owDS1825 (OneWire.v)

This module implements the 1-wire bus operations for talking to the Maxim DS1825 serial-number/temperature chip. The 1-wire bus is a single-wire bi-directional bus standard originally authored by Dallas Semiconductor. It has a master/slave architecure, capable of handling multiple slave devices. This module implements a bus master for talking to a single slave device. In particular, it detects the device, can read the ROM data (serial number), and can read the scratch-pad memory of the DS1825, including temperature. It implements the ROM commands 0x33 (read ROM data) and 0xcc (skip ROM operation), and the function commands 0x44 (temperature conversion) and 0xbe (read scratch-pad memory). As per the DS1825 data sheet, no data are retrieved by the function data block following the function command 0x44 for temperature conversion: that block is skipped. Similarly, the 'ROM data' block is skipped following the ROM skip command (0xcc).

This module requires a 1-MHz clock and control signals timed for that clock. The done output is pulsed when the operation is finished, including waiting for the temperature conversion to finish.

Below is a plot of temperature readings over several hours.

Ports | | Notes | Example |

Ports

clk	input	The clock input.
reset	input	The reset input.
go	input	Pulse to initiate a bus cycle.
dq	in/out	The bidirectional 1-wire bus, configured to be pulled high when not driven (the high-impedance state).
romcmd	input	The eight-bit ROM command.
romdata	output	The 64-bit ROM data consisting of the 8-bit family code, and 48-bit serial number, and 8-bit CRC.
funcmd	input	The eight-bit function command.
fundata	output	The 72-bit function data consisting of the temperature and other data.
busy	output	Indicates that a bus cycle is in progress. At the time it is deasserted, either the `done` or `error` port is pulsed.
done	output	Indicates that a device was detected and the bus cycle was completed.
error	output	Indicates at the end of the reset/presence sequence that a device was not detected on the bus and the bus cycle was aborted. A pulse from the `error` port flags this event without a corresponding pulse from the `done` port.
	output

Parameters

There are currently no parameters.

Notes

The temperature resolution is fixed at 0.0625°C. Reduced resolution and shorter conversion times are available from the chip, but not implemented by this interface. The signed temperature scale is in degrees C with binary point four bits above the least-significant bit of the 12-bit temperature field in fundata. The duration of temperature conversion is about 0.65 s. The precision of the temperature reading is as good as the resolution.
Although other clock frequencies cannot be used by this version of the module, modifying the timing parameters of the instances of the blocks in the block diagram for values specific to a given clock frequency can probably provide this flexibility.
See also: 1-wire overview, the DS1825 chip, the owWrite module, the owRead module, and the owResetPresence module.

Example instantiation in Verilog

tri1			onewire;	// 1-wire bus
wire			owGo, owBusy, owDone, owError;
wire		[ 7:0]	owFamCode, owCRCr, owConfigreg, owCRCf, owFill8;
wire	signed	[11:0]	owTemperature;  // temperature
wire		[47:0]	owSerNum;	// serial number
wire		[15:0]	owAlarm;	// alarm bits
wire		[ 3:0]	owFill4;	// filler
wire		[23:0]	owFill24;	// filler
owDS1825 owdevice (
    .clk(owClk),		.reset(owReset),
    .dq(onewire),		.go(owGo),
    .romcmd(8'h33),	// read ROM command
    .romdata({owCRCr, owSerNum, owFamCode}),
    .funcmd(8'hbe),	// comvert command
    .fundata({owCRCf, owFill24, owConfigreg, owAlarm, owFill4, owTemperature}),
    .busy(owBusy),		.done(owDone),
    .error(owError)
);

2/3/2015