Description of the Command Codes
3-19
MSP53C691 Software Description
3.5.4.4
Playing Single Channel FM Synthesis in Four-Bit Transfer Mode
The MSP53C691 can play single-channel FM in four-bit transfer mode.
Sending commands to perform other functions (volume control, toggle I/O,
etc.) is also supported in this mode. The FM data is supplied to the
MSP53C691 from the master over the four-bit data lines (DATA0–DATA3). The
DATA/COMMAND line is pulled high while transferring FM data. The
MSP53C691 sends a request back to the master (by pulling OUTRDY low and
transferring four nibbles to the master) whenever it needs more FM data. The
master then must send a command back to the MSP53C691 informing how
many bytes of FM data are going to be sent. Then the master can send the
promised number of bytes to the MSP53C691 in response to each lowering
of INRDY. The following is a typical sequence of events for FM transfer:
1) The MSP53C691 pulls INRDY low and waits for a command from the mas-
ter.
2) The master sends the command header 0x04 to the MSP53C691. Note
that writing 0x04 to the data bus ensures that the DATA4 line is low, indicat-
ing that the value on data lines DATA0–DATA3 is a command.
Most Significant
DATA LINES
DATA4
DATA3
DATA2
DATA1
DATA0
0
0
1
0
0
First command header sent
3) After receiving the header, the MSP53C691 pulls INRDY low to expect the
level 1 parameter from the master.
4) The master sends 0x4 as the level 1 parameter to indicate four-bit transfer
mode for playing FM.
Most Significant
DATA LINES
DATA4
DATA3
DATA2
DATA1
DATA0
0
0
1
0
0
Level 1 parameter sent
5) The MSP53C691 interprets the command and initializes itself to get ready
to speak FM.
6) The MSP53C691 pulls OUTRDY low, indicating that it must send a nibble
back to the master.
7) The master reads the data from the bus (DATA0–DATA3).The
MSP53C691 pulls OUTRDY low three more times to send a total of four
nibbles. The four nibbles sent to the master are 1, 8, 2, and 3 respectively.
The 1 and 8 are interpreted as 18H (24 decimal) bytes needed to fill up the
FM buffer initially. The 2 and 3 are interpreted as 23, characterizing the FM
data request to the master.
Summary of Contents for MSP53C691
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