The following methods are used to
monitor and diagnose the control the LED control and feedback loop:
- [SM_21] DAC to ADC Loopback
Test: Checks that the ADCs and DACs used to regulated LED brightness and
current limit are functioning properly. Sets DAC levels and then measures the
output of the DAC via ADC. If the ADC measurements do not match the expected DAC
levels, the test fails. This test can be configured to run at start-up and it
can be executed by command after the software is changed to Stand-by mode using
the host command. Upon failure, software will not transition to Display Mode
even if it is commanded by the host. An error will also be logged.
- [SM_22] Photo Feedback
Monitor: Checks the connection of the photodiode used to regulate LED
brightness. The disconnection of the photodiode could result in a very bright
image. The DLP4620S-Q1 chipset has two modes of
operation—continuous mode for high brightness and discontinuous mode for mid and
low brightness. In continuous mode, this test compares the output of the
photodiode amplifier versus the LED current. A high LED current and low
photodiode reading on for all LEDs indicates that the photodiode is
disconnected. In discontinuous mode, LED current measurements are not possible,
so a different method is used. The COMPOUT signal indicates whether the
photodiode measurement is below or above the target brightness level of the
LEDs. If the photodiode measurement never reaches the target LED brightness for
all LEDs in one frame, the photodiode is considered disconnected. To prevent
false failures, the number of consecutive frames that the error must persist for
it to be considered a real failure is configurable in flash. The number of
frames can be increased for preventing false failures. However, the number of
frames needed to indicate an error and respond to it should be less than the
Fault Tolerant Time Interval (FTTI) specified by the OEM. Upon failure,
emergency shutdown will be executed and an error will be logged. This test is
always executed during display mode.
- [SM_6] Back End Functional
Test (BEFT): Tests the sequencer block by executing a specialized LED
sequence, but without turning on LEDs. Tests the sequencer block's ability to
access and execute commands from sequencer memory. Makes sure that the sequencer
can properly control LED illumination signals in continuous and discontinuous
mode. This test can be configured to run at start-up, and it can be executed by
command after the software is changed to Stand-by mode using the host command.
In case of test failure, software will not transition to Display Mode even if it
is commanded by the host. An error will also be logged.