AVR Freaks

 I cant tell if the master is driving the clock or if the slave is driving the clock for those 8 additional cycles.

If you really want to know, you can install some small series resistors for the slave chip...so when it pulls the I2C lines low, it won't get quite to solid gnd compared to master.   Don't go overboard with too much---you don't want to create an erroneous voltage level, 100mv or so is fine!!  Some chips may offer enough difference by themselves to discern without resistors, but you have to have an eagle eye.

Note also, this is for scoping the lines...a logic analyzer (at least not a $10k one), will just show 0/1.

it's handling the failure cases that accounts for the majority of the development work!

That leads to the we saw it was working 2 months ago at the concept demo, what's all the engineering "delay"??...well it was "working" perfectly fine & quite usable under ideal conditions....however:

it really should verify that the file was written correctly.  

it really should check that the desired file name is legit & request a re-entry

it really should warn if an exiting file will be overwritten

it really should abort & warn (rather than stall) if the file cannot be opened

it really should apply some error detection/CRC to the contents

etc

Same thing not just for files, but most anything; reading a keypad & entering a bad value, or the keypad becomes shorted, gets stuck keys, etc.

Yes, you saw the motor spinning a month ago, but there was not yet overtemperature shutdown, torque limiting to prevent gear stripping, backup limits for speed sensor failure, etc

So you can get "the basics" operating quickly, but that is often the tip of the iceberg.  It's not even that these things haven't been thought about, they simply haven't yet been put in place to get a demo running asap.

void functions have no return so no opportunity to let the caller know if they worked or not. In a function that is trying to communicate to an external device that may not even be there or that may be in some "error" condition that means it cannot respond correctly it's good to be a able to return an error code to highlight the problem. Otherwise the calling code is just going to do this(), that(), the_other() without realising that this() did not succeed so everything after that was pointless.

 

In Fleury for example you have:

D:\fleury_i2c>grep i2c_.*( i2cmaster.h | grep extern
extern void i2c_init(void);
extern void i2c_stop(void);
extern unsigned char i2c_start(unsigned char addr);
extern unsigned char i2c_rep_start(unsigned char addr);
extern void i2c_start_wait(unsigned char addr);
extern unsigned char i2c_write(unsigned char data);
extern unsigned char i2c_readAck(void);
extern unsigned char i2c_readNak(void);
extern unsigned char i2c_read(unsigned char ack);

Things like i2c_start() have an unsigned char return and can return for example:

 @retval   0   device accessible 
 @retval   1   failed to access device 
 */
extern unsigned char i2c_start(unsigned char addr);

Having cited Fleury as a good example. He then presents:

 int main(void)
 {
     unsigned char ret;

     i2c_init();                             // initialize I2C library

     // write 0x75 to EEPROM address 5 (Byte Write) 
     i2c_start_wait(Dev24C02+I2C_WRITE);     // set device address and write mode
     i2c_write(0x05);                        // write address = 5
     i2c_write(0x75);                        // write value 0x75 to EEPROM
     i2c_stop();                             // set stop conditon = release bus


     // read previously written value back from EEPROM address 5 
     i2c_start_wait(Dev24C02+I2C_WRITE);     // set device address and write mode

     i2c_write(0x05);                        // write address = 5
     i2c_rep_start(Dev24C02+I2C_READ);       // set device address and read mode

     ret = i2c_readNak();                    // read one byte from EEPROM
     i2c_stop();

     for(;;);
 }

as an example that completely ignores any return values that the functions may be giving - this is NOT a good example !!

My eyes are open now. 👀

 

Thanks clawson, awneil, understood everything.