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MCP41010E/P Datasheet(PDF) 19 Page - Microchip Technology |
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MCP41010E/P Datasheet(HTML) 19 Page - Microchip Technology |
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19 / 32 page  2003 Microchip Technology Inc. DS11195C-page 19 MCP41XXX/42XXX 5.4 Daisy-Chain Configuration Multiple MCP42XXX devices can be connected in a daisy-chain configuration, as shown in Figure 5-4, by connecting the SO pin from one device to the SI pin on the next device. The data on the SO pin is the output of the 16-bit shift register. The daisy-chain configuration allows the system designer to communicate with sev- eral devices without using a separate CS line for each device. The example shows a daisy-chain configura- tion with three devices, although any number of devices (with or without the same resistor values) can be configured this way. While it is not possible to use a MCP41XXX at the beginning or middle of a daisy-chain (because it does not provide the serial data out (SO) pin), it is possible to use the device at the end of a chain. As shown in the timing diagram in Figure 5-3, data will be clocked-out of the SO pin on the falling edge of the clock. The SO pin has a CMOS push-pull output and will drive low when CS goes high. SO will not go to a high-impedance state when CS is held high. When using the daisy-chain configuration, the maxi- mum clock speed possible is reduced to ~5.8 MHz, because of the propagation delay of the data coming out of the SO pin. When using the daisy-chain configuration, keep in mind that the shift register of each device is automatically loaded with zeros whenever a command is executed (CS = high). Because of this, the first 16 bits that come out of the SO pin once the CS line goes low will always be zeros. This means that when the first command is being loaded into a device, it will always shift a NOP command into the next device on the chain because the command bits (and all the other bits) will be zeros. This feature makes it necessary only to send command and data bytes to the device farthest down the chain that needs a new command. For example, if there were three devices on the chain and it was desired to send a command to the device in the middle, only 32 bytes of data need to be transmitted. The last device on the chain will have a NOP loaded from the previous device so no registers will be affected when the CS pin is raised to execute the command. The user must always ensure that multiples of 16 clocks are always provided (while CS is low), as all commands will abort if the number of clocks provided is not a multiple of 16. FIGURE 5-3: Timing Diagram for Daisy-Chain Configuration. SO SI SCK CS Data Registers for all devices are loaded on Rising Edge of CS 2 3 4 567 89 10 1 D PP 111213141516 X C XX X First 16 bits shifted out C D D D DDD D will always be zeros 23 45 678 9 10 1 D PP 111213141516 X C XX X Command and Data for Device 3 C D D D DDD D start shifting out after the first 16 clocks D PP XC X X XC D D D D D D D 23 45 67 89 10 1 D PP 111213141516 X C XX X C D D D DDD D D PP XC X X X C DD DD D D D Command Byte for Device 3 Data Byte for Device 1 Command Byte for Device 1 Command Byte for Device 2 Data Byte for Device 2 Data Byte for Device 3 Command and Data for Device 2 start shifting out after the first 32 clocks † There must always be multiples of 16 clocks while CS is low or commands will abort. ‡ The serial data out pin (SO) is only available on the MCP42XXX device. |
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