DS28CN01 Maxim, DS28CN01 Datasheet - Page 8

• Manufacturer Part Number: DS28CN01
• Description: The DS28CN01 combines 1024 bits of EEPROM with challenge-and-response authentication security implemented with the Federal Information Publications (FIPS) 180-1/180-2 and ISO/IEC 10118-3 Secure Hash Algorithm (SHA-1)
• Manufacturer: Maxim

1Kb I
This description of the SHA-1 computation is adapted
from the Secure Hash Standard SHA-1 document that
can be downloaded from the NIST website. Refer to the
full version of the data sheet for more details.
SDA is an open-drain output on the DS28CN01 that
requires a pullup resistor (see the Typical Operating
Circuit ) to realize high logic levels. Because the
DS28CN01 uses SCL only as input (no clock stretch-
ing), the master can drive SCL either through an open-
drain/collector output with a pullup resistor or a
push-pull output.
According to the I
be able to sink at least 3mA at a V
SMBus specification requires a current sink capability
of 4mA at +0.4V. The DS28CN01 can sink at least 4mA
at +0.4V V
This DC characteristic determines the minimum value of
the pullup resistor: R
maximum operating voltage of +5.5V, the minimum
Figure 6. I
8
_______________________________________________________________________________________
SHA-1 Computation Algorithm
2
C Fast-Mode Pullup-Resistor Selection Chart
1200
1000
OL
800
600
400
200
0
2
1.5
over its entire operating voltage range.
C/SMBus EEPROM with SHA-1 Engine
Applications Information
SDA and SCL Pullup Resistors
2
C specification, a slave device must
PMIN
Pullup Resistor R
MINIMUM R
2.0
ABRIDGED DATA SHEET
= (V
P
CC
- 0.4V)/4mA. With a
2.5
OL
of +0.4V. The
P
3.0
Sizing
PULLUP VOLTAGE (V)
3.5
MAXIMUM LOAD AT MINIMUM R
value for the pullup resistor is 1.275kΩ. The “Minimum
R
resistor changes with the operating (pullup) voltage.
For I
sured from 30% to 70% of the pullup voltage. The maxi-
mum bus capacitance C
time must not exceed 300ns. Assuming maximum rise
time, the maximum resistor value at any given capaci-
tance C
ln(7/3)). For a bus capacitance of 400pF the maximum
pullup resistor would be 885Ω.
Since an 885Ω pullup resistor, as would be required to
meet the rise time specification and 400pF bus capaci-
tance, is lower than R
approach is necessary. The “Maximum Load” line in
Figure 6 is generated by first calculating the minimum
pullup resistor at any given operating voltage
(“Minimum R
bus capacitance that yields a rise time of 300ns.
Only for pullup voltages of +4V and lower can the maxi-
mum permissible bus capacitance of 400pF be main-
tained. A reduced bus capacitance of 300pF is
acceptable for the entire operating voltage range. The
corresponding pullup resistor value at the voltage is
indicated by the “Minimum R
P
” line in Figure 6 shows how the minimum pullup
2
C systems, the rise time and fall time are mea-
4.0
B
is calculated as: R
P
” line) and then calculating the respective
4.5
P
FAST MODE
B
PMIN
is 400pF. The maximum rise
5.0
P
” line.
at +5.5V, a different
PMAX
5.5
= 300ns/(C
600
500
400
300
200
100
0
B
x