System Clock Test - Renesas RL78 Series Application Note

RL78 Family
2.4

System Clock Test

Two self test modules (hardware and software base) are provided for the RL78 self test library in order to be able to test
the internal system clock (CPU and Peripheral clocks). The software measurement module is included for backward
compatibility with previous products and also to allow for any RL78 devices where the Timer Array does not include
the additional hardware capability, or that the timer is used by the application and is not available to be used as part of
the MCU self tests. These modules can be used by the application to detect the correct operation and deviation in the
main system clock during operation of the application. Please note that if the internal low speed oscillator is used for
measurement, the accuracy of the system clock measurement will be reduced due the greater tolerance of the internal
low speed oscillator. Therefore only the relative operation of the system clock can be obtained, which should still be
sufficient to establish that the system clock is operating correctly and within acceptable limits.
The principle behind both measurement approaches is that if the operation frequency of the main clock deviates during
runtime from a predefined range, then this can be detected by the system. The accuracy of the measurement obviously
depends on the accuracy of the reference clock source. For example an external signal input or 32 KHz crystal can
provide a more accurate measurement of the system clock than the internal low speed oscillator. This however does
require the extra components.
A "Pass / Fail" status of the test is returned. Also implemented is a "No Reference Clock" detection scheme which
returns a different status value to the normal test, in order to identify the appropriate fault state. Both the software and
hardware measurement function use the same return status format.
The modules compare the measured (captured) time is within a reference window (upper and lower limit values) using
the user defined reference values set in the "stl_clocktest_h" header file. This header file defines the reference values
for both software and hardware measurements and also the input test port pin for the software measurement.
1. Hardware Measurement
All current RL78 devices include an option in the Timer Array Unit (TAU) that provides additional input capture
sources that are designed to be able test the system clock operation. The extra capture inputs are selected as part of the
"safety" register (TIS0) and include the following:-
The example shown below is for the R5F100LE device, which has this feature implemented on TAU channel 5. The
timer channel used for other RL78 family members can vary, however the principles of this test capability remain the
same.
R01AN0749EG0201 Rev.2.01
Mar 04, 2014
VDE Certified IEC60730/60335 Self Test Library
•
The internal Low-speed oscillator (fiL)
•
External 32KHz Oscillator (Sub Clock) (fsub)
•
External signal input (TIOn)
Figure 7 Timer Array Unit Channel 5 Configuration
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