DS1390-DS1393 MAXIM [Maxim Integrated Products], DS1390-DS1393 Datasheet - Page 18

• Manufacturer Part Number: DS1390-DS1393
• Description: Low-Voltage SPI/3-Wire RTCs with Trickle Charger
• Manufacturer: MAXIM [Maxim Integrated Products]

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
Low-Voltage SPI/3-Wire RTCs with
Trickle Charger
Figure 9. DS1390/DS1391 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 impedance until 8 bits of data are ready to
be shifted out during a read.
18
Disable
MODE
BACKUP
Read
Write
____________________________________________________________________
V
TRICKLE-CHARGE REGISTER (8Fh WRITE, 0Fh READ)
CC
CSZ
. Also, assume that the trickle charger has
TCS3
BIT 7
H
L
L
NOTE: ONLY 1010b ENABLES CHARGER
CC
SCLK Falling
SCLK Falling
SCLK Rising
SCLK Rising
CPOL* = 1,
TCS2
CPOL = 0,
CPOL = 1,
CPOL = 0,
BIT 6
and a super cap is connected to
1 0F 16 SELECT
Disabled
SCLK
Input
TCS1
BIT 5
TCS0
Disabled
BIT 4
Data Bit
Latch
Input
SDI
X
BIT 3
DS1
SELECT
1 OF 2
Impedance
Impedance
Next Data
Bit Shift**
SDO
High
High
BIT 2
DS0
ROUT1
BIT 1
SELECT
1 OF 3
V
therefore be calculated as follows:
As the super cap changes, the voltage drop between
V
current decreases.
Figure 10. Serial Clock as a Function of Microcontroller Clock-
Polarity Bit
ROUT0
CC
CC
BIT 0
DATA LATCH (WRITE/INTERNAL STROBE)
DATA LATCH (WRITE/INTERNAL STROBE)
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
I
MAX
and V
and V
SHIFTED OUT DURING A READ.
= (3.3V - diode drop) / R2
BACKUP
BACKUP
TCS
DS
ROUT
SHIFT DATA OUT (READ)
SHIFT DATA OUT (READ)
SCLK WHEN CPOL = 0
SCLK WHEN CPOL = 1
0-1
0-3
0-1
= DIODE SELECT
= TRICKLE-CHARGE SELECT
= RESISTOR SELECT
. The maximum current I
decreases and therefore the charge
2k
CS
1.3mA
250
2k
4k
R1
R2
R3
(3.3V - 0.7V) /
V
MAX
BACKUP
would