Thanks. Yes, that's a pain.

Are there any other patterns apart from write then read? I guess it might end up being easier to just have a new function that did just that.

Or perhaps:

I2C.writeTo({address:0x55, stop:false}, data);
I2C.readFrom({address:0x55, stop:false}, count);

I don't know of any other patterns, but it's possible I guess. Maybe keep writeTo and readFrom as-is for a higher-level capability where it doesn't matter, but also build beginTransmission,write,requestFrom,read and endTransmission to match Arduino when greater control is needed. A bit like fopen/fread/fclose vs. open/read/close in C.

It would be handy to have an I2C.readRegister(address, register, count=1) convenience function, although I don't know how widespread Repeated Start is. I've only programmed a few I2C devices, and this is the first one I've encountered that needs Repeated Start. I don't know whether other devices allow it, prefer it or prohibit it. Apparently it is an official thing, though: http://www.i2c-bus.org/repeated-start-condition/

I'm sort of against re-implementing Arduino's functions, because they tend to complicate things and provide more opportunity for error. Also a silicon problem on the STM32F1 means that you can totally lock up the I2C peripheral if you send a STOP at the wrong time, so it's good not to let people have that choice :)

I don't think that allowing an object instead of a numeric address would cause any backwards compatibility problems though - at least not unless people were doing something really nasty.

Just tried to fix this... it should work like:

I2C.writeTo({address:0x55, stop:false}, data);
I2C.readFrom({address:0x55, stop:false}, count);

I don't have anything to test with though, so please let me know how this works.

A build should be up soon at: http://www.espruino.com/binaries/git/commits/a939ef6285ca9c6f639d4edaaf86fe7606176829

Looks like this may not work - but I'll have to get some hardware to test with first.

@gfwilliams : unfortunately I'm not going to have a chance to test this for a while, thanks to real life. However, I'd be happy to buy you an MPL3115A2 if you want to have a go with it... if so, email or DM me with postal address.

@gfwilliams I just tested this with 1v70:

    var _read_reg = function (_i2c, _reg) {
        _i2c.writeTo({address:MPL3115A2_ADDRESS /* 0x60 */, stop:false}, _reg);
        var bs = _i2c.readFrom(MPL3115A2_ADDRESS, 1);
        return bs[0];
    };

    var whoami = _read_reg(_i2c, MPL3115A2_WHO_AM_I /* 0x0C */);
    console.log("WHOAMI (should return 196 apparently): "+whoami);

returns:

WHOAMI: undefined
Uncaught InternalError: Timeout on I2C Read BUSY

I've been using https://github.com/sparkfun/MPL3115A2_Breakout/blob/master/firmware/MPL3115A2/MPL3115A2.ino#L355 as a reference.

I'm currently using BMP180 instead as it's cheaper, more accurate, and simpler. However the MPL3115A2 does have on-board altitude calculation (lower memory footprint?) and has interrupt lines (ultra low power capability). The I2C Repeated Start is, however, probably worth fixing for other devices I suppose.

Hi - thanks for testing it out (and for the offer of a sensor)... I've actually just bought one though - when it arrives I'll see if I can fix the repeated start issue. It should only be a few lines I hope!

Incidentally, while hacking the libraries on nrf51, I noticed that the MPU6050 code write/read routine for that SDK uses Repeated Start too. I believe it's optional on that sensor.

I'm trying to get an MMA8451 (http://www.adafruit.com/datasheets/Freescale_MMA8451QR1.pdf) working and it doesn't seem like the repeated start is working properly?
I get

 Uncaught InternalError: Timeout on I2C Read BUSY

when I try to read after writing an address with stop set to false. Reading gives me the same 7 bytes every time.

Relevant Arduino code:
https://github.com/adafruit/Adafruit_MMA8451_Library/blob/master/Adafruit_MMA8451.cpp

Ok, thanks. Will try and look into it when I get back to the UK.

What board was this on? The I2C implementations are different on different chips

What board were you using? a Pico?

yup, using a pico

Fixed. On the MPL3115, the following code now works:

function r(addr) {
  i2c.writeTo({address:0x60, stop:false}, addr);
  return i2c.readFrom(0x60,1)[0];
}

var i2c = I2C3;
i2c.setup({sda:B4,scl:A8});
r(0x0c)==196; // WHOAMI

Just came back to this project, using 1v85 and I cannot get it to connect on I2C1. On I2C3 (on a different board) it works fine. Unfortunately for my current project i'm using those pins for SPI so a way to get this working with I2C1 would be super helpful. Same Uncaught InternalError: Timeout on I2C Write BUSY error as before. Using the code snippet from above but different (obviously) addresses.

You sure it's not that you're missing pullup resistors or having SDA/SCL mixed? The code is identical for I2C1/2 and 3 - so the chances of it working on one and not the other are pretty minimal.