Datasheet

7) The t
2
wave is detected and the width of the t
2
pulse
is measured and stored as the t
2
time. The wave
number for the measurement of the t
2
wave width is
set by the T2WV[5:0] bits in the TOF2 register.
8) The preferred number of stop hits are then detect-
ed. For each hit, the measured TOF is stored in
the appropriate HITxUPINT and HITxUPFrac or
HITxDNINT and HITxDNFRAC registers. The num-
ber of hits to detect is set by the STOP[2:0] bits in
the TOF2 register. The wave number to measure for
each stop hit is set by the HITx wave select bits in
the TOF3, TOF4, and TOF5 registers.
9) After receiving all of the programmed hits, the
MAX35101 calculates the average of the recorded
hits and stores this to AVGUPINT and AVGUPFrac
or AVGDNInt and AVGDNFrac. The ratio of t
1
/t
2
and
t
2
/t
ideal
are calculated and stored in the WVRUP or
WVRDN register.
10) Once all of the hit data, wave ratios, and averages
become available in the Results registers, the TOF
bit in the Interrupt Status register is set and the INT
pin is asserted (if enabled) and remains asserted
until the Interrupt Status register is accessed by the
microprocessor with a read register command.
The computation of the total time of flight is performed
by counting the number of full and fractional 4MHz clock
cycles that elapsed between the launch start and a hit
stop as shown in Figure 4.
Each TOF measurement result is comprised of an
integer portion and a fractional portion. The integer
portion is a binary representation of the number of
t
4MHz
periods that contribute to the time results. The
fractional portion is a binary representation of one
t
4MHz
period quantized to a 16-bit resolution. The maxi-
mum size of the integer is 7FFFh or (2
15
-1) x t
4MHz
or ~ 8.19 ms. The maximum size of the fraction is:
16
4MHz
16
21
FFFFh or t . or ~ 249.9961 ns.
2
−
×
Table 1. Two’s Complement TOF_DIFF
Conversion Example
Figure 4.Start/Stop for Time-to-Digital Timing
REGISTER VALUE CONVERTER VALUE
TOF_DIFFInt
(hex)
TOF_DIFFFrac
(hex)
TOF DIFF VALUE
(ns)
7FFF FFFF 8,191,999.9962
001C 0403 7,003.9177
0001 00A1 250.6142
0000 0089 0.5226
0000 0001 0.0038
0000 0000 0.0000
FFFF FFFF -0.0038
FFFF FFC0 -0.2441
FFFE 1432 -480.2780
FF1C 8001 -56,874.9962
8000 0000 -8,192,000.0000
2 3 4
N
4 MHz CLOCK
START SIGNAL
(INTERNALLY GENERATED
WHEN ACOUSTIC SIGNAL
IS TRANSMITTED)
STOP SIGNAL
(GENERATED UPON
ACOUSTIC SIGNAL
RECEPTION)
1
INTEGER TOF RESULTS PORTION
1 LSB = T
4MHZ
FRACTIONAL TOF RESULTS PORTION
1 LSB = T
4MHz
/(2^16)
TOTAL TIME OF FLIGHT = INTEGER + FRACTIONAL
Figure 4
MAX35101 Time-to-Digital Converter with Analog Front-End
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