BU9873 Datasheet, PDF (25/40 Pages) Rohm – I2C BUS Serial Interface RTC with High-precision Oscillation Adjustment

BU9873
Datasheet
Configuration of Application Circuit
1. Quartz Crystal
Typical frequency of quartz crystal is 32.768 kHz. Generally, quartz crystal units have basic characteristics including an
equivalent series resistance (R₁) indicating the oscillation margin and a load capacitance (C_L) indicating the degree of their
center frequency. We recommended characteristics of crystal units for the BU9873 as follows:
(1) Max. R₁ is up to 80kΩ.
(2) C_L value of 6 to 12.5pF.
The stability of quartz crystal units' oscillation depends on the parasitic capacitance of the circuit board and external
oscillator capacitors (C_Gout and C_Dout). The best C_L value we recommended is 6 to 8pF, but if C_L value is more than 8pF,
please use the oscillation adjustment method and refer to P17 “Oscillation Adjustment Function with Digital Method”.
Install the quartz crystal unit in the closest position to the real-time clock ICs. Avoid using any long parallel lines to wire the
OSCIN and OSCOUT pins. Avoid laying any signal lines or power lines in the vicinity of the oscillation circuit.
2. Pull Up Resistor R_PU
(1) Pull up resistance of SDA and INTRB terminal
SDA and INTRB output is NMOS open drain, so requires pull up resistance. As for this resistance value (R_PU), select
an appropriate value from microcontroller V_IL, I_L, and V_OL-I_OL characteristics of this IC. If R_PU is large, operating
frequency is limited. The smaller the R_PU, the larger is the consumption current.
(a) Maximum value of R_PU
The maximum value of R_PU is determined by the following factors.
• SDA rise time to be determined by the capacitance (C_BUS) of bus line of R_PU and SDA should be t_R or lower. And
AC timing should be satisfied even when SDA rise time is slow.
• The bus electric potential _○A to be determined by input leak total (I_L) of device connected to bus at output of 'H'
to the SDA line and R_PU should sufficiently secure the input 'H' level (V_IH) of microcontroller and EEPROM
including recommended noise margin 0.2Vcc.
VCC - ILRPU - 0.2VCC ≥VIH
∴RPU ≤ ^{0.8VCC - VIH}
IL
Ex.) Vcc =3V IL=10µA VIH=0.7 Vcc
thus
0.8 ×3 -0.7 ×3
∴RPU ≤ _{10 ×10 -6}
≤30 [kΩ ]
Microcontroller
BU9873
RPU
A
SDA or INTRB
terminal
IL
IL
Bus line
capacity
C_BUS
Figure 28. I/O circuit diagram
(b) Minimum value of R_PU
The minimum value of R_PU is determined by the following factors.
• When IC outputs LOW, it should be satisfied that V_OLMAX=0.4V and I_OLMAX=1mA.
VCC -VOL
RPU
≤IOL
∴RPU
≥^{VCC -VOL}
IOL
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