I have a problem with booting ADSP-21992 from an 8-bit external flash via EMI. When switching on the power supply, VDD_INT and VDD_EXT voltages stabilize on their correct values, the oscillator get stabilized, too and after about 1.5 ms the DSP's internal "power on reset" circuit deasserts the RESET signal. DSP starts to boot the code from the external memory, however, after several hundreds of microseconds it suddenly hangs, letting the bootstrapping unfinished. Then, when I assert and deassert the RESET signal "manually" with a piece of wire, the booting procedure starts again and completes normally, the DSP boots to my program and everything is OK. When I switch to EEPROM >4KB boot, connect an external EEPROM (with the same program) to the SPI bus and boot from it instead of flash, everything goes all right.
During my attempts to fix this problem, I experimented with crystal load capacities bringing it to the state, in which it is not able to start oscillations without some external shock (touching XTAL pin with finger etc.). However, after introducing such a shock and starting the oscillator, the DSP normally booted up even via EMI without need to assert and deassert the RESET signal manually.
I have ADSP-21992BSTZ, 32 MHz external crystal (using DSP's internal TTL oscillator), BYPASS pin unconnected (thus, bypass mode selected via the internal pull-up) and POR connected to the RESET (none other connections to those pins). Power supplies are soft-start DC/DC convertors.
Any guess what to do?
por1 - switching on power supplies; pink - VDD_INT, violet - VDD_EXT, yellow - CLK_IN, green - POR (scale: 1 ms - 1 V)
por2 - address bus activity after deasserting the RESET signal by POR
por3 - address bus activity after asserting and deasserting the RESET signal again, but manually
thank you for the advice. I tried to connect VDD_INT and VDD_EXT directly to my dual lab linear power supply, but the result is the same. Unlike the power supply of my PCB (MAX 1775 dual step-down), it rises both voltages simultaneously, but even slower (about 10 milliseconds). So, I will try to obtain some quicker power supply for the next testing.
Is there some technical note about powering ADSP-21992? I followed the datasheet for the design purposes, but no specification is given for the power voltages rise time, only a remark that they should be provided either simultaneously or VDD_INT first and then the VDD_EXT, what is fullfilled in both cases.
Yes, you are right - MAX 1775 takes its higher output and uses it as an input for the lower output step-down block, so VDD_INT is derived from VDD_EXT.
I don't see any "parasitic" signals from other devices connected to the DSP bus during the startup phase and the number of bytes read via EMI after that the DSP hangs is always exactly the same: 42 (according to the bus activity monitoring).