ADBMS1818CSWAZ-R7 datasheet(79/92 Pages) AD

79 / 92 page

Data Sheet

ADBMS1818

APPLICATIONS INFORMATION

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Rev. B | 79 of 92

if (remainder & 0x4000)

{

remainder = ((remainder << 1));

remainder = (remainder ^ CRC15_POLY)

}

else

{

remainder = ((remainder << 1));

}

pec15Table[i] = remainder&0xFFFF;

}

unsigned int16 pec15 (char *data , int len)

{

int16 remainder,address;

remainder = 16;//PEC seed

for (int i = 0; i < len; i++)

{

address = ((remainder >> 7) ^ data[i]) & 0xff;//calculate PEC table

address remainder = (remainder << 8 ) ^ pec15Table[address];

}

return (remainder*2);//The CRC15 has a 0 in the LSB so the final

value must be multiplied by 2

}

isoSPI IBIAS and ICMP Setup

The ADBMS1818 allows the isoSPI links of each application to

be optimized for power consumption or for noise immunity. The

power and noise immunity of an isoSPI system is determined by

the programmed IB current, which controls the isoSPI signaling

currents. IB can range from 100 μA to 1 mA. Internal circuitry scales

up this bias current to create the isoSPI signal currents equal to

be 20 × IB. A low IB reduces the isoSPI power consumption in the

ready and active states, whereas a high IB increases the amplitude

of the differential signal voltage VA across the matching termination

resistor, RM. The IB current is programmed by the sum of the RB1

and RB2 resistors connected between the 2 V IBIAS pin and GND,

as shown in Figure 103. The receiver input threshold is set by the

ICMP voltage that is programmed with the resistor divider created

by the RB1 and RB2 resistors. The receiver threshold is half of the

voltage present on the ICMP pin.

The following guidelines must be followed when setting IB (100 μA

to 1 mA) and the receiver comparator threshold voltage VICMP/2:

RM=Transmission Line Cℎaracteristic

Impedance Z0

Signal Amplitude =

VA = 20 × IB × RM/2

Receiver Comparator Threshold (VTCMP) = K × VA

Voltage on ICMP Pin (VCIMP) = 2 × VTCMP

RB2 = VICMP/IB

RB1 = (2/IB) - (RB2)

Select IB and K (signal amplitude VA to receiver comparator thresh-

old ratio) according to the application:

►

For lower power links: IB = 0.5 mA and K = 0.5.

►

For full power links: IB = 1 mA and K = 0.5.

►

For long links (>50m): IB = 1 mA and K = 0.25.

For applications with little system noise, setting IB to 0.5 mA is a

good compromise between power consumption and noise immuni-

ty. Using this IB setting with a 1:1 transformer and RM = 100 Ω,

RB1 must be set to 3.01 k, and RB2 set to 1 kΩ. With a typical

CAT5 twisted pair, these settings allow communication up to 50 m.

For applications in very noisy environments or that require cables

longer than 50 m, it is recommended to increase IB to 1 mA. Higher

drive current compensates for the increased insertion loss in the

cable and provides high noise immunity. When using cables over

50 m and a transformer with a 1:1 turns ratio and RM = 100 Ω, RB1

is 1.5 k, and RB2 is 499 Ω.

The maximum clock rate of an isoSPI link is determined by the

length of the isoSPI cable. For cables 10 m or less, the maximum

1 MHz SPI clock frequency is possible. As the length of the cable

increases, the maximum possible SPI clock rate decreases. This

dependence is a result of the increased propagation delays that

can create possible timing violations. Figure 102 shows how the

maximum data rate reduces as the cable length increases when

using a CAT5 twisted pair.

Cable delay affects three timing specifications: tCLK, t6, and t7. In

the electrical characteristics table, each of these specifications is

derated by 100 ns to allow for 50 ns of cable delay. For longer

cables, the minimum timing parameters may be calculated as

shown below:

tCLK, t6, and t7 > 0.9 μs + 2 × tCABLE (0.2 m per ns)

ADBMS1818CSWAZ-R7 数据表(PDF) 79 Page - Analog Devices

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