DS1390 Datasheet, PDF (18/24 Pages) Maxim Integrated Products – Low-Voltage SPI/3-Wire RTCs with Trickle Charger

Low-Voltage SPI/3-Wire RTCs with
Trickle Charger
TRICKLE-CHARGE REGISTER (8Fh WRITE, 0Fh READ)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
TCS3 TCS2 TCS1 TCS0
DS1
BIT 2
BIT 1
BIT 0
DS0 ROUT1 ROUT0
1 0F 16 SELECT
NOTE: ONLY 1010b ENABLES CHARGER
1 OF 2
SELECT
1 OF 3
SELECT
V_CC
TCS_0-3 = TRICKLE-CHARGE SELECT
DS_0-1 = DIODE SELECT
ROUT_0-1 = RESISTOR SELECT
R1
250Ω
R2
2kΩ
R3
4kΩ
V_BACKUP
Figure 9. DS1390/DS1391 Programmable Trickle Charger
Table 6. SPI Pin Function
MODE CSZ
SCLK
SDI
SDO
Disable H
Input
Disabled
Input
Disabled
High
Impedance
CPOL* = 1,
SCLK Rising
Write L
CPOL = 0,
SCLK Falling
Data Bit
Latch
High
Impedance
CPOL = 1,
SCLK Falling
Read L
CPOL = 0,
X
Next Data
Bit Shift**
SCLK Rising
*CPOL is the clock-polarity bit set in the control register of the
host microprocessor.
**SDO remains at high impedance until 8 bits of data are ready to
be shifted out during a read.
CS
DATA LATCH (WRITE/INTERNAL STROBE)
SHIFT DATA OUT (READ)
SCLK WHEN CPOL = 0
DATA LATCH (WRITE/INTERNAL STROBE)
SHIFT DATA OUT (READ)
SCLK WHEN CPOL = 1
NOTE 1: CPHA BIT POLARITY (IF APPLICABLE) MAY NEED TO BE SET ACCORDINGLY.
NOTE 2: CPOL IS A BIT SET IN THE MICROCONTROLLER'S CONTROL REGISTER.
NOTE 3: SDO REMAINS AT HIGH IMPEDANCE UNTIL 8 BITS OF DATA ARE READY TO BE
SHIFTED OUT DURING A READ.
Figure 10. Serial Clock as a Function of Microcontroller Clock-
Polarity Bit
The user determines diode and resistor selection
according to the maximum current desired for battery
or super cap charging. The maximum charging current
can be calculated as illustrated in the following exam-
ple. Assume that a system power supply of 3.3V is
applied to V_CC and a super cap is connected to
V_BACKUP. Also, assume that the trickle charger has
been enabled with a diode and resistor R2 between
V_CC and V_BACKUP. The maximum current I_MAX would
therefore be calculated as follows:
I_MAX = (3.3V - diode drop) / R2 ≈ (3.3V - 0.7V) /
2kΩ ≈ 1.3mA
As the super cap changes, the voltage drop between
V_CC and V_BACKUP decreases and therefore the charge
current decreases.
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