how to implement digital trigger in a MCU based oscilloscope

I am trying to build a scope based on a MCU with PC to display waveform. The MCU integrates ADC, RAM and transmits captured and processed data to PC. The only thing I can think of to implement digital trigger inside MCU is that after each ADC sample, the core will read it and calculate if it is a trigger condition.
I wonder if it is the right way? Does any body know how commercial MCU based scope does this, e.g. bitscope Micro

Hi,

I'd use an FPGA for this.
It's more suitable. faster, direct access to RAM....

Klaus

Thanks for the reply.

I know that people use FPGA or CPLD for digital trigger. I will do a FPGA (do everything in FPGA) scope later on.
But now I am building a MCU version, so looking for reference from the commercial MCU based scope...

ADC -> DMA -> RAM will work the same way as ADC -> FPGA -> RAM <- FPGA <- MCU, frequency will be a bit lower, but cost promise to be really cheap. Expected sample rate will be 1-3 MSPS.
Another option: fast external ADC -> FSMC -> DMA -> RAM. Can give you up to 20-30 MSPS still without using FPGA.
But, anyway, this will be just a toys compare to use FGPA and 200+ MSPS.
Trigger itself is just a fast analog comparator which compare the input voltage with reference one, given by DAC, usually connected by SPI (or used internal DAC in MCU). Comparator triggers the digital input which triggers the DMA to start transfer.
Then DMA triggers the interrupt with transfer complete flag. After that buffer simply being copied to host or printed out to LCD.

thank you for the reply.
I have 2 more questions.
1. if I want to do some complex digital trigger, FPGA must be used, it that correct?
2. For external ADC, how can I interface it with an MCU which does not have FSMC or any standard parallel ADC interface(CMOS/LVDS)? Can I just program GPIO to do the job? Is there any restriction on that?