Cirrus Logic AN298 - Manual (Page 12)

12 AN298RC14
3 Firmware Messaging
3 Firmware Messaging
While using the CS485xx, it is necessary to communicate with the DSP in order to control or monitor the various
downloaded firmware modules. We refer to this process of communication firmware messaging. The purpose of this
section is to cover the types and formats of these firmware messages. In general, with firmware messaging, the user can
control the firmware module running on the DSP and subsequently perform various tasks such as:
• Configure the module after firmware download (e.g. Kickstarting the DSP, etc.)
• Change runtime parameters (e.g. adjusting the Volume, switching Pro Logic II modes, etc.)
• Obtain information from the DSP (e.g. current state of the firmware registers, data stream information, etc.)
3.1 Communication Overview
From a “micro-programmer” point of view, the CS485xx firmware modules can be thought of as a blocks of several 32-bit
registers (variables) that either control the behavior of the firmware or store information about the state of the firmware at
the time of operation. Each register has a unique index. Access to the register involves a combination of a specified opcode
for that firmware module together with the register index. For each firmware module, the following opcodes are available:
• Write Opcode - Issues a command to write to a specific module.
• Read Opcode - Issues a command to read from a specific module.
• Read Response Opcode - Indicates module and index that has been read.
These available opcodes permit the following types of communication with the CS485xx DSP:
• Writing to the DSP
• Solicited read from the DSP
• Unsolicited read from the DSP
The following sub-sections cover the communication types listed above.
3.1.1 Writing to the DSP
A write session with the CS485xx consists of one 8-byte message from the host to the CS485xx. In essence, the write
message consists of a 32-bit command word followed by a 32-bit data word (that is, data to be written to the register). The
command word is formed by combining the write opcode for that module together with the index of the register that needs
to be written. The 32-bit Data Word is the value of the data intended to fill that register.
The following diagram shows the format of a write message:
3.1.2 Solicited Read
A solicited read can be thought of as a request to read the contents of a specific register. This is comprised of a 32-bit
solicited read command word which is a request to read a specific index (register) in a given module. The DSP, upon
receiving this message, responds by sending back a 32-bit Read Response opcode and the requested 32-bit data word
contained in the register.
Write Command Word:
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
OPCODE[31:16] INDEX[15:0]
Write Data Word:
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
DATA WORD[31:0]