Rabbit 2000 - Program Loading Process Overview; Program Loading Process Details

6 Rabbit 2000 Microprocessor
program reaches address zero the boot mode is disabled and instruction fetching resumes
at address zero.
Wait states are automatically inserted during the fetching of bytes 3, 5 and 7 to wait for
the serial or parallel port ready. The wait states continue indefinitely until the serial port is
ready. This will cause the processor to be in the middle of an instruction fetch until the
next character is ready. While the processor is in this state the chip select, but not the out-
put enable, will be enabled if the memory mapping registers are such as to normally
enable the chip select for the boot ROM address. The chip select will stay low for
extended periods while the processor is waiting for the serial or parallel port data to be
ready. Additionally, the chip select will go low when a write is performed to an I/O
address if the address is such as to enable that chip select if it were a write to a memory
address.
3.2 Program Loading Process Overview
On start up, Dynamic C first uses the PC’s DTR line on the serial port to assert the Rabbit
RESET line and put the processor in cold-boot mode. Next, Dynamic C uses a four stage
process to load a user program:
1. Load an initial loader (coldloader) via triplets sent at 2400 baud from the PC in to a tar-
get in cold-boot mode.
2. Run the initial loader and load a secondary loader (pilot BIOS) at 19200 baud.
3. Run the secondary loader and load the BIOS (as Dynamic C compiles it)
4. Run the BIOS and load the user program at 115200 baud (after Dynamic C compiles it
to a file).
3.3 Program Loading Process Details
When Dynamic C starts, the following sequence of events takes place:
1. The serial port is opened at with the DTR line low, closed, then reopened with the DTR
line high at 2400 baud. This pulses the reset line on the target low (the programming
cable inverts the DTR line) and prepares the PC to send triplets.
2. A group of triplets defined in the file COLDLOAD.BIN consisting of 2 address bytes
and a data byte are sent to the target. The first few bytes sent are sent to I/O addresses to
set the up MMU and MIU and do system initialization. The MMU is set up so that
RAM is mapped to 0x00000, and flash is mapped 0x80000.
3. The remaining triplets place a small program at memory location 0x00000. The last
triplet sent is 0x80,0x24,0x80, which tells the CPU to ignore the SMODE pins and start
running code at address 0x00000.
4. The PC now bumps the baud rate on the serial port being used to 19200.
5. The primary loader measures the crystal speed to determine what divisor is needed to
set a baud rate of 19200. The divisor is stored at address 0x4002 for later use by the
BIOS, and the programming port is set up to be a 19200 baud serial port.