Datasheet
ADE7854/ADE7858/ADE7868/ADE7878 Data Sheet
Rev. G | Page 66 of 100
default values. The ADE78xx computes the cyclic redundancy
check (CRC) based on the IEEE802.3 standard. The registers
are introduced one-by-one into a linear feedback shift register
(LFSR) based generator starting with the least significant bit (as
shown in Figure 82). The 32-bit result is written in the
CHECKSUM register. After power-up or a hardware/software
reset, the CRC is computed on the default values of the registers
giving the results presented in the Table 23.
Table 23. Default Values of CHECKSUM and of Internal
Registers CRC
Part No.
Default Value of
CHECKSUM
CRC of Internal
Registers
ADE7854 0x44C48F8 0x391FBDDD
ADE7858 0xD6744F93 0x3E7D0FC1
ADE7868 0x93D774E6 0x23F7C7B1
ADE7878 0x33666787 0x2D32A389
Figure 83 shows how the LFSR works. The MASK0, MASK1,
COMPMODE, gain, CFMODE, CF1DEN, CF2DEN, CF3DEN,
CONFIG, MMODE, ACCMODE, LCYCMODE, and HSDC_CFG
registers, and the six 8-bit reserved internal registers form the
bits [a
255
, a
254
,…, a
0
] used by LFSR. Bit a
0
is the least significant
bit of the first internal register to enter LFSR; Bit a
255
is the most
significant bit of the MASK0 register, the last register to enter
LFSR. The formulas that govern LFSR are as follows:
b
i
(0) = 1, i = 0, 1, 2, …, 31, the initial state of the bits that form
the CRC. Bit b
0
is the least significant bit, and Bit b
31
is the most
significant.
g
i
, i = 0, 1, 2, …, 31 are the coefficients of the generating
polynomial defined by the IEEE802.3 standard as follows:
G(x) = x
32
+ x
26
+ x
23
+ x
22
+ x
16
+ x
12
+ x
11
+ x
10
+ x
8
+ x
7
+
x
5
+ x
4
+ x
2
+ x + 1 (49)
g
0
= g
1
= g
2
= g
4
= g
5
= g
7
= 1
g
8
= g
10
= g
11
= g
12
= g
16
= g
22
=g
23
= g
26
= 1 (50)
All of the other g
i
coefficients are equal to 0.
FB(j) = a
j – 1
XOR b
31
(j – 1) (51)
b
0
(j) = FB(j) AND g
0
(52)
b
i
(j) = FB(j) AND g
i
XOR b
i − 1
(j – 1), i = 1, 2, 3, ..., 31 (53)
Equation 51, Equation 52, and Equation 53 must be repeated for
j = 1, 2, …, 256. The value written into the CHECKSUM register
contains the Bit b
i
(256), i = 0, 1, …, 31. The value of the CRC,
after the bits from the reserved internal register have passed
through LFSR, is obtained at Step j = 48 and is presented in the
Table 23.
Two different approaches can be followed in using the CHECK-
SUM register. One is to compute the CRC based on the relations
(47) to (53) and then compare the value against the CHECKSUM
register. Another is to periodically read the CHECKSUM register.
If two consecutive readings differ, it can be assumed that one of
the registers has changed value and therefore, the ADE7854,
ADE7858, ADE7868, or ADE7878 has changed configuration.
The recommended response is to initiate a hardware/software
reset that sets the values of all registers to the default, including
the reserved ones, and then reinitialize the configuration registers.
31 0 0 15 0 15 0 01531
255 248 240 232 224 216
7070707 0
0
07 07
40 32 24 16 8 7
MASK0 MASK1 COMPMODE CFMODEGAIN
INTERNAL
REGISTER
INTERNAL
REGISTER
INTERNAL
REGISTER
INTERNAL
REGISTER
INTERNAL
REGISTER
INTERNAL
REGISTER
LFSR
GENERATOR
08510-055
Figure 82. CHECKSUM Register Calculation
b
0
LFSR
FB
g
0
g
1
g
2
g
31
b
1
g
3
b
2
b
31
a
255
, a
254
,....,a
2
, a
1
, a
0
08510-056
Figure 83. LFSR Generator Used in CHECKSUM Register Calculation
INTERRUPTS
The ADE7854/ADE7858/ADE7868/ADE7878 have two interrupt
pins,
IRQ0
and
IRQ1
. Each of the pins is managed by a 32-bit
interrupt mask register, MASK0 and MASK1, respectively. To
enable an interrupt, a bit in the MASKx register must be set to
1. To disable it, the bit must be cleared to 0. Two 32-bit status
registers, STATUS0 and STATUS1, are associated with the inter-
rupts. When an interrupt event occurs in the ADE78xx, the
corresponding flag in the interrupt status register is set to a Logic 1
(see Table 37 and Table 38). If the mask bit for this interrupt in
the interrupt mask register is Logic 1, then the
IRQx
logic output