Functional Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-20
SYM53C040 DATA MANUAL VERSION 2.0
PRELIMINARY
Power On Configuration Options
The power on configuration of the SYM53C040 can be customized for different
applications using the AD5, AD1-0, and A11-8 pi
ns. Table 2-2 summarizes the
functionality that is enabled or disabled by using pullup resistors on these pins.
Automatic branch generation
At reset, the microcontroller core fetches its first instruction from address 0000h. Because
the interrupt vectors are mapped to locations 0003h through 0032h, an unconditional
jump instruction must be issued as the first instruction fetched from address 0000h. The
SYM53C040 address decode logic automatically generates this jump instruction when the
microcontroller core accesses address locations 0000h through 0002h during its initial
instruction fetch, if automatic branch generation is enabled. Three address destinations are
possible for this initial automatic jump, configurable by external pullup resistors on the
AD0 and AD1 pins. The states of these pins are checked on chip reset. In the SYM53C040,
the AD0 and AD1 signal pins have internal pulldown resistors, so these pins power up
deasserted if no external pullup is used. They power up asserted with an external pullup
resistor. If an external pullup resistor is detected on either of these pins, the internal
Table 2-2
Power on configuration pins and options
Signal Name
Function
Pullup Use
AD1-0
Automatic Branch generation
See Table 2-3.
AD5
Serial ROM automatic download
Enable with external pullup
A10-8
Download configuration address
See
A11
Download serial port select
Pullup resistor changes serial
ROM download from two-wire
serial port 0 to port 1.
The values of these pins are latched into the Power On Configuration Registers (System registers
FF01h, FF03h)
A12 and A15 are test mode enables; external pull-ups should not be used.