Motorola DSP56012 - Manual (Page 134)

4-54 DSP56012 User’s Manual MOTOROLA
Parallel Host Interface
Host Interface (HI)
4.4.8.2.3 Host to DSP—Bootstrap Loading Using the HI
The circuit shown in Figure 4-31 will cause the DSP to boot through the HI on power
up. During the bootstrap program, the DSP looks at the MODC, MODB, and MODA
bits. If the MODC:MODB:MODA bits = 001, the DSP will load from the HI. Data is
written by the host processor in a pattern of four bytes, with the high byte being a
dummy and the low byte being the low byte of the DSP word (see
Figure 4-31 and
Figure 4-30 ). Figure 4-32 on page 4-57 shows how an 8-, 16-, 24-, or 32-bit word in
the host processor maps into the HI registers. The HI register at address $4 is not
used and will read as 0. It is not necessary to use address $4, but since many host
processors are 16- or 32-bit processors, address $4 will often be used as part of the 16-
or 32-bit word. The low order byte (at $7) should always be written last, since writing
to it causes the HI to initiate the transfer of the word to the HORX. Data is then
transferred from the HORX to the DSP program memory. If the host processor needs
to terminate the bootstrap loading before 512 words have been downloaded, it can
set the HF0 bit in the ICR. The DSP will then terminate the download and start
executing at location P:$0000. Since the DSP56012 is typically faster than the host
processor, handshaking during the data transfer is normally not required.
Figure 4-30 Transmit/Receive Byte Registers
Host
Data Read—00000000
Write—XXXXXXXX
Host
Transmit/Receive
Byte Registers
Host Byte
Address
0 0 0 0 0 0 0 0
TXH/RXH
High Byte
TXM/RXM
Middle Byte
TXL/RXL
Low Byte
8-bit Transfer
16-bit Transfer
24-bit Transfer
32-bit Transfer, Lowest 24 Bits Are Significant
Access to
Low Byte
Initiates
Transfer
NOTE: Access the low byte last
7 0
4
5
6
7
31
High Middle Low
24 23 16 15 8 7 0
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