Action
Configure the TWI Slave address and bit rate
Syntax
CONFIG TWISLAVE = address , BTR = value , BITRATE = value , SAVE=option [,GENCALL=value]
Remarks
Address |
The slave address that is assigned to the slave chip. This must be an Even number. The address 0 is the general call address and may not be used. While a slave address is 7 bit since bit 0 is used to indicate read/write, BASCOM uses byte notation where you can ignore the last bit. The last bit will be set by BASCOM automatically. |
BTR |
Bytes to receive. With this constant you specify how many bytes will be expected when the slave receives bytes. |
Bit rate |
This is the I2C/TWI clock frequency. Most chips support 400 KHz (400000) but all I2C chips support 100000. |
SAVE |
SAVE = NOSAVE : this can be used when you do not change a lot of registers in the interrupt. SAVE : SAVE : this is best to be used when you do not use ASM in the TWI interrupt. See the explanation below. |
GENCALL |
General call address activated or not. When you specify 1 or YES, the General call address will be activated which mean that the slave will respond not only to it's own address, but also to the general call address 0. When you omit the option or specify 0 or NO, the general call address will not be honored. |
The variables Twi , Twi_btr and Twi_btw are created by the compiler. These are all bytes
The TWI interrupt is enabled but you need to enabled the global interrupt
The TWI Slave code is running as an interrupt process. Each time there is a TWI interrupt some slave code is executed. Your BASIC code is called from the low level slave code under a number of events. You must include all these labels in your Slave application. You do not need to write code in all these sub routines.
Label |
Event |
Twi_stop_rstart_received |
The Master sent a stop(i2CSTOP) or repeated start. Typical you do not need to do anything here. |
Twi_addressed_goread |
The master has addressed the slave and will now continue to send data to the slave. You do not need to take action here. |
Twi_addressed_gowrite |
The master has addressed the slave and will now continue to receive data from the slave. You do not need to take action here. |
Twi_gotdata |
The master has sent data. The variable TWI holds the received value. The byte TWI_BTW is an index that holds the value of the number of received bytes. The first received byte will have an index value of 1. |
Twi_master_needs_byte |
The master reads from the slave and needs a value. The variable TWI_BTR can be inspected to see which index byte was needed. With the CONFIG BTR, you specify how many bytes the master will read. |
The TWI Slave code will save all used registers. But as it will call your BASIC application as the TWI interrupt occurs, your BASIC code could be in the middle of a PRINT statements.
When you then execute another PRINT statement , you will destroy registers.
So keep the code in the sub routines to a minimum, and use SAVE option to save all registers.
While two printing commands will give odd results (print 12345 and 456 in the middle of the first print will give 1234545) at least no register is destroyed.
A typical configuration is shown below.
To test the above hardware, use the samples : twi-master.bas and twi-slave.bas
Optional you can use i2cscan.bas to test the general call address.
See also
ASM
NONE
Example1(master)
'-------------------------------------------------------------------------------
' (c) 2004 MCS Electronics
' This demo shows an example of the TWI
' Not all AVR chips have TWI (hardware I2C)
'-------------------------------------------------------------------------------
'The chip will work in TWI/I2C master mode
'Connected is a PCF8574A 8-bits port extender
$regfile = "M88def.dat" ' the used chip
$crystal = 8000000 ' frequency used
$baud = 19200 ' baud rate
$lib "i2c_twi.lbx" ' we do not use software emulated I2C but the TWI
Config Scl = Portc.5 ' we need to provide the SCL pin name
Config Sda = Portc.4 ' we need to provide the SDA pin name
'On the Mega88, On the PCF8574A
'scl=PC5 , pin 28 pin 14
'sda=PC4 , pin 27 pin 15
I2cinit ' we need to set the pins in the proper state
Config Twi = 100000 ' wanted clock frequency
'will set TWBR and TWSR
'Twbr = 12 'bit rate register
'Twsr = 0 'pre scaler bits
Dim B As Byte , X As Byte
Print "TWI master"
Do
Incr B ' increase value
I2csend &H0 , B ' send the value to general call address
I2csend &H70 , B ' send the value
Print "Error : " ; Err ' show error status
I2creceive &H70 , X ' get a byte
Print X ; " " ; Err ' show error
Waitms 500 'wait a bit
Loop
End
Example2(slave)
'-------------------------------------------------------------------------------
' (c) 2004 MCS Electronics
' This demo shows an example of the TWI in SLAVE mode
' Not all AVR chips have TWI (hardware I2C)
' IMPORTANT : this example ONLY works when you have the TWI slave library
' which is a commercial add on library, not part of BASCOM
'Use this sample in combination with i2cscan.bas and/or twi-master.bas
'-------------------------------------------------------------------------------
$regfile = "M88def.dat" ' the chip we use
$crystal = 8000000 ' crystal oscillator value
$baud = 19200 ' baud rate
Print "MCS Electronics TWI-slave demo"
Config Twislave = &H70 , Btr = 1 , Bitrate = 100000 , Gencall = 1
'In i2c the address has 7 bits. The LS bit is used to indicate read or write
'When the bit is 0, it means a write and a 1 means a read
'When you address a slave with the master in bascom, the LS bit will be set/reset automatic.
'The TWAR register in the AVR is 8 bit with the slave address also in the most left 7 bits
'This means that when you setup the slave address as &H70, TWAR will be set to &H0111_0000
'And in the master you address the slave with address &H70 too.
'The AVR TWI can also recognize the general call address 0. You need to either set bit 0 for example
'by using &H71 as a slave address, or by using GENCALL=1
'as you might need other interrupts as well, you need to enable them all manual
Enable Interrupts
'this is just an empty loop but you could perform other tasks there
Do
nop
Loop
End
'A master can send or receive bytes.
'A master protocol can also send some bytes, then receive some bytes
'The master and slave must match.
'the following labels are called from the library
Twi_stop_rstart_received:
Print "Master sent stop or repeated start"
Return
Twi_addressed_goread:
Print "We were addressed and master will send data"
Return
Twi_addressed_gowrite:
Print "We were addressed and master will read data"
Return
'this label is called when the master sends data and the slave has received the byte
'the variable TWI holds the received value
Twi_gotdata:
Print "received : " ; Twi
Return
'this label is called when the master receives data and needs a byte
'the variable twi_btr is a byte variable that holds the index of the needed byte
'so when sending multiple bytes from an array, twi_btr can be used for the index
Twi_master_needs_byte:
Print "Master needs byte : " ; Twi_btr
Twi = 65 ' twi must be filled with a value
Return
'when the mast has all bytes received this label will be called
Twi_master_need_nomore_byte:
Print "Master does not need anymore bytes"
Return