Add feature to skip id, when only one device on the bus available

[uzi18]

Please add also this common feature.

https://github.com/pstolarz/OneWireNg/blob/7cb5d9ae929b1b4ddbfca22fde840be349d9fcd3/src/drivers/DSTherm.cpp#L28-L51

[pstolarz]

Do you mean DSTherm::readScratchpad() variant like this one?

OneWireNg::ErrorCode readScratchpad(Scratchpad *scratchpad)
{
    OneWireNg::Id id;
    OneWireNg::ErrorCode ec;

    ec = _ow.readSingleId(id);
    if (ec == OneWireNg::EC_SUCCESS) {
        ec = readScratchpad(id, scratchpad);
    }
    return ec;
}

[uzi18]

@pstolarz exactly.

[pstolarz]

The routine has 1 performance flaw. It needs to retrieve id of the connected sensor before communication with the sensor starts. In case of single-sensor environment the retrieval is simply redundant. For performance reason It's better to retrieve the connected sensor id once by calling readSingleId() and next use it afterwards with readScratchpad(OneWireNg::Id&, Scratchpad*). This way the performance is improved by eliminating the unnecessary calls. For this reason I'd not recommend to use the proposed routine in single-sensors environments. Do you still see any value of the routine?

[pstolarz]

I probably found other (better) solution of the problem for single-sensor setups - it's possible to use SKIP ROM command (0xCC) instead of MATCH ROM (0x55) and this way eliminate sending the id at all. Of course, the solution will only work on single-sensor environment since on multi-sensors setups this will cause transmission interference and will result with CRC error.

[uzi18]

@pstolarz I see what You mean. In fact what I need is to simply issue commands like:

• RESET,CMD_SKIP_ROM , CMD_CONVERT_T (this is covered) and next after a while
• RESET,CMD_SKIP_ROM , CMD_READ_SCRATCHPAD (this not) So maybe for one sensor it could be possible to readScratchpad(Scratchpad*) with call to _ow.addressAll() instead _ow.readSingleId(id)

Something as simple as:

OneWireNg::ErrorCode readScratchpad(Scratchpad *scratchpad)
{
    OneWireNg::ErrorCode ec;

    ec = _ow.addressAll();
    if (ec == OneWireNg::EC_SUCCESS) {
                uint8_t cmd[1 + Scratchpad::LENGTH] = { 
             CMD_READ_SCRATCHPAD, 
             /* the read scratchpad will be placed here (9 bytes) */ 
             0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff 
         }; 

         _ow.touchBytes(cmd, sizeof(cmd)); 

         if (OneWireNg::crc8(&cmd[1], Scratchpad::LENGTH - 1) == 
             cmd[Scratchpad::LENGTH]) 
         { 
             new (scratchpad) Scratchpad(_ow, id, &cmd[1]); 
         } else 
             ec = OneWireNg::EC_CRC_ERROR; 
    }
    return ec;
}

What do you think?

[uzi18]

code duplication could be resolved by reuse of small private method

[pstolarz]

That's exactly what I meant with my previous comment. That's solution is fine, I'll implement this. Thanks for the concept.

[uzi18]

@pstolarz same for MAX31850 Idea is to enable simple solution for such case.

[pstolarz]

same for MAX31850

will be (almost) automatically by class inheritance.

I propose to name the new routine readScratchpadAll() to keep consistency with the existing functions: convertTempAll(), writeScratchpadAll(), copyScratchpadAll() etc. Beside this "All" postfix the routine is intended to be used only on single-sensor setups (which of course will be documented). What do you think?

[uzi18]

@pstolarz ok, I don't remember how it is implemented in silicon, but it was something like "first" device will respond.

Simple example should be also available for Arduino people.

For example this problem in DallasTemperature lib was resolved by sequence: RESET,CMD_SEARCH_ROM,read rom,RESET,CMD_MATCH_ROM , CMD_READ_SCRATCHPAD

[pstolarz]

For example this problem in DallasTemperature lib was resolved by sequence: RESET,CMD_SEARCH_ROM,read rom,RESET,CMD_MATCH_ROM , CMD_READ_SCRATCHPAD

This sequence simply means - search ids and read them one-by-one. There is no improvement as in our case.

[pstolarz]

I don't remember how it is implemented in silicon, but it was something like "first" device will respond

All devices will respond, which means you get garbage on the bus if more than one sensor is connected.

[uzi18]

For example this problem in DallasTemperature lib was resolved by sequence: RESET,CMD_SEARCH_ROM,read rom,RESET,CMD_MATCH_ROM , CMD_READ_SCRATCHPAD

This sequence simply means - search ids and read them one-by-one. There is no improvement as in our case.

They just use first found id, but this has also some overhead.

[pstolarz]

Last commit contains the enhancement in question. Unfortunately it was not possible to use the approach with SKIP ROM command, since the library need to know type of sensor to correctly interpret its scratchpad. For this reason its id is needed. But I found other mean to reduce unnecessary use of READ ROM command. Details in the new DSTherm::readScratchpadSingle() function in-line doc.