Implementing a boot off SD with hardware?

rwgast_logicdesign

Id like to make a feature where I can pop in an sd-card with a program/os like CP/M, sphinix, or a prop demo and the prop runs it from the SD-Card. Now I know there is code out there that makes it possible to boot from the sd card but this isnt feasible all the time. If I just wanna boot the thing up and run a defualt eprom program i want that to work too.

Id like to make a boot off sd switch, I dont really know how one would implement this though? My only guess is to wire up a second eeprom with the sd bootloader written on to it use a dpdt switch to change the i2s bus wires from oone eeprom to the other?

Mike Green

You could do this by having two EEPROMs and an SPDT switch. The EEPROMs have an address select line, sometimes several of them. This sets what group of addresses the EEPROM will respond to. Most such select lines have built-in pulldown resistors to Vss so the default address is %000. One contact of the SPDT switch goes to one EEPROM's A0 select line and the other contact to the 2nd EEPROM's A0 select line. The common connection goes through a 10K resistor to Vdd. In one switch position, one EEPROM will be device #0 and the other device #1. In the other switch position, the EEPROMs are reversed.

You should also have a place for a jumper connected to one EEPROM's /WP (write protect) line (and ground). Once you've downloaded an SD card loader or other default program into the EEPROM (like the CP/M interpreter or Sphinx or Spinix), you can prevent the EEPROM from being modified.

Other solutions include using the SD card inserted switch to provide the select signal for one EEPROM and an inverter to provide the signal for the other EEPROM. This would run an SD card loader if a card is present or the contents of the other EEPROM if not.