Re: I2C Single Master: peripheral or bit banging?

From	Bernd Linsel <bl1@gmx.com>
Newsgroups	comp.arch.embedded
Subject	Re: I2C Single Master: peripheral or bit banging?
Date	2020-11-20 11:38 +0100
Organization	news.netcologne.de
Message-ID	<rp86am$nr9$1@newsreader4.netcologne.de> (permalink)
References	<rp7vjk$edg$1@dont-email.me>

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On 20.11.2020 09:43, pozz wrote:
> I hate I2C for several reasons. It's only two-wires bus, but for this 
> reason it is insidious.
> 
> I usually use hw peripherals when they are available, because it's much 
> more efficient and smart and because it's the only possibility in many 
> cases.
> Actually we have MCUs with abundant UARTs, timers and so on, so there's 
> no real story: choose a suitable MCU and use that damn peripheral.
> So I usually start using I2C peripherals available in MCUs, but I found 
> many issues.
> 
> I have experience with AVR8 and SAMC21 by Atmel/Microchip. In both cases 
> the I2C peripheral is much more complex than UART or similar serial 
> lines. I2C Single Master, that is the most frequent situation, is very 
> simple, but I2C Multi Master introduces many critical situations.
> I2C peripherals usually promise to be compatible with multi-master, so 
> their internal state machine is somewhat complex... and often there's 
> some bug or situations that aren't expected that leave the code stucks 
> at some point.
> 
> I want to write reliable code that not only works most of the time, but 
> that works ALL the time, in any situations (ok, 99%). So my first test 
> with I2C is making a temporary short between SCL and SDA. In this case, 
> I2C in SAMC21 (they named it SERCOM in I2C Master mode) hangs forever. 
> The manual says to write ADDR register to start putting the address on 
> the bus and wait for an interrupt flag when it ends. This interrupt is 
> never fired up. I see the lines goes down (because START bit is putting 
> low SDA before SCL), but the INTFLAG bits stay cleared forever. Even 
> error bits in STATUS register (bus error, arbitration lost, any sort of 
> timeout...) stay cleared and the BUSSTATE is IDLE. As soon as the short 
> is removed, the state-machine goes on.
> 
> Maybe I'm wrong, so I studied Atmel Software Framework[1] and Arduino 
> Wire libraries[2]. In both cases, a timeout is implemented at the driver 
> level.
> 
> Even the datasheet says:
> 
>    "Note:  Violating the protocol may cause the I2C to hang. If this
>    happens it is possible to recover from this state by a
>    software reset (CTRLA.SWRST='1')."
> 
> I think the driver code should trust the hw, between them there's a 
> contract, otherwise it's impossibile. For a UART driver, you write DATA 
> register and wait an interrupt flag when a new data can be written in 
> the register. If the interrupt nevers fire, the driver hangs forever.
> But I have never seen a UART driver that uses a timeout to recover from 
> a hardware that could hang. And I used UARTs for many years now.
> 
> 
> Considering all these big issues when you want to write reliable code, 
> I'm considering to wipe again the old and good bit banging technique.
> For I2C Single Master scenario, it IS very simple: put data low/high 
> (three-state), put clock low/high. The only problem is to calibrate the 
> clock frequency, but if you a free timer it will be simple too.
> 
> What is the drawback of bit banging? Maybe you write a few additional 
> lines of code (you have to spit off 9 clock pulses by code), but I don't 
> think much more than using a peripheral and protect it with a timeout.
> But you earn a code that is fully under your control and you know when 
> the I2C transaction starts and you can be sure it will end, even when 
> there are some hw issues on the board.
> 
> 
> 
> 
> 
> [1] 
> https://github.com/avrxml/asf/blob/68cddb46ae5ebc24ef8287a8d4c61a6efa5e2848/sam0/drivers/sercom/i2c/i2c_sam0/i2c_master.c#L406 
> 
> 
> [2] 
> https://github.com/acicuc/ArduinoCore-samd/commit/64385453bb549b6d2f868658119259e605aca74d 
> 

1. The interrupt will only fire if a connected slave acknowledges the 
address. If you want to catch the situation of a non-acknowledged start 
& address byte, you have to set up a timer that times out.

2. I²C is asynchronous, you don't need to keep a fixed bit rate. Just 
pulse SCL as fast as you can/need (within the spec). Clients can adjust 
the speed pulling down SCL when they can't keep up with the master's speed.

3. As you not only have to bit-bang SCL & SDA according to the protocol, 
but also monitor SCL before you go on, even implementing a s/w I²C 
master correctly is not trivial; additionally, the CPU load is remarkable.

If you have difficulties using the I²C peripherals, just have a peek on 
the according linux driver sources. They are often very reliable (at 
least for chip families that are a bit mature), and if there exist any 
issues, they are documented (cf. Raspi's SPI driver bug in the first 
versions).

Regards
Bernd

Thread

I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-20 09:43 +0100
  Re: I2C Single Master: peripheral or bit banging? Bernd Linsel <bl1@gmx.com> - 2020-11-20 11:38 +0100
    Re: I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-20 12:45 +0100
      Re: I2C Single Master: peripheral or bit banging? Bernd Linsel <bl1@gmx.com> - 2020-11-20 15:57 +0100
  Re: I2C Single Master: peripheral or bit banging? Richard Damon <Richard@Damon-Family.org> - 2020-11-20 08:09 -0500
    Re: I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-20 15:15 +0100
      Re: I2C Single Master: peripheral or bit banging? Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-11-20 17:25 +0200
        Re: I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-20 17:33 +0100
          Re: I2C Single Master: peripheral or bit banging? Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-11-20 19:39 +0200
            Re: I2C Single Master: peripheral or bit banging? Dimiter_Popoff <dp@tgi-sci.com> - 2020-11-20 23:25 +0200
              Re: I2C Single Master: peripheral or bit banging? Grant Edwards <invalid@invalid.invalid> - 2020-11-20 23:09 +0000
                Re: I2C Single Master: peripheral or bit banging? Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-11-21 11:46 +0200
            Re: I2C Single Master: peripheral or bit banging? David Brown <david.brown@hesbynett.no> - 2020-11-21 12:06 +0100
              Re: I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-22 17:48 +0100
                Re: I2C Single Master: peripheral or bit banging? David Brown <david.brown@hesbynett.no> - 2020-11-22 19:44 +0100
                Re: I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-23 08:44 +0100
                Re: I2C Single Master: peripheral or bit banging? David Brown <david.brown@hesbynett.no> - 2020-11-23 09:42 +0100
                Re: I2C Single Master: peripheral or bit banging? David Brown <david.brown@hesbynett.no> - 2020-11-23 09:44 +0100
                Re: I2C Single Master: peripheral or bit banging? Richard Damon <Richard@Damon-Family.org> - 2020-11-23 07:25 -0500
                Re: I2C Single Master: peripheral or bit banging? Reinhardt Behm <rbehm@hushmail.com> - 2020-11-24 18:15 +0800
  Re: I2C Single Master: peripheral or bit banging? stephen@mpeforth.com (Stephen Pelc) - 2020-11-20 14:53 +0000
  Re: I2C Single Master: peripheral or bit banging? John Speth <johnspeth@yahoo.com> - 2020-11-20 06:56 -0800
  Re: I2C Single Master: peripheral or bit banging? Michael Kellett <mk@mkesc.co.uk> - 2020-11-21 09:10 +0000
    Re: I2C Single Master: peripheral or bit banging? Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2020-11-21 11:48 +0200
    Re: I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-22 17:35 +0100
      Re: I2C Single Master: peripheral or bit banging? Michael Kellett <mk@mkesc.co.uk> - 2020-11-22 20:21 +0000
  Re: I2C Single Master: peripheral or bit banging? Mike Perkins <spam@spam.com> - 2020-11-25 17:43 +0000
    Re: I2C Single Master: peripheral or bit banging? pozz <pozzugno@gmail.com> - 2020-11-26 08:54 +0100
      Re: I2C Single Master: peripheral or bit banging? Mike Perkins <spam@spam.com> - 2020-11-26 14:34 +0000

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