Rainbow Electronics DS1875 - Manual (Page 24)

DS1875
24 ______________________________________________________________________________________
Dual-range functionality consists of two modes of opera-
tion: fine mode and coarse mode. Each mode is calibrat-
ed for a unique transfer function, hence the term, dual
range. Table 6 highlights the registers related to MON3.
Fine mode is equivalent to the other MON channels. Fine
mode is calibrated using the gain, offset, and right-shift-
ing registers at locations shown in Table 6 and is ideal
for relatively small analog input voltages. Coarse mode is
automatically switched to when the input exceeds the
threshold (to be discussed in a subsequent paragraph).
Coarse mode is calibrated using different gain and offset
registers, but lacks right-shifting (since coarse mode is
only used on large input signals). The gain and offset
registers for coarse mode are also shown in Table 6.
With the use of right-shifting, the fine mode full scale is
programmed to (1/2N)th the coarse mode full scale. The
DS1875 will now autorange to choose the range that
gives the best resolution for the measurement. To elimi-
nate chatter, 6.25% of hysteresis is applied when the
input resides at the boundary of the two ranges. See
Figure 10. Additional information for each of the registers
can be found in the
Memory Map section.
Dual range operation is transparent to the end user. The
results of MON3 analog-to-digital conversions are still
stored/reported in the same memory locations (68–69h,
Lower Memory) regardless of whether the conversion
was performed in fine mode or coarse mode.
When the DS1875 is powered up, analog-to-digital con-
versions begin in a round-robin fashion. Every MON3
timeslice begins with a fine mode analog-to-digital con-
version (using fine mode’s gain, offset, and right-shift-
ing settings). See the flowchart in Figure 10. Then,
depending on whether the last MON3 timeslice resulted
in a coarse-mode conversion and also depending on
the value of the current fine conversion, decisions are
made whether to use the current fine-mode conversion
result or to make an additional conversion (within the
same MON3 timeslice), using coarse mode (using
coarse mode’s gain and offset settings, and no right-
shifting) and reporting the coarse-mode result. The
flowchart also illustrates how hysteresis is implement-
ed. The fine-mode conversion is compared to one of
MON3
TIMESLICE
END OF MON3
TIMESLICE
PERFORM FINE-
MODE CONVERSION
REPORT FINE
CONVERSION RESULT
REPORT COARSE
CONVERSION RESULT
DID PRIOR MON3
TIMESLICE RESULT IN A
COARSE CONVERSION?
(LAST RSSI = 1?)
LAST RSSI = 0 LAST RSSI = 1
WAS CURRENT FINE-
MODE CONVERSION
≥ 93.75% OF FS?
PERFORM COARSE-
MODE CONVERSION
DID CURRENT FINE-
MODE CONVERSION
REACH MAX?
N
Y
Y
Y
N
N
Figure 10. RSSI Flowchart
Table 6. MON3 Configuration Registers
REGISTER FINE MODE COARSE MODE
MON3 FINE SCALE 98h–99h, Table 02h 9Ch–9Dh, Table 02h
MON3 FINE OFFSET A8h–A9h, Table 02h ACh–ADh, Table 02h
RIGHT SHIFT0/1 8Eh–8Fh, Table 02h —
CONFIG (RSSI_FC, RSSI_FF bits) 89h, Table 02h
MON3 VALUE 68h–69h, Lower Memory
PON Triplexer and SFP Controller