User Manual

f = nominal crystal frequency
k = parabolic curvature constant (-0.035ppm/°C2
±0.005ppm/°C2 for 32.768kHz watch crystals)
T
0
= turnover temperature (+25°C ±5°C for 32.768kHz
watch crystals)
T = temperature of interest (°C)
For example: What is the worst-case change in oscilla-
tor frequency from +25°C ambient to +45°C ambient?
∆f
drift
= 32,768
✕
(-0.04
✕
(1
✕
10
-6
))
✕
(20-45)
2
= -0.8192Hz
What is the worst-case timekeeping error per second?
Error due to temperature drift:
∆t
drift
= {[1 / [(f+∆f
drift
) / 32768]]-1s} / 1s
∆t
drift
= {[1 / [(32768 - 0.8192) / 32768]]-1}/1s
= 0.000025s/s
Error due to +25°C initial crystal tolerance of ±20ppm:
∆f
initial
= 32,768
✕
(-20
✕
((1
✕
10
-6
)) = -0.65536Hz
∆t
initial
= {[1 / [(f+∆f
initial
) / 32768]]-1s} / 1s
∆t
initial
= {[1 / [(32768-0.65536) / 32768]]-1} / 1s
= 0.000025s/s
Total timekeeping error per second:
MAX6901
3-Wire Serial RTC in a TDFN
______________________________________________________________________________________ 13
Figure 3. Burst Mode Data Transfer
A0
A1 A2 A3 A4 A5 A6 A7
D0
D1 D2
D3 D4 D5
D6 D7
CS
SCLK
I/O
ADDRESS/COMMAND BYTE I/O DATA BYTE 1
D0
D1 D2
D3 D4 D5
D6 D7
I/O DATA BYTE N
R11 1 111 1
N = 8 FOR TIMEKEEPING REGISTER BURST
N = 31 MAX FOR RAM BURST
A0 A1 A2 A3 A4 A5 A6 A7 D0 D1 D2 D3 D4 D5 D6
CS
SCLK
I/O
D7
D0 D1 D2 D3 D4 D5 D6 D7
0111 11R1
R = RAM/ REGISTER SELECT BIT
RAM = 1, REGISTER = 0
R = RAM/ REGISTER SELECT BIT
RAM = 1, REGISTER = 0
ADDRESS/COMMAND BYTE I/O DATA BYTE 1 I/O DATA BYTE N
N = 8 FOR TIMEKEEPING REGISTER BURST
N = 31 MAX FOR RAM BURST
(b) 3-WIRE BURST WRITE:
()