I'm trying to figure out how to program the baud rate for my I2C bus on the HCS12. The S12IICV2 document describes two equations that deal with tap points and what not, but I have no idea what a tap point means. Any help would be appreciated. My micro will have a 25MHz square wave input clock, which I think translates to a 12.5MHz bus clock. With those numbers, how can I compute the necessary values to program the IBFD register for 100Kbp/s and 400Kbp/s?? Thanks. 

I2C (IIC) and HCS12
Started by ●August 5, 2002
Posted by ●August 5, 2002
Here is a 24Mhz setup, change it for yours (25Mhz). ;Initialize IIC  ;100kbps ;IBC7 IBC6 IBC5 IBC4 IBC3 IBC2 IBC1 IBC0 ; 0 1 ;MUL ; 0 1 1 ;SCL_Tap=8 SDA_Tap=2 ; 0 1 1 ;scl2tap=6 tap2tap=8 ;SCL Divider = MUL x {2 x (scl2tap + [(SCL_Tap  1) x tap2tap] + 2)} ; 2*{2*(6+[(81)*8]+2)}%6 ;24MHz / 256 = 93.75kHz ; ;SDA Hold = MUL x {scl2tap + [(SDA_Tap  1) x tap2tap] + 3} ; 2*{2*(6+[(21)*8]+2)}d ;24MHz / 64 = 375kHz ; movb #%01011011,IBFD movb #%10111111,IBFD movb #%10000000,IBCR > I'm trying to figure out how to program the baud rate for my I2C bus > on the HCS12. The S12IICV2 document describes two equations that > deal with tap points and what not, but I have no idea what a tap > point means. Any help would be appreciated. > > My micro will have a 25MHz square wave input clock, which I think > translates to a 12.5MHz bus clock. With those numbers, how can I > compute the necessary values to program the IBFD register for > 100Kbp/s and 400Kbp/s?? > > Thanks. 

Posted by ●August 6, 2002
So the 93.75KHz value you have is the clock for the I2C? And since the standard rate is 100Kbps, should you try to get that value as close to 100K as possible? Also, does any of this really matter if all I'm going to be using the MCU for is as a slave?  In 68HC12@y..., "zeta_alpha2002" <zeta_alpha2002@y...> wrote: > Here is a 24Mhz setup, change it for yours (25Mhz). > > ;Initialize IIC  >  > ;100kbps > ;IBC7 IBC6 IBC5 IBC4 IBC3 IBC2 IBC1 IBC0 > ; 0 1 ;MUL > ; 0 1 1 ;SCL_Tap=8 > SDA_Tap=2 > ; 0 1 1 ;scl2tap=6 > tap2tap=8 > > ;SCL Divider = MUL x {2 x (scl2tap + [(SCL_Tap  1) x tap2tap] + 2)} > ; 2*{2*(6+[(81)*8]+2)}%6 > ;24MHz / 256 = 93.75kHz > ; > ;SDA Hold = MUL x {scl2tap + [(SDA_Tap  1) x tap2tap] + 3} > ; 2*{2*(6+[(21)*8]+2)}d > ;24MHz / 64 = 375kHz > ; movb #%01011011,IBFD > movb #%10111111,IBFD > movb #%10000000,IBCR > > I'm trying to figure out how to program the baud rate for my I2C > bus > > on the HCS12. The S12IICV2 document describes two equations that > > deal with tap points and what not, but I have no idea what a tap > > point means. Any help would be appreciated. > > > > My micro will have a 25MHz square wave input clock, which I think > > translates to a 12.5MHz bus clock. With those numbers, how can I > > compute the necessary values to program the IBFD register for > > 100Kbp/s and 400Kbp/s?? > > > > Thanks. 

Posted by ●August 6, 2002
> So the 93.75KHz value you have is the clock for the I2C? Yes > the standard rate is 100Kbps, should you try to get that value as > close to 100K as possible? No  it does not have to be precisely/close as to 100K. Some older devices like EEPROM will only work upto 100K. Newer devices from 100K to 1M. It is device dependent on what is the max clock speed on the target device. > Also, does any of this really matter if > all I'm going to be using the MCU for is as a slave? Who is the master? You have to look how much can HS12 be clocked at max speed then clock speed to that. Haven't looked at that because in our apps we are only talking to RTC and EEPROM. > > >  In 68HC12@y..., "zeta_alpha2002" <zeta_alpha2002@y...> wrote: > > Here is a 24Mhz setup, change it for yours (25Mhz). > > > > ;Initialize IIC  >  > >  > > ;100kbps > > ;IBC7 IBC6 IBC5 IBC4 IBC3 IBC2 IBC1 IBC0 > > ; 0 1 ;MUL > > ; 0 1 1 ;SCL_Tap=8 > > SDA_Tap=2 > > ; 0 1 1 ;scl2tap=6 > > tap2tap=8 > > > > ;SCL Divider = MUL x {2 x (scl2tap + [(SCL_Tap  1) x tap2tap] + 2)} > > ; 2*{2*(6+[(81)*8]+2)}%6 > > ;24MHz / 256 = 93.75kHz > > ; > > ;SDA Hold = MUL x {scl2tap + [(SDA_Tap  1) x tap2tap] + 3} > > ; 2*{2*(6+[(21)*8]+2)}d > > ;24MHz / 64 = 375kHz > > > > > > ; movb #%01011011,IBFD > > movb #%10111111,IBFD > > movb #%10000000,IBCR > > > > > > > I'm trying to figure out how to program the baud rate for my I2C > > bus > > > on the HCS12. The S12IICV2 document describes two equations that > > > deal with tap points and what not, but I have no idea what a tap > > > point means. Any help would be appreciated. > > > > > > My micro will have a 25MHz square wave input clock, which I think > > > translates to a 12.5MHz bus clock. With those numbers, how can I > > > compute the necessary values to program the IBFD register for > > > 100Kbp/s and 400Kbp/s?? > > > > > > Thanks. 