Datasheet

ADC121C021, ADC121C021Q, ADC121C027

SNAS415F –JANUARY 2008–REVISED MARCH 2013

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Standard-Fast Mode

In Standard-Fast mode, the master generates a start condition by driving SDA from high to low while SCL is

high. The start condition is always followed by a 7-bit slave address and a Read/Write bit. After these 8 bits have

been transmitted by the master, SDA is released by the master and the ADC121C021 either ACKs or NACKs the

address. If the slave address matches, the ADC121C021 ACKs the master. If the address doesn't match, the

ADC121C021 NACKs the master.

For a write operation, the master follows the ACK by sending the 8-bit register address pointer to the ADC. Then

the ADC121C021 ACKs the transfer by driving SDA low. Next, the master sends the upper 8-bits to the

ADC121C021. Then the ADC121C021 ACKs the transfer by driving SDA low. For a single byte transfer, the

master should generate a stop condition at this point. For a 2-byte write operation, the lower 8-bits are sent by

the master. The ADC121C021 then ACKs the transfer, and the master either sends another pair of data bytes,

generates a Repeated Start condition to read or write another register, or generates a Stop condition to end

communication.

A read operation can take place either of two ways:

If the address pointer is pre-set before the read operation, the desired register can be read immediately following

the slave address. In this case, the upper 8-bits of the register, set by the pre-set address pointer, are sent out

by the ADC. For a single byte read operation, the Master sends a NACK to the ADC and generates a Stop

condition to end communication after receiving 8-bits of data. For a 2-Byte read operation, the Master continues

the transmission by sending an ACK to the ADC. Then the ADC sends out the lower 8-bits of the ADC register.

At this point, the master either sends an ACK to receive more data or sends a NACK followed by a Stop or

Repeated Start. If the master sends an ACK, the ADC sends the next data byte, and the read cycle repeats.

If the ADC121C021address pointer needs to be set, the master needs to write to the device and set the address

pointer before reading from the desired register. This type of read requires a start, the slave address, a write bit,

the address pointer, a Repeated Start (if appropriate), the slave address, and a read bit (refer to Figure 30).

Following this sequence, the ADC sends out the upper 8-bits of the register. For a single byte read operation, the

Master must then send a NACK to the ADC and generate a Stop condition to end communication. For a 2-Byte

write operation, the Master sends an ACK to the ADC. Then, the ADC sends out the lower 8-bits of the ADC

Repeated Start. If the master sends an ACK, the ADC sends another pair of data bytes, and the read cycle will

repeat. The number of data words that can be read is unlimited.

High-Speed (Hs) Mode

For Hs-mode, the sequence of events to begin communication differs slightly from Standard-Fast mode.

Figure 26 describes this in further detail. Initially, the bus begins running in Standard-Fast mode. The master

generates a Start condition and sends the 8-bit Hs master code (00001XXX) to the ADC121C021. Next, the

ADC121C021 responds with a NACK. Once the SCL line has been pulled to a high level, the master switches to

Hs-mode by increasing the bus speed and generating a second Repeated Start condition (driving SDA low while

SCL is pulled high). At this point, the master sends the slave address to the ADC121C021, and communication

continues as shown above in the "Basic Operation" Diagram (see Figure 25).

When the master generates a Repeated Start condition while in Hs-mode, the bus stays in Hs-mode awaiting the

slave address from the master. The bus continues to run in Hs-mode until a Stop condition is generated by the

master. When the master generates a Stop condition on the bus, the bus must be started in Standard-Fast mode

again before increasing the bus speed and switching to Hs-mode.

Product Folder Links: ADC121C021 ADC121C021Q ADC121C027