MAX6909 MAXIM [Maxim Integrated Products], MAX6909 Datasheet - Page 12

• Manufacturer Part Number: MAX6909
• Description: I2C-Compatible Real-Time Clocks with uP Supervisor and NV RAM Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

I
Supervisor and NV RAM Controller
Table 1. Crystal Manufacturers and Part Numbers
Timekeeping accuracy of the MAX6909/MAX6910 is
dependent on the frequency stability of the external
crystal. To determine frequency stability, use the para-
bolic curve of Figure 3 and the following equations:
where:
∆f = change in frequency from +25°C (Hz)
f = nominal crystal frequency (Hz)
k = parabolic curvature constant (-0.035 ±0.005ppm/°C
T
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?
Figure 3. Frequency Stability and Temperature
12
Caliber Electronics
ECS INC International
Fox Electronics
M-tron
Raltron
0
2
= turnover temperature (+25°C ±5°C for 32.768kHz
for 32.768kHz watch crystals)
C-Compatible Real-Time Clocks with µP
______________________________________________________________________________________
watch crystals)
MANUFACTURER
-100
-150
-200
-250
-50
0
TYPICAL TEMPERATURE CHARACTERISTICS
-50
-40
(k = -0.035ppm/°C
∆f = f
-30
-20
-10
✕
TEMPERATURE (°C)
0
k
10
AWS2A-32.768KHz,
AWS2B-32.768KHz
ECS-.327-6.0-17
FSM327
SX2010/ SX2020
RSE-32.768-6-C-T
✕
20
(T
30
0
2
40
; T
- T)
50
O
= +25°C)
PART
60
2
70
80
90
100
2
RANGE (°C)
-20 to +70
-10 to +60
-40 to +85
-20 to +75
-10 to +60
TEMP
What is the worst-case timekeeping error per second?
1) Error due to temperature drift:
2) Error due to +25°C initial crystal tolerance of ±20ppm:
3) Total timekeeping error per second:
Figure 4. Oscillator Functional Schematic
∆t
∆t
∆t
drift
∆f
total
∆t
initial
∆t
initial
initial
{[1 / [(32,768Hz - 0.8192Hz) / 32,768]] - 1s} /
∆f
drift
= 0.000025s / s + 0.000020s / s = 0.000045 s / s
drift
= {[1 / [32,768 - 0.65536 / 32,768]] - 1} /
= 32,768Hz
12pF
= {[1 / [(f + ∆f
MAX6909
{[1 / [(f + ∆f
Cg
(20°C - 45°C)
= 32,768Hz
∆t
1s = 0.000025s / s
1s = 0.000020s / s
total
(pF)
C
6
6
6
6
6
L
= ∆t
X1
drift
✕
EXTERNAL
CRYSTAL
initial
(-20ppm) = -0.65536Hz
✕
drift
2
Rf
) / 32,768]] - 1s} / 1s
= -0.8192Hz
(-0.04ppm/°C)
) / 32,768]] - 1} / 1s
+ t
X2
+25°C FREQUENCY
TOLERANCE (ppm)
initial
±20
±20
±20
±20
±20
Rd
Cd
12pF
2
✕