Datasheet

A TOF up measurement has pulses launched from the
LAUNCH_UP pin, which is connected to the downstream
transducer. The ultrasonic pulse is received at the
upstream transducer, which is connected to the STOP_
UP pin. A TOF down measurement has pulses launched
from the LAUNCH_DN pin, which is connected to the
upstream transducer. The ultrasonic pulse is received at
the downstream transducer, which is connected to the
STOP_DN pin.
TOF measurements can be initiated by sending either the
TOF_UP, TOF_DN, or TOF_DIFF commands. TOF_DIFF
measurements can also be automatically executed using
event timing mode commands EVTMG1 or EVTMG2.
The steps involved in a single TOF measurement are
described here and shown in Figure 3.
1) The 4MHz oscillator and LDO is enabled with a pro-
grammable settling delay time set by the CLK_S[2:0]
bits in Calibration and Control register.
2) A common-mode bias is enabled on the STOP pin.
This bias charge time is set by the CT[1:0] bits in the
TOF1 register.
3) Once the bias charge time has expired, the pulse
launcher drives the appropriate LAUNCH pin with
a programmable sequence of pulses. The number
of pulses launched is set by the PL[7:0] bits in the
TOF1 register. The frequency of these 50% duty-
cycle pulses is set by the DPL[3:0] bits, also in the
TOF1 register. The start of these launch pulses gen-
erates a start signal for the time-to-digital converter
(TDC) and is considered to be time zero for the TOF
measurement. This is denoted by the start signal in
the start/stop TDC timing (Figure 3).
4) After a programmable delay time set in TOF
Measurement Delay register, the comparator and hit
detector at the appropriate STOP pin are enabled.
This delay allows the receiver to start recording hits
when the received wave is expected, eliminating
possible false hits from noise in the system.
5) Stop hits are detected according to the programmed
preferred edge of the acoustic signal sequence
received at the STOP pin according to the setting
of the STOP_POL bit in the TOF1 register. The first
stop hit is detected when a wave received at the
STOP pin exceeds the comparator offset voltage,
which is set in the TOF6 and TOF7 registers. This
first detected wave is wave number 0. The width of
the wave’s pulse that exceeds the comparator offset
voltage is measured and stored as the t1 time.
6) The offset of the comparator then automatically and
immediately switches to the comparator return off-
set, which is set in the TOF6 and TOF7 registers.
Figure 3. Time-of-Flight Sequence
T
1
HIT1
(2)
BIAS APPLIED
TO STOP PIN
(3)
LAUNCH
PULSES
TOF
START
(4)
TOF
MEASUREMENT
DELAY
(5)
T
1
WAVE
(6)
COMPARE
RETURN
(7)
T
2
WAVE
HIT2 HIT
3
HIT4 HIT
5
HIT
6
(8)
STOP
HITS
(9)
CALCULATIO
NS
TOF
COMMAND
RECEIVED
(1)
4 MHZ
STARTUP
LAUNCH PIN
STOP PIN
COMPARATOR OFFSET
COMPARATOR OFFSET RETURN
TIME OF FLIGHT MEASUREMENTS
WAVE
NUMBER
765431 20 8 9 10 11
TOF MEASUREMENT SEQUENCE
STOP HITS SELECTED = 6, STOP POLARITY = POSITIVE EDGE
AVG = ∑(HIT[1:6]) ÷ 6
SELECTED WAVES FOR HITS:
T2 = 4 HIT1 = 6 HIT2 = 7 HIT3 = 8 HIT4 = 9 HIT5 = 10 HIT6 = 11
(2)
BIAS
CHARGE TIME
(4)
ENABLE
RECEIVER
(10)
INT
ASSERTED
T
2
INT PIN
Figure 3
MAX35101 Time-to-Digital Converter with Analog Front-End
www.maximintegrated.com
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