Rainbow Electronics DS2781 - Manual (Page 23)

DS2781: Two-Cell Standalone Fuel Gauge IC
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Table 3. PARAMETER EEPROM MEMORY BLOCK 1
ADDRESS
(HEX)
DESCRIPTION
ADDRESS
(HEX)
DESCRIPTION
60 CONTROL - Control Register 70 AE Segment 4 Slope
61 AB - Accumulation Bias 71 AE Segment 3 Slope
62 AC - Aging Capacity MSB 72 AE Segment 2 Slope
63 AC - Aging Capacity LSB 73 AE Segment 1 Slope
64 VCHG - Charge Voltage 74 SE Segment 4 Slope
65 IMIN - Minimum Charge Current 75 SE Segment 3 Slope
66 VAE - Active Empty Voltage 76 SE Segment 2 Slope
67 IAE - Active Empty Current 77 SE Segment 1 Slope
68 Active Empty 40 78 RSGAIN - Sense Resistor Gain MSB
69 RSNSP - Sense Resistor Prime 79 RSGAIN - Sense Resistor Gain LSB
6A Full 40 MSB 7A RSTC - Sense Resistor Temp. Coeff.
6B Full 40 LSB 7B COB – Current Offset Bias
6C Full Segment 4 Slope 7C TBP34
6D Full Segment 3 Slope 7D TBP23
6E Full Segment 2 Slope 7E TBP12
6F Full Segment 1 Slope 7F Reserved
1-WIRE BUS SYSTEM
The 1-Wire bus is a system that has a single bus master and one or more slaves. A multidrop bus is a 1-Wire bus
with multiple slaves. A single-drop bus has only one slave device. In all instances, the DS2781 is a slave device.
The bus master is typically a microprocessor in the host system. The discussion of this bus system consists of four
topics: 64-bit net address, hardware configuration, transaction sequence, and 1-Wire signaling.
64-BIT NET ADDRESS
Each DS2781 has a unique, factory-programmed 1-Wire net address that is 64 bits in length. The first eight bits are
the 1-Wire family code (3Dh for DS2781). The next 48 bits are a unique serial number. The last eight bits are a
cyclic redundancy check (CRC) of the first 56 bits (see Figure 18). The 64-bit net address and the 1-Wire I/O
circuitry built into the device enable the DS2781 to communicate through the 1-Wire protocol detailed in the 1-Wire
Bus System section of this data sheet.
Figure 18. 1-Wire Net Address Format
8-BIT CRC 48-BIT SERIAL NUMBER
8-BIT FAMILY
CODE (3Dh)
MSb LSb
CRC GENERATION
The DS2781 has an 8-bit CRC stored in the most significant byte of its 1-Wire net address. To ensure error-free
transmission of the address, the host system can compute a CRC value from the first 56 bits of the address and
compare it to the CRC from the DS2781. The host system is responsible for verifying the CRC value and taking
action as a result. The DS2781 does not compare CRC values and does not prevent a command sequence from
proceeding as a result of a CRC mismatch. Proper use of the CRC can result in a communication channel with a
very high level of integrity.
The CRC can be generated by the host using a circuit consisting of a shift register and XOR gates as shown in
Figure 18, or it can be generated in software. Additional information about the Dallas 1-Wire CRC is available in