DS1390 Maxim, DS1390 Datasheet - Page 19

• Manufacturer Part Number: DS1390
• Description: The low-voltage serial-peripheral interface (SPI™) DS1390/DS1391/DS1394 and the low-voltage 3-wire DS1392/DS1393 real-time clocks (RTCs) are clocks/calendars that provide hundredths of a second, seconds, minutes, hours, day, date, month, and ye
• Manufacturer: Maxim

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
V
been enabled with a diode and resistor R2 between
Figure 8. DS1390–DS1394 Programmable Trickle Charger
Table 6. SPI Pin Function
* CPOL is the clock-polarity bit set in the control register of the host microprocessor.
** SDO remains at high-Z until 8 bits of data are ready to be shifted out during a read.
BACKUP
Disable
MODE
Read
Read
Write
Write
V
TRICKLE-CHARGE REGISTER (8Fh WRITE, 0Fh READ)
CC
TCS3
BIT 7
. Also, assume that the trickle charger has
NOTE: ONLY 1010b ENABLES CHARGER
CC
TCS2
BIT 6
1 0F 16 SELECT
and a super cap is connected to
CPHA
X
TCS1
0
0
1
1
BIT 5
TCS0
BIT 4
Low-Voltage SPI/3-Wire RTCs with
____________________________________________________________________
High
Low
Low
Low
Low
BIT 3
DS1
CS
SELECT
1 OF 2
BIT 2
DS0
CPOL* = 0, SCLK Rising;
CPOL* = 1, SCLK Rising;
CPOL = 0, SCLK Falling;
CPOL = 1, SCLK Falling;
CPOL = 1, SCLK Falling
CPOL = 0, SCLK Falling
CPOL = 1, SCLK Rising
CPOL = 0, SCLK Rising
ROUT1
BIT 1
Input Disabled
SELECT
1 OF 3
SCLK
ROUT0
V
therefore be calculated as follows:
As the super cap changes, the voltage drop between
V
current decreases.
BIT 0
CC
CC
I
MAX
and V
and V
= (3.3V - diode drop) / R2 ≈ (3.3V - 0.7V) /
TCS[3:0] = TRICKLE-CHARGE SELECT
DS[1:0] = DIODE SELECT
ROUT[1:0] = RESISTOR SELECT
BACKUP
BACKUP
Trickle Charger
Input Disabled
Data Bit Latch
Data Bit Latch
. The maximum current I
decreases and therefore the charge
2kΩ ≈ 1.3mA
SDI
X
X
250Ω
2kΩ
4kΩ
R1
R2
R3
Next Data Bit Shift**
Next Data Bit Shift**
High-Z
High-Z
High-Z
V
SDO
BACKUP
MAX
would
19