Datasheet

Diagnostic State Machine
t
WIDTH
= Four 4-MHz
Clock Cycles
Diagnostic State
Machine
DIAG_END Pin
DIAG_EN Register Bit = 1
Diagnostic Ends
t
DIAG
Diagnostic Starts
AFE4490
SBAS602F –DECEMBER 2012–REVISED OCTOBER 2013 www.ti.com
Diagnostics Module
The diagnostics module, when enabled, checks for nine types of faults sequentially. The results of all faults are
latched in 11 separate flags. At the end of the sequence, the state of the 11 flags are combined to generate two
interrupt signals: PD_ALM for photodiode-related faults and LED_ALM for transmit-related faults. The status of all
flags can also be read using the SPI interface. Table 4 details each fault and flag used. Note that the diagnostics
module requires all AFE blocks to be enabled in order to function reliably.
Table 4. Fault and Flag Diagnostics
(1)
MODULE SEQ. FAULT FLAG1 FLAG2 FLAG3 FLAG4 FLAG5 FLAG6 FLAG7 FLAG8 FLAG9 FLAG10 FLAG11
— — No fault 0 0 0 0 0 0 0 0 0 0 0
Rx INP cable
1 1
shorted to LED cable
Rx INM cable
2 1
shorted to LED cable
Rx INP cable
PD 3 shorted to GND 1
cable
Rx INM cable
4 shorted to GND 1
cable
5 PD open or shorted 1 1
Tx OUTM line
6 shorted to GND 1
cable
Tx OUTP line
LED
7 shorted to GND 1
cable
8 LED open or shorted 1 1
9 LED open or shorted 1
(1) Resistances below 10 kΩ are considered to be shorted.
Figure 77 shows the timing for the diagnostic function.
Figure 77. Diagnostic Timing Diagram
By default, the diagnostic function takes t
DIAG
= 8 ms to complete after the DIAG_EN register bit is enabled. By
setting the EN_SLOW_DIAG register bit (CONTROL2 register, bit D8) the diagnostic time can be increased to
16 ms.
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