PSB7238 SIEMENS [Siemens Semiconductor Group], PSB7238 Datasheet - Page 40

• Manufacturer Part Number: PSB7238
• Description: Joint Audio Decoder-Encoder - Multimode
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

PSB 7238
Interfaces and Memory Organization
3.3.2.2
Mailbox
The mailbox is implemented as physically two separate 256-byte memory blocks. Only
least significant bytes are used. One is read-only by the DSP and write-only by the host,
the other is write-only by the DSP and read-only by the host.
Figure 14
Since the two memories are totally independent, data transfer from host to DSP can take
place simultaneously with data transfer from DSP to host (full duplex operation).
The mailbox is seen from the host as an I/O device. Thus, to read or write a byte in the
mailbox, the host accesses a single location (separate for read and for write mailbox).
The address is given by an address register directly programmable by the host. This
address is autoincremented every time an access by the host to the mailbox I/O address
is performed. Thus, for sequential, fast access, the mailbox is seen as a 256-byte, full
duplex FIFO. For random accesses to the mailbox the host has to reprogram the address
register(s). This is summarized in Figure 15.
Figure 15
Semiconductor Group
40
Data Sheet 1998-07-01