Digital oscilloscope - part 4
Now that the oscilloscope skeleton is working, it is easy to add more functionality.
Edge-slope trigger
Let's add the ability to trigger on a rising-edge or falling-edge.
Any oscilloscope can do that.
We need one bit of information to decide with direction we want to trigger on.
Let's use bit-0 of the data sent by the PC.
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assign Trigger = (RxD_data[0] ^ Threshold1) & (RxD_data[0] ^ ~Threshold2);
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That was easy.
More options
Let's add the ability to control the trigger threshold.
That's an 8-bits value.
Then we require horizontal acquisition rate control, filtering control...
That requires multiple control bytes from the PC to control the oscilloscope.
The simplest approach is to use the "async_receiver" gap detection feature.
The PC sends control bytes in burst, and when it stops sending, the FPGA detects it and assert an "RxD_gap" signal.
wire RxD_gap;
async_receiver async_rxd(.clk(clk), .RxD(RxD), .RxD_data_ready(RxD_data_ready), .RxD_data(RxD_data), .RxD_gap(RxD_gap));
reg [1:0] RxD_addr_reg;
always @(posedge clk) if(RxD_gap) RxD_addr_reg <= 0; else if(RxD_data_ready) RxD_addr_reg <= RxD_addr_reg + 1;
// register 0: TriggerThreshold
reg [7:0] TriggerThreshold;
always @(posedge clk) if(RxD_data_ready & (RxD_addr_reg==0)) TriggerThreshold <= RxD_data;
// register 1: "0 0 0 0 HDiv[3] HDiv[2] HDiv[1] HDiv[0]"
reg [3:0] HDiv;
always @(posedge clk) if(RxD_data_ready & (RxD_addr_reg==1)) HDiv <= RxD_data[3:0];
// register 2: "StartAcq TriggerPolarity 0 0 0 0 0 0"
reg TriggerPolarity;
always @(posedge clk) if(RxD_data_ready & (RxD_addr_reg==2)) TriggerPolarity <= RxD_data[6];
wire StartAcq = RxD_data_ready & (RxD_addr_reg==2) & RxD_data[7];
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We've also added a 4 bits register (HDiv[3:0]) to control the horizontal acquisition rate.
When we want to decrease the acquisition rate, either we discard samples coming from the ADC, or we filter/downsample them at the frequency we are interested in.
More and more features
As you can see, there are lots of features that can be added.
The interesting thing is that you can design the oscilloscope the way you need it - maybe a special trigger mechanism? a special filtering function?
Your turn to experiment.
This page was last updated on April 12 2005.