commands are always associ-
ated with one particular PWM channel:
•
PWM_WRITE — load one word into the script command
file at a specified address.
FIGURE 12. PWM Script Execution Engine
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•
•
Please note: The PWM_STOP command might not take im-
mediate effect if the current command being executed is a
command with long execution time. If a PWM_STOP com-
mand is sent when the PWM engine is running a long RAMP
command, the PWM will only stop after the RAMP is com-
pleted.
The script commands have their own fixed-length 16-bit for-
mat and encoding unrelated to the variable-length, byte-
based format used for host commands. A script command is
sent by the host to the LM8322 as a parameter to the
PWM_WRITE
PWM_WRITE command specifies an address in the script
command file for receiving the command.
14.1 COMMAND QUEUE
After the host issues a PWM_START command, script com-
mands are read from the script command file into a command
queue which consists of a command file output register, com-
mand buffer, and active command register. This allows one
command to be active while another command is queued in
the command buffer, which allows seamless back-to-back
command execution.
A command loaded into the command file output register is
synchronized to the 32.768 kHz clock and stored in the com-
mand buffer. If no command is currently active, the command
passes through to the active command register. In this case,
another command can be read from the script command file,
which is queued in the command buffer. On completion of the
currently active command, the contents of the command
buffer are transferred to the active command register, and the
command buffer may then receive a new command.
The host does not have direct access to any of the registers
in the command queue. The operations which read script
commands from the script command file occur automatically
after the host issues the PWM_START command.
Script execution stops when the host sends a PWM_STOP
command or when the script engine executes an END com-
mand with the Reset bit set to 1. Executing an END command
with the Reset bit set to “1” or the reception of a PWM_STOP
command asserts IRQ to the host.
14.2 PWM TIMER OPERATION
The timers implement a fixed 256-cycle period with a pro-
grammable duty cycle and programmable ramp-up/ramp-
down of the duty cycle. Figure 13 shows the architecture of a
PWM timer.
PWM_START — start execution of the script.
PWM_STOP — stop execution of the script.
command.
Another
parameter
to
the