I2C Problems

Murmele — Thu, 05 Jul 2012 02:34:43 GMT

Hi,

I made a programm for the MC9S08AW32 to read the temperature of a LM92. I send the first byte and then i got the Acknowledge. Then I send the second byte but then i don't get the second acknowledge. Does somebody know why this doesn't work?

Re: I2C Problems

Ingo_Michael — Fri, 13 Jul 2012 12:26:48 GMT

Hello Murele!

I seen that you choosed a wrong SCL divider during your IIC initialisation.

Dec 64 seems to be not allowed as SCL divider. Please take a look in the Datasheet for

MC9S08AW60, MC9S08AW48, MC9S08AW32, MC9S08AW16 Rev 2 12/2006 on page 220 and 221.

Calculate the IIC Baudrate as follows

IIC baud rate = bus speed (Hz) / (mul x SCL divider)

Good Luck!

Re: I2C Problems

bigmac — Tue, 17 Jul 2012 19:15:30 GMT

Hello,

The baud rate divisor setting of 0xBF will set the divisor to 4*3840 = 15360. Since the minimum allowable clock rate for the LM92 is 1kHz, this setting is potentially problematic for a MCU bus frequency below 15.36 MHz.

I am not sure of the position in the sequence where you perceive that there is a problem. The IIC sequence to read a 16-bit register value should be as follows -

Set master transmit mode & generate START
Send LM92 address, write mode - error exit if no ACK from slave
Send register pointer value ( zero for temperature register) - error exit if no ACK from slave
Generate repeat START
Send LM92 address, read mode - error exit if no ACK from slave
Set master receive mode, with ACK returned to slave after next transfer
Discard garbage read value - start read transfer for MS data byte
Set master receive mode, with no ACK returned to slave after next transfer
Read MS byte of returned data and store - start read transfer for LS data byte
Generate STOP
Read LS byte of returned data and store

Your code is very difficult to read, but may be deficient in the above marked areas. If an ACK error is detected, a STOP should be generated so as to release the bus.

The following code snippet demonstrates how to improve code readability and efficiency. The original code, shown first, requires 18 bytes of code. This occurs in numerous places.

     lda   #ADRESS_SLAVE_WRITE
     sta   IIC1D
NoInterrupt_pending
     brset 1,IIC1S,Interrupt_pending
     jmp   NoInterrupt_pending
Interrupt_pending
     bset 1,IIC1S
NACK_4
     brclr 0,IIC1S,ACK_4
     jmp   NACK_4
ACK_4

This code could be directly replaced as follows, and would require smaller code (11 bytes). The previously used jump instructions are unnecessary. Maybe you were thinking of PIC branching limitations that do not apply here? With bit manipulation and bit test operations, the individual bits should always be named, to provide readability.

MOV #ADRESS_SLAVE_WRITE,IIC1D ; Start data transfer
BRCLR IIC1S_IICIF,IIC1S,* ; Wait until flag is set

BSET IIC1S_IICIF,IIC1S ; Clear flag
BRSET IIC1S_RXAK,IIC1S,* ; This does not provide an error exit

The last instruction above would then need to be altered to provide error exit -

BRSET IIC1S_RXAK,IIC1S,ERROR ; Error exit if no ACK from slave

Regards,

Mac

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