MCP413X datasheet(49/88 Pages) MICROCHIP

49 / 88 page

DS22060B-page 49

MCP413X/415X/423X/425X

7.2

Data Byte

Only the Read Command and the Write Command use

the Data Byte, see Figure 7-1. These commands

concatenate the 8-bits of the Data Byte with the one

data bit (D8) contained in the Command Byte to form

9-bits of data (D8:D0). The Command Byte format

supports up to 9-bits of data so that the 8-bit resistor

network can be set to Full-Scale (100h or greater). This

allows wiper connections to Terminal A and to

Terminal B.

The D9 bit is currently unused, and corresponds to the

position on the SDO data of the CMDERR bit.

7.3

Error Condition

The CMDERR bit indicates if the four address bits

received (AD3:AD0) and the two command bits

received (C1:C0) are a valid combination (see

Table 4-1). The CMDERR bit is high if the combination

is valid and low if the combination is invalid.

SPI commands that do not have a multiple of 8 clocks

are ignored.

Once an error condition has occurred, any following

commands are ignored. All following SDO bits will be

low until the CMDERR condition is cleared by forcing

the CS pin to the inactive state (VIH).

7.3.1

ABORTING A TRANSMISSION

All SPI transmissions must have the correct number of

SCK pulses to be executed. The command is not

executed until the complete number of clocks have

been received. If the CS pin is forced to the inactive

state (VIH) the serial interface is reset. Partial com-

mands are not executed.

SPI is more susceptible to noise than other bus

protocols. The most likely case is that this noise

corrupts the value of the data being clocked into the

MCP4XXX or the SCK pin is injected with extra clock

pulses. This may cause data to be corrupted in the

device, or a command error to occur, since the address

and command bits were not a valid combination. The

extra SCK pulse will also cause the SPI data (SDI) and

clock (SCK) to be out of sync. Forcing the CS pin to the

inactive state (VIH) resets the serial interface. The SPI

interface will ignore activity on the SDI and SCK pins

until the CS pin transition to the active state is detected

(VIH to VIL or VIH to VIHH).

Note 1: When data is not being received by the

MCP4XXX, It is recommended that the

CS pin be forced to the inactive level (VIL)

2: It is also recommended that long

continuous command strings should be

broken down into single commands or

shorter continuous command strings.

This reduces the probability of noise on

the SCK pin corrupting the desired SPI

commands.

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