Microcode From Ram Initialization Sequence - Motorola MC68302 User Manual

RISC Microcode from RAM
designed to comply with the requirements of the Hayes AT command set; however, it can
be used with simpler character schemes as well (such as a carriage return).
The SCC receiver synchronizes on the falling edge of the START bit. Once a start bit is de-
tected, each bit received is processed by the AutoBaud controller. The AutoBaud controller
measures the length of the START bit to determine the receive baudrate and compares the
length to values in a user supplied lookup table. After the baudrate is determined, the Auto-
Baud controller assembles the character and compares it against two user-defined charac-
ters. If a match is detected, the AutoBaud controller interrupts the host and returns the
determined nominal start value from the lookup table. The AutoBaud controller continues to
assemble the characters and interrupt the host until the host stops the reception process.
The incoming message should contain a mixture of even and odd characters so that the user
has enough information to decide on the proper character format (length and parity). The
host then uses the returned nominal start value from the lookup table, modifies the SCC
Configuration Register (SCON) to generate the correct baudrate, and reprograms the SCC
to UART mode.
Many rates are supported including: 150, 300, 600, 1200, 2400, 4800, 9600, 14.4K, 19.2K,
38.4K, 57.6K, 64K, 96K, and 115.2K. To estimate the performance of the AutoBaud micro-
code package, the performance table in Appendix A of the MC68302 user's manual can be
used. The maximum full-duplex rate for a BISYNC channel is one-tenth of the system clock
rate. So a 16.67 MHz 68302 can support 115.2k autobaudrate with another low-speed chan-
nel (<50 kbps) and a 20 MHz MC68302 can support 115.2k AutoBaudrate with 2 low-speed
channels. The performance can vary depending on system loading, configuration, and
echoing mode.
C.6 MICROCODE FROM RAM INITIALIZATION SEQUENCE
1. Perform a total system reset of the MC68302.
2. Write $0700 to the BAR. The base address of the internal dual-port RAM after this ac-
tion is $700000 (hex). If a different base address is desired, the S-record file address-
es should be modified to the desired address.
3. Load the S-record file data into the internal dual-port RAM. (In a production environ-
ment, the microcode may be copied from EPROM directly to the internal dual-port
RAM.)
4. Write $0001 to address $0F8 in supervisory space.
5. Write a software reset command to the CR.
6. Continue with the normal initialization sequence.
C-4 MC68302 USER'S MANUAL MOTOROLA