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[/] [zipcpu/] [trunk/] [rtl/] [peripherals/] [zipjiffies.v] - Rev 128
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//////////////////////////////////////////////////////////////////////////////// // // Filename: zipjiffies.v // // Project: Zip CPU -- a small, lightweight, RISC CPU soft core // // Purpose: This peripheral is motivated by the Linux use of 'jiffies'. // A process, in Linux, can request to be put to sleep until a certain // number of 'jiffies' have elapsed. Using this interface, the CPU can // read the number of 'jiffies' from this peripheral (it only has the // one location in address space), add the sleep length to it, and // write the result back to the peripheral. The zipjiffies peripheral // will record the value written to it only if it is nearer the current // counter value than the last current waiting interrupt time. If no // other interrupts are waiting, and this time is in the future, it will // be enabled. (There is currrently no way to disable a jiffie interrupt // once set.) The processor may then place this sleep request into a // list among other sleep requests. Once the timer expires, it would // write the next jiffy request to the peripheral and wake up the process // whose timer had expired. // // Quite elementary, really. // // Interface: // This peripheral contains one register: a counter. Reads from the // register return the current value of the counter. Writes within // the (N-1) bit space following the current time set an interrupt. // Writes of values that occurred in the last 2^(N-1) ticks will be // ignored. The timer then interrupts when it's value equals that time. // Multiple writes cause the jiffies timer to select the nearest possible // interrupt. Upon an interrupt, the next interrupt time/value is cleared // and will need to be reset if the CPU wants to get notified again. With // only the single interface, there is no way of knowing when the next // interrupt is scheduled for, neither is there any way to slow down the // interrupt timer in case you don't want it overflowing as often and you // wish to wait more jiffies than it supports. Thus, currently, if you // have a timer you wish to wait upon that is more than 2^31 into the // future, you would need to set timers along the way, wake up on those // timers, and set further timer's until you finally get to your // destination. // // // Creator: Dan Gisselquist, Ph.D. // Gisselquist Technology, LLC // //////////////////////////////////////////////////////////////////////////////// // // Copyright (C) 2015, Gisselquist Technology, LLC // // This program is free software (firmware): you can redistribute it and/or // modify it under the terms of the GNU General Public License as published // by the Free Software Foundation, either version 3 of the License, or (at // your option) any later version. // // This program is distributed in the hope that it will be useful, but WITHOUT // ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or // FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License // for more details. // // License: GPL, v3, as defined and found on www.gnu.org, // http://www.gnu.org/licenses/gpl.html // // //////////////////////////////////////////////////////////////////////////////// // module zipjiffies(i_clk, i_ce, i_wb_cyc, i_wb_stb, i_wb_we, i_wb_data, o_wb_ack, o_wb_stall, o_wb_data, o_int); parameter BW = 32, VW = (BW-2); input i_clk, i_ce; // Wishbone inputs input i_wb_cyc, i_wb_stb, i_wb_we; input [(BW-1):0] i_wb_data; // Wishbone outputs output reg o_wb_ack; output wire o_wb_stall; output wire [(BW-1):0] o_wb_data; // Interrupt line output reg o_int; // // Our counter logic: The counter is always counting up--it cannot // be stopped or altered. It's really quite simple. Okay, not quite. // We still support the clock enable line. We do this in order to // support debugging, so that if we get everything running inside a // debugger, the timer's all slow down so that everything can be stepped // together, one clock at a time. // reg [(BW-1):0] r_counter; always @(posedge i_clk) if (i_ce) r_counter <= r_counter+1; // // Writes to the counter set an interrupt--but only if they are in the // future as determined by the signed result of an unsigned subtract. // reg int_set, new_set; reg [(BW-1):0] int_when, new_when; wire signed [(BW-1):0] till_when, till_wb; assign till_when = int_when-r_counter; assign till_wb = new_when-r_counter; initial new_set = 1'b0; always @(posedge i_clk) // Delay things by a clock to simplify our logic if ((i_wb_cyc)&&(i_wb_stb)&&(i_wb_we)) begin new_set <= 1'b1; new_when<= i_wb_data; end else new_set <= 1'b0; initial o_int = 1'b0; initial int_set = 1'b0; always @(posedge i_clk) begin o_int <= 1'b0; if ((i_ce)&&(int_set)&&(r_counter == int_when)) begin // Interrupts are self-clearing o_int <= 1'b1; // Set the interrupt flag int_set <= 1'b0;// Clear the interrupt end if ((new_set)&&(till_wb > 0)&&((till_wb<till_when)||(~int_set))) begin int_when <= new_when; int_set <= ((int_set)||(till_wb>0)); end end // // Acknowledge any wishbone accesses -- everything we did took only // one clock anyway. // always @(posedge i_clk) o_wb_ack <= (i_wb_cyc)&&(i_wb_stb); assign o_wb_data = r_counter; assign o_wb_stall = 1'b0; endmodule
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