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User manual Rev. 2 — 1 August 2017 16 of 24
NXP Semiconductors
UM10729
PCA9955A/B demonstration board OM13483
8.2 Test of LED open or short error detection
The PCA9955A/B can detect an LED open or short condition at its open-drain LED
outputs. Users recognize these faults by reading the status of a pair of error bits (ERRx) in
error flag registers (EFLAGn) for each channel.
The user can poll the ERROR status bit (bit 6 in MODE2 register) to check if there is a
fault condition in any of the 16 channels. The EFLAGn registers can then be read to
determine which channels are at fault and the type of fault in those channels.
1. Copy the ‘PCA9955A/B Open or Short test’ text file as shown below. From the ‘File’
drop-down menus select ‘Open’, and from the ‘open data file’ window select the
‘PCA9955A/B Open or Short test’ text file.
Expert Mode Data File
00,Write,Yes,5,06,Comments: software reset
02,Write,Yes,0,98,80,80,Comments: set current to 50% of max in IREF0/IREF1
02,Write,Yes,50,02,05,Comments: set LED0 and LED1 are fully ON
02,Write,Yes,1,01,11,Comments: Clear all error status bits in EFLAGn registers by
writing bit4=1 in MODE2
02,Read,Yes,0,01,Comments: read ERROR status (bit6) in MODE2
02,Write,Yes,0,46,Comments: set EFLAG0 register address
02,Read,Yes,0,00,Comments: Read EFLAG0 status (bit1/0 for LED 0 and bit 3/2 for
LED1 error status)
02,Write,Yes,1,01,11,Comments: Clear all error status bits in EFLAGn registers by
writing bit4=1 in MODE2
02,Write,Yes,0,98,00,00,Comments: set output current to zero in IREF0/IREF1
Sequence:01,02,03,04,05,06,07,08,09
2. After opening the ‘PCA9955A/B Open or Short test’ text file, the Win-I2CUSB GUI in
Expert mode screen will be displayed as shown in Figure 9 .
3. Click the ‘Send All’ button. All the valid messages on the screen is sent in order of the
row number (Msg #). The action is performed one time.
4. To verify the read data on message line 5 for ERROR status and line 7 for EFLAG
status.
5. To open the J12 and J16 for open-circuit test on LED0 and LED1, repeat steps 3 and
4 to find the error report in message line 5 and line 7.
6. To short the J15 and J17 for short-circuit test on LED0 and LED1, repeat steps 3 and
4 to find the error report in message line 5 and line 7.