Datasheet

LTC1955
15
1955fd
For more information www.linear.com/LTC1955
Interfacing to a Microcontroller
The serial port of the LTC1955 can be connected directly
to a 68HC11 style microcontroller’s serial port. The mcro
-
controller should be configured as the master device and
its clock’s idle state should be set to high (MSTR = 1,
CPOL = 1 and CPHA = 0 for the MC68HC11 family).
Figure
4 shows the recommended configuration and
direction of data flow. Note that an additional I/O line
is necessary for LD to load the data once it has shifted
around the loop. Command data is latched into the com
-
mand register on the falling edge of the LD
signal. The
LTC1955 will begin to act on new command data as soon
as LD goes low. Any general purpose microcontroller I/O
line can be configured to control the LD pin.
The status of the LTC1955 is returned over the serial
port. Status data is latched into the shift register on the
rising edge of the LD pin. Whenever the system is wait
-
ing for status data from the LTC1955, its LD pin should
be held low
.
Daisychained Operation
For applications requiring more than two card sockets,
the serial port of the LTC1955 is designed to be easily
daisychained. The D
OUT
pin of one LTC1955 can be con-
nected directly to the D
IN
pin of another LTC1955. Rather
than sending two 8-bit bytes before asserting LD, the
microcontroller should send two 8-bit bytes per device. LD
should only be asserted after all devices have been updated.
Figure 6 shows three LTC1955s cascaded in daisychain
fashion. In this case, the microcontroller would write six
8-bit bytes before asserting the LD pin. Alternatively, if
two serial ports are available on the microcontroller, then
two LTC1955s can be controlled independently.
If the DATA lines of two or more LTC1955s are connected
together, the static pull-up current will be the sum of the
devices. The static current can be brought back to the level
of a single LTC1955 by setting bit D3 on all but one of the
LTC1955s to 1 (see Table 1). Bit D3 disables the pull-up
current source on the DATA pin. This will help prevent V
OL
problems in multiple LTC1955 applications when driving
the DATA or I/O pins low.
CARD A
CARD B
1955 F04
D
IN
D
OUT
SCLK
LD
LTC1955
MOSI
MISO
SCK
I/O
µCONTROLLER
Figure 4. Microcontroller Interface
applicaTions inForMaTion