. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Power On Configuration Options
SYM53C040 DATA MANUAL VERSION 2.0
2-21
PRELIMINARY
pulldown resistor will be disabled to reduce power dissipation. The destination of the first
instruction jump is determined by the power on values of the AD0 and AD1 pins, as
Address locations 0033h and 0000h correspond to the internal RAM. Address locations
8000h and 4000h correspond to external memory accesses. Using no external pullup
resistors on AD0 and AD1 disables the automatic branch instruction generation, and the
microcontroller fetches its first instruction from address 0000h. If automatic branch
generation is disabled, a branch instruction must be downloaded into address 0000h, using
an external serial ROM. The power-on setting of AD0 and AD1 can be read in the Power
on Configuration register Zero (System register FF01) bits 0 and 1, respectively.
External serial ROM configuration
The SYM53C040 can attempt to download firmware from a serial ROM at power on,
through the two-wire serial interface. The serial ROM download is performed immediately
at power on or after a reset, and the microcontroller will hold off from fetching its first
instruction until the download completes.
The initial ROM download will be skipped if no external pullup is used on the AD5 signal
pin (the pin has an internal pulldown). If a pullup is detected on the AD5 signal pin, the
download will be attempted and the internal pulldown will be disabled to reduce power
dissipation.
Serial ROM chip address
The chip address of the serial ROM to be used in the power on download can be defined by
using pullup resistors on the AD8, AD9, and AD10 signal pins. There are eight possible
chip addresses for most popular 2-wire serial EEPROM devic
es. Table 2-4 describes the
address of the serial EEPROM device from which the SYM53C040 will attempt the initial
Table 2-3
External pullup values for automatic branch generation
External pullup on
AD1/AD0
First instruction
branch destination
pullup/pullup
8000h
pullup/none
4000h
none/pullup
0033h
none/none
fetched from 0000h