Rabbit 6000 User’s Manual
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The processor checks the SMODE and SYSCFG pins after the /RESET signal is inactive. Table 3-2 sum-
marizes what happens.
If both SMODE pins are zero, the Rabbit 6000 begins fetching instructions from the memory device
mapped into memory bank 0. When SYSCFG is low, memory bank 0 is set to /CS0 and /OE0. If SYSCFG
is high, memory bank 0 is set to /CS3 and /OE0. In both cases, the internal SRAM is selected in 16-bit
mode. If a 16-bit memory is used in memory bank 0, the first section of code must immediately select the
16-bit bus mode. Chapter 5 provides a short sample program to do this.
If either of the SMODE pins is high, the processor will enter the bootstrap mode and accept triplets from
Serial Port A, the serial flash bootstrap port, or the slave port, depending on the SMODE pin selection. It is
good practice to place pulldown resistors on the SMODE pins to ensure the proper operation of your
design.
In the bootstrap mode, the processor inhibits the normal memory fetch, and instead fetches instructions
from a small internal boot ROM. This program reads triplets of three bytes from the selected peripheral.
The first byte is the most-significant byte of a 16-bit address, the second byte is the least-significant byte
of the address, and the third byte is the data to be written. If the uppermost bit of the address is 1, then the
address is assumed to be an internal register address instead of a memory address, and the data are written
to the appropriate register instead. For example, a triplet of (0x04, 0x34, 0x5A) will write 0x5A to logical
memory address 0x0434, while a triplet of (0x80, 0x34, 0x5A) will write 0x5A to processor register 0x34.
Processor registers with addresses above 0xFF are not accessible in the bootstrap mode.
The boot ROM program waits for data to be available; each byte received automatically resets the watch-
dog timer with a 2-second timeout. Bytes must be received quickly enough to prevent timeout (or the
watchdog must be disabled).
The device checks the state of the SMODE pins each time it jumps back to the start of the ROM program
and responds according to the current state. In addition, by setting bit 7 of the Slave Port Control Register
(SPCR) high, the processor can be told to ignore the state of the SMODE pins and continue normal opera-
tion.
Table 3-2. SMODE Pin Settings
SMODE Pins [1,0]
SYSCFG
Operation
00
0
No bootstrap; code is fetched from address 0x0000
on /CS0, /OE0.The internal SRAM is enabled as a
16-bit memory device.
00
1
No bootstrap; code is fetched from address 0x0000
on /CS3, /OE0. The internal SRAM is enabled as a
16-bit memory device.
01
x
Bootstrap from the slave port.
10
x
Bootstrap from Serial Port A, serial flash mode.
11
x
Bootstrap from Serial Port A, asynchronous mode.
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