Jameco Electronics Rabbit 3000 - Manual (Page 104)

User’s Manual 95
7.9 System Reset
The Rabbit 3000 contains a master reset input (pin 46), which initializes everything in the
device except for the Real-Time Clock (RTC). This reset is delayed until the completion
of any write cycles in progress to prevent potential corruption of memory. If no write
cycles are in progress the reset takes effect immediately. The reset sequence requires a
minimum of 128 cycles of the fast oscillator to complete, even if no write cycles were in
progress at the start of the reset. Reset forces both the processor clock and the peripheral
clock in the divide-by-eight mode. Note that if the processor is being clocked from the 32
kHz clock, the 128 cycles of the fast oscillator will probably not be sufficient to allow any
writes in progress to be completed before the reset sequence completes and the clocks
switch to divide-by-eight mode.
During reset /CS1 is high impedance and all of the other memory and I/O control signals
are held inactive (High). After the /RESET signal becomes inactive (High) the processor
begins fetching instructions and the memory control signals begin normal operation. Note
that the default values in the Memory Bank Control Registers select four wait states per
access, so the initial program fetch memory reads are 48 clock cycles long (8 x (2 + 4)).
Software can immediately adjust the processor timing to whatever the system requires.
/CS1 is high-impedance during reset (and during power-down, when only VBAT is pow-
ered) to allow an external RAM connected to /CS1 to be powered by VBAT. This is possi-
ble because the /CS1 pin is powered by VBAT. In this case an external pull-up resistor (to
VBAT) is required on /CS1 to keep the RAM deselected during power-down. If the exter-
nal RAM connected to /CS1 is not powered by VBAT, so that any information held within
it is lost during power-down, no pull-up resistor on /CS1 is appropriate, as this would add
leakage (through the protection diode) to drain VBAT. The RESOUT signal, which is
High during reset and power-down, can be used to control an external power switch to dis-
connect VDD from supplying VBAT.
The default selection for the memory control signals consists of /CS0 and /OE0, and
writes are disabled. This selection can also be immediately programmed to match the
hardware configuration. A typical sequence would be to speed up the clock to full speed,
followed by selection of the appropriate number of wait states and the chip select signals,
output enable signals and write enable signals. At this point software would usually check
the system status to determine what type of reset just occurred and begin normal opera-
tion.
The default values for all of the peripheral control registers are shown with the following
register listing. The registers within the CPU affected by reset are the Stack Pointer (SP),
the Program Counter (PC), the IIR register, the EIR register, and the IP register. The IP
register is set to all ones (disabling all interrupts), while all of the other listed CPU regis-
ters are reset to all zeros.
Table 7-14 describes the state of the I/O pins after an external reset is recognized by the
Rabbit CPU. Note that the /RESET signal must be held low for three clocks for the proces-
sor to begin the reset sequence. There is no facility to tri-state output lines such as the
address lines and the memory and I/O control lines.