ADC

Model of the MonthWe can update you automatically when this page changes.To receive regular notification of updates to our Model of the Month section, click here.Analog-to-Digital ConverterOK. We’ve had a few requests for a Verilog Model of the Month, so here it is. This month we’ll present a model of an ADC. You might remember that we modelled an ADC in our April Model of the Month, so this allows us to contrast VHDL coding with Verilog coding.The example we present is for a 16-bit ADC, but you can easily modify the digital output wordlength for any desired accuracy of ADC.The 16-bit ADC model is built around a function, ADC_16b_10v_bipolar. Note there are no conversion operators in this code as there was with the VHDL code —Verilog simply doesn’t need them due to its absence of data typing.Note that we must specify a range for the returned value from the function in Verilog, in VHDL, a range constraint is not mandatory. Real-valued ports are not allowed in Verilog so we must use a 64-bit input port to maintain an accurate representation of the analogue signals.All of the required Verilog code is here, there are no references to other external code modules (there are eight referenced packages in the VHDL code). In the VHDL model there are frequent references to conversion functions, in the Verilog code we can use the built-in $rtoi function. The charge_ovr reg is intended to be accessed hierarchically from the testbench in order to determine whether reset_charge needs to be re-activated. A note for VHDL coders; hierarchical naming is allowed in Verilog —remember that in VHDL you always have to use ports and generics to allow hierarchy traversal.You are welcome to use the source code we provide but you must keep the copyright notice with the code (see the Acknowledgements page for more details).// 16-bit Analogue-Digital Converter//// +-----------------------------+// | Copyright 1996 DOULOS |// | designer : Tim Pagden |// | opened: 7 Jun 1996 |// +-----------------------------+`timescale 1 ns / 1 psmodule ADC_16bit (analog_in,digital_out);parameter conversion_time = 25.0, // conversion_time in ns// (see `timescale above)charge_limit = 1000000; // = 1 millioninput[63:0] analog_in;// double-precision representation of a real-valued input port; a fix that enabl output[15:0] digital_out;reg[15:0] delayed_digitized_signal;reg[15:0] old_analog,current_analog;reg[4:0] changed_bits;reg[19:0] charge;reg charge_ovr;reg reset_charge;/* SIGNALS:-analog_in = 64-bit representation of a real-valued signalanalog_signal = real valued signal recovered from analog_inanalog_limited = analog_signal, limited to the real-valued input range of the ADC digital_out = digitized 16bit 2's complement quantization of analog_limited*//* function to convert analog_in to digitized_2s_comp_signal.Takes analog_in values from (+10.0 v - 1LSB) to -10.0 v and convertsthem to values from +32767 to -32768 respectively */function[15:0] ADC_16b_10v_bipolar;parameter max_pos_digital_value = 32767,max_in_signal = 10.0;input[63:0] analog_in;reg[15:0] digitized_2s_comp_signal;real analog_signal,analog_abs,analog_limited;integer digitized_signal;beginanalog_signal = $bitstoreal (analog_in);if (analog_signal < 0.0)beginanalog_abs = -analog_signal;if (analog_abs > max_in_signal)analog_abs = max_in_signal;analog_limited = -analog_abs;endelsebeginanalog_abs = analog_signal;if (analog_abs > max_in_signal)analog_abs = max_in_signal;analog_limited = analog_abs;endif (analog_limited == max_in_signal)digitized_signal = max_pos_digital_value;elsedigitized_signal = $rtoi (analog_limited * 3276.8);if (digitized_signal < 0)digitized_2s_comp_signal = 65536 - digitized_signal;elsedigitized_2s_comp_signal = digitized_signal;ADC_16b_10v_bipolar = digitized_2s_comp_signal;endendfunction/* This function determines the number of digital bit changes fromsample to sample; can be used to determine power consumption if required.Task power_determine not yet implemented */function[4:0] bit_changes;input[15:0] old_analog,current_analog;reg[4:0] bits_different;integer i;beginbits_different = 0;for (i=0;i<=15;i=i+1)if (current_analog[i] != old_analog[i])bits_different = bits_different + 1;bit_changes = bits_different;endendfunction/* Block to allow power consumption to be measured (kind of). Reset_charge is used always @ (posedge reset_charge)begincharge = 0;charge_ovr = 0;end/* This block only triggered when analog_in changes by an amount greater than 1LSB always @ (ADC_16b_10v_bipolar (analog_in))begincurrent_analog = ADC_16b_10v_bipolar (analog_in); // digitized_signalchanged_bits = bit_changes (old_analog,current_analog);old_analog = current_analog;charge = charge + (changed_bits * 3);if (charge > charge_limit)charge_ovr = 1;end/* Block to implement conversion_time tpd; always block use to showdifference between block and assign coding style */always# conversion_time delayed_digitized_signal = ADC_16b_10v_bipolar (analog_in); assign digital_out = delayed_digitized_signal;endmoduleTo download the HDL source code for this month'sModel of the Month, click here.VHDL and Verilog source code for other Models of the Monthcan be downloaded from here.Verilog for ASIC DesignASIC Design and Project ServicesOther Doulos Training CoursesDoulos Home PageCopyright 1995-1996 DoulosThis page was last updated 10th June 1996.We welcome your e-mail comments. 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