Datasheet

Calibration Operation
For more accurate results, calibration of the TDC can
be performed. Calibration allows the MAX35101 to per-
form a calibration measurement that is based upon the
32.768kHz crystal, which is the most accurate clock in the
system. This calibration is used when a ceramic oscillator
is used in place of an AT-cut crystal for the 4MHz refer-
ence. The MAX35101 automatically generates START
and STOP signals based upon edges of the 32.768kHz
clock. The number of 32.768kHz clock periods that are
used and then averaged are selected with the CAL_
PERIOD[3:0] bits in the Calibration and Control register.
The TDC measures the number of 4MHz clock pulses that
occur during the 32.768kHz pulses. The measured time of
a 32.768kHz clock pulse is reported in the CalibrationInt
and CalibrationFrac Results registers. These results can
then be used as a gain factor for calculating actual time-
to-digital converter measurement if the CAL_USE bit in
the Event Timing 2 Register is set.
Following is a description of an example calibration. Each
TDC measurement is a 15-bit fixed-point integer value
concatenated with a 16-bit fractional value binary repre-
sentation of the number of t
4MHz
periods that contribute
to the time result, the actual period of t
4MHz
needs to be
known. If the CAL_PERIOD[3:0] bits in the Calibration
and Control register are set to 6, then 6 measurements
of 32.768kHz periods are measured by the TDC and
then averaged. The expected measured value would be
30.5176µs/250ns = 122.0703125 t
4MHz
periods. Assume
that the 4MHz ceramic resonator is actually running at
4.02MHz. The TDC measurement unit would then mea-
sure 30.5176µs/248.7562ns = 122.6806641 t
4MHz
peri-
ods and this result would be returned in the Calibration
Results register. For all TDC measurements, a gain value
of 122.0703125/122.6806641 = 0.995024876 would then
be applied.
Calibration is performed at the following events:
● When the Calibration command is sent to the
MAX35101. At the completion of this calibration, the
CAL bit in the Interrupt Status register and the INT pin
asserts (if enabled).
● During event timing operation, automatic calibrations
can be performed before executing TOF or tempera-
ture measurements. This is selectable with the CAL_
CFG[2:0] bits in the Event Timing 2 register. Upon
completion of an automatic calibration during event
timing, the result is updated in the Calibration Results
register, but the CAL bit in the Interrupt Status register
is not set and the INT pin does not assert.
Error Handling During Calibration
Since calibration can be set to be automatic by configur-
ing the CAL_CFG[2:0] bits in the Event Timing 2 register,
any errors that occur during the Calibrate command stop
the CalibrationInt and the CalibrationFrac Results regis-
ters from being updated with new calibration coefficients.
The results for the previous Calibration data remain in
these two registers and are used for scaling measured
results. If the calibration error is caused by the internal
calibration time measurement exceeding the time set by
the TIMOUT[2:0] bits in the TOF2 register, then the TO
bit in the Interrupt Status register is set and the INT pin
asserts (if enabled).
RTC, Alarm, Watchdog, and Tamper Operation
RTC Operation
The MAX35101 contains a real-time clock that is driven
by a 32kHz oscillator. The time and calendar information
is obtained by reading the appropriate register words.
The time and calendar are set or initialized by writing
the appropriate register words. The contents of the time
and calendar registers are in the Binary-Coded Decimal
(BCD) format. The clock/calendar provides hundredths
of seconds, tenths of seconds, seconds, minutes, hours,
day, date, month, and year information. The date at the
end of the month is automatically adjusted for months with
fewer than 31 days, including corrections for leap year
valid up to 2100. The clock operates in either the 24-hour
or 12-hour format with AM/PM indicator. The MAX35101
real-time clock can be programmed for either 12-hour or
24-hour formats. If using the 24-hour format, Bit6 (12 HR
MODE) of the Mins_Hrs register should be cleared to 0
and then Bit5 represents the 20-hour indicator. If using the
12-hour format, Bit6 should be set to 1 and Bit5 represents
AM (if 0) or PM (if 1). The day-of-week register increments
at midnight. Values that correspond to the day of week
are user defined but must be sequential (i.e., if 0 equals
Sunday, then 1 equals Monday, and so on). Illogical time
and date entries result in undefined operation.
Alarm Operation
The MAX35101 real-time clock provides one program-
mable alarm. The alarm is activated when either the AM1
or AM2 bits in the Real-Time Clock register are set. Based
upon these bits, an alarm can occur when either the
minutes and/or hours programmed in the Alarm register
match the current value in the Mins_Hrs register. When
an alarm occurs, the AF bit in the Interrupt Status register
is set and the INT device pin asserts (if enabled).
MAX35101 Time-to-Digital Converter with Analog Front-End
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