Rainbow Electronics DS8314 - Manual (Page 9)

Voltage Supervisor
The voltage supervisor monitors the V DD supply. A
220µs reset pulse (t W ) is used internally to keep the
device inactive during power-on or power-off of the
V
DD supply. See Figure 2.
The DS8313/DS8314 card interface remains inactive
regardless of the levels on the command lines until
duration t
W after V DD has reached a level higher than
V TH2 + V HYS2 . When V DD falls below V TH2 , the
DS8313/DS8314 execute a card deactivation sequence
if their card interface is active.
Clock Circuitry
The card clock signal (CLK) is derived from a clock sig-
nal input to XTAL1 or from a crystal operating at up to
20MHz connected between XTAL1 and XTAL2. The
output clock frequency of CLK is selectable through
inputs CLKDIV1 and CLKDIV2. The CLK signal frequen-
cy can be f
XTAL
, f
XTAL
/2, f
XTAL
/4, or f
XTAL
/8. See Table
1 for the frequency generated on the CLK signal given
the inputs to CLKDIV1 and CLKDIV2.
Note that CLKDIV1 and CLKDIV2 must not be changed
simultaneously; a delay of 10ns minimum between
changes is needed. The minimum duration of any state
of CLK is eight periods of XTAL1.
The frequency change is synchronous: during a transition
of the clock divider, no pulse is shorter than 45% of the
smallest period, and the first and last clock pulses about
the instant of change have the correct width. When
changing the frequency dynamically, the change is effec-
tive for only eight periods of XTAL1 after the command.
The f XTAL duty factor depends on the input signal on
XTAL1. To reach a 45% to 55% duty factor on CLK,
XTAL1 should have a 48% to 52% duty factor with tran-
sition times less than 5% of the period.
With a crystal, the duty factor on CLK can be 45% to
55% depending on the circuit layout and on the crystal
characteristics and frequency. In other cases, the duty
factor on CLK is guaranteed between 45% and 55% of
the clock period.
I/O Transceivers
I/O and I/OIN are pulled high with an 11k Ω resistor (I/O
to V CC and I/OIN to V DD ) in the inactive state. The first
side of the transceiver to receive a falling edge
becomes the master. When a falling edge is detected
(and the master is decided), the detection of falling
edges on the line of the other side is disabled; that side
then becomes a slave. After a time delay t
D(EDGE) , an n
transistor on the slave side is turned on, thus transmit-
ting the logic 0 present on the master side.
When the master side asserts a logic 1, a p transistor
on the slave side is activated during the time delay t
PU
and then both sides return to their inactive (pulled up)
states. This active pullup provides fast low-to-high tran-
sitions. After the duration of t
PU , the output voltage
depends only on the internal pullup resistor and the
load current. Current to and from the card I/O lines is
limited internally to 15mA. The maximum frequency on
these lines is 1MHz.
Inactive Mode
The DS8313/DS8314 power up with the card interface
in the inactive mode. Minimal circuitry is active while
waiting for the host to initiate a smart card session.
• All card contacts are inactive (approximately 200
Ω
to GND).
• The I/OIN pin in the high-impedance state (11k Ω
pullup resistor to V DD ).
• Voltage generators are stopped.
• XTAL oscillator is running (if included in the device).
• Voltage supervisor is active.
• The internal oscillator is running at its low frequency.
Activation Sequence
After power-on and the reset delay, the host microcon-
troller can monitor card presence with signals OFF and
CMDVCC, as shown in Table 2.
DS8313/DS8314
Smart Card Interface
_______________________________________________________________________________________ 9
Table 1. Clock Frequency Selection
CLKDIV1 CLKDIV2 f CLK
0 0 f XTAL /8
0 1 f XTAL /4
1 1 f XTAL /2
1 0 f XTAL
Table 2. Card Presence Indication
OFF CMDVCC STATUS
High High Card present.
Low High Card not present.