CM118B Datasheet, PDF (16/25 Pages) C-Media Electronics – Highly Integrated & Configurable USB Audio Single Chip

CM118B
Highly Integrated & Configurable USB Audio Single Chip
The figure below shows an example of two bytes read transfer. Because the CM118B has auto-increment
function, the second DATA byte will be the register data on the next address.
Note: The USB host tries to get new HID data every 2mS. It’s quite slow. If the continuous write transfers are
too close in terms of time, the former transfer may have no effect.
The figure below shows typical transactions between MCU and t h e CM118B. After a START, MCU should send
7-bit slave address (0111000) first and then the 8^th bit denotes a read transfer when it’s high; or a write transfer
when it’s low.
In a write transfer, MCU keeps acting as the transmitter. The CM118B regards the first DATA byte as the start
register address (it’s better to be 0x00). The next four DATA bytes are the contents that MCU writes to the register
addresses. In a read transfer, two transactions are necessary. MCU resets start register address by the first
transaction. Then MCU changes in order to be the receiver during the second transaction to get four bytes of data.
Note: Bits 0~3 of the first HID byte always reflect the button activity, so they cannot be written by MCU.
Datasheet Revision: 1.00
Page 16 / 25
www.cmedia.com.tw
Copyright© C-Media Electronics Inc.