Connectivity considerations – Kontron COMe Starterkit Eval T2 User Manual
Page 122
COM Express Interfaces
Specification. The circuitry in Figure 47 below shows how to connect an Atmel 'AT24C04' 4kbit
EEPROM to the General Purpose I2C bus on the COM Express Carrier Board
(http://www.atmel.com). According to the COM Express specification, the I2C address lines A2,
A1 and A0 of the system configuration EEPROM must be pulled high. Depending on the
EEPROM size this leads to the I2C addresses 1010 111x (2kbit), 1010 110x (4kbit), or 1010 100x
(8kbit).
Figure 47:
System Configuration EEPROM Circuitry
The EEPROM stores configuration information for the system of the Carrier Board. The data
structure used is defined in the The Specification recommends but
does not require the use of this system configuration EEPROM. The Module BIOS may check
the Carrier Board configuration EEPROM but is not required to do so by the Specification.
The Atmel AT24C02 with 2Kb organized as 256 x 8 is a suitable device in an 8-pin SOIC
package. For applications that require additional ROM or memory capacity such as 8Kb (1K x 8)
or 16Kb (2K x 8), an Atmel AT24C08A may be used.
The COM Express Specification requires a minimum capacity of 2Kb. The Atmel AT24C02
meets this minimum capacity.
Address inputs A0, A1, A2 are pulled high. This creates the I2C address 1010 111x, which is
required by the COM Express Specification. EEPROM devices internally set I2C address lines
A6, A5, A4, A3 to binary value 1010.
WP (write protect) is pulled low for normal read/write.
2.18.3.
Connectivity Considerations
The maximum amount of capacitance allowed on the Carrier General Purpose I2C bus lines
(I2C_DAT, I2C_CK) is specified by your Module vendor. The Carrier designer is responsible for
ensuring that the maximum amount of capacitance is not exceeded and the rise/fall times of the
signals meet the I2C bus specification. As a general guideline, an IC input has 8pF of
capacitance, and a PCB trace has 3.8pF per inch of trace length.
PICMG
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COM Express
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Carrier Board Design Guide
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VC C _3V3_S BY
VC C _3V3_S BY
CEX
I2C_D AT
I2C_C K
R 137
10k
R 137
10k
C 176
100n
C 176
100n
U 21
A 0
1
A 1
2
A 2
3
G N D
4
S D A
5
S C L
6
W P
7
V C C
8
24C02
CEX