@CHIP-RTOS C Library - RTOS API
This function installs a callback function
which executes periodically within the 1000 Hz (default rate)
Real-Time Interrupt (RTI).
void RTX_Install_RTI_Hook (
unsigned char delay )
- Far pointer to callback procedure to be invoked
from within 1000 Hz RTI. Set this argument to NULL to remove
typedef unsigned char huge (far * RTI_HOOK
- Number of 1000 Hz RTI before first call.
Set to 1 for callback on very next RTI. Set to 0 for 256 RTI later.
- -- none --
- External interrupts are masked and then restored to their state on
The term 'tick' used here refers to a 1 millisecond period
when the system timer RTI is operated at its default rate
of 1000 Hz. Note that this RTI rate can be configured to
higher rates with the
setting in the CHIP.INI, in which case tick time will be
under 1 millisecond.
The callback function is free to use any register except for the
DS and DI registers. These two registers must be
This API was provided for applications which require
a precise and variable timing for high frequency front end
hardware sampling, for example.
The least significant byte of the callbacks return value
(AL register) will be used as the delay count in milliseconds
to the next callback. Examples (assuming 1000Hz RTI):
- Return 1 to execute again on very next RTI, 1 millisecond later.
- Return zero for a delay of 256 milliseconds.
- Return 5 to execute at a 200 Hz rate.
The system supports only a single RTI hook. Installing a second
overwrites the first. A callback can be removed by installing a
NULL callback vector.
Unlike with the RTX timer
callback functions, which execute after
a task switch to the kernel task, this RTI hook executes directly
in the context of the interrupt. No task switch occurs prior
to the callback and execution proceeds on the stack of what ever task
was interrupted. Consequently the callback should be conservative
with its stack space usage.
This interrupt has been registered with the RTX kernel, so the user is
free to use the task triggering RTX API functions from within
the callback such as:
However any time consuming activity must be avoided and it is not
legal to call any RTOS API which could block or result in current
task being suspended. As a rough guideline, the dwell within
this callback should be held to under 50 us. Either very carefully
written C code, or assembly code is recommended here.
As true for any installed callback function, it is essential
that an application program remove its RTI hook callback
(vector set to zero) before program termination.
- This library function invokes a RTOS software interrupt.
Refer to this RTOS API function's
for more details.
- RTX API Overview
Supported since or modified in @CHIP-RTOS version
Supported by @CHIP-RTOS C Library since version
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