Hbusout
Syntax
Hbusout Control, { Address }, [ Variable {, Variable…} ]
or
Hbusout Variable
Overview
Transmit a value to the I2C bus using the microcontroller's on-board MSSP1 module, by first sending the control and optional address out of the clock pin (SCL), and data pin (SDA). Or alternatively, if only one operator is included after the Hbusout command, a single value will be transmitted, along with an Ack reception.
Parameters
Variable is a user defined variable or constant. Control may be a constant value or a Byte sized variable expression. Address may be a constant, variable, or expression.
The Hbusout command operates as an I2C master and may be used to interface with any device that complies with the 2-wire I2C protocol.
The most significant 7-bits of control byte contain the control code and the slave address of the device being interfaced with. Bit-0 is the flag that indicates whether a read or write command is being implemented.
For example, if we were interfacing to an external EEPROM such as the 24LC32, the control code would be %10100000 or $A0. The most significant 4-bits (1010) are the EEPROM's unique slave address. Bits 2 to 3 reflect the three address pins of the EEPROM. And Bit-0 is clear to signify that we wish to write to the EEPROM. Note that this bit is automatically cleared by the Hbusout command, regardless of its initial value.
Example
' Send a byte to the I2C bus.
Dim MyByte as Byte ' We'll only read 8-bits
Dim Address as Word ' 16-bit address required
Symbol Control = %10100000 ' Target an EEPROM
Address = 20 ' Write to address 20
MyByte = 200 ' The value place into address 20
Hbusout Control, Address, [MyByte] ' Send the byte to the EEPROM
DelayMs 10 ' Allow time for allocation of byteAddress, is an optional parameter that may be an 8-bit or 16-bit value. If a variable is used in this position, the size of address is dictated by the size of the variable used (Byte or Word). In the case of the above EEPROM interfacing, the 24LC32 EEPROM requires a 16-bit address. While the smaller types require an 8-bit address. Make sure you assign the right size address for the device interfaced with, or you may not achieve the results you intended.
The value sent to the bus depends on the size of the variables used. For example: -
Dim MyWord as Word ' Create a Word size variable
Hbusout Control, Address, [MyWord]Will send a 16-bit value to the bus. While: -
Dim MyByte as Byte ' Create a Byte size variable
Hbusout Control, Address, [MyByte]Will send an 8-bit value to the bus.
Using more than one variable within the brackets allows differing variable sizes to be sent. For example: -
Dim MyByte as Byte
Dim MyWord as Word
Hbusout Control, Address, [MyByte, MyWord]Will send two values to the bus, the first being an 8-bit value dictated by the size of variable Var1 which has been declared as a byte. And a 16-bit value, this time dictated by the size of the variable MyWord which has been declared as a word. Of course, Bit type variables may also be used, but in most cases these are not of any practical use as they still take up a byte within the EEPROM.
A string of characters can also be transmitted, by enclosing them in quotes: -
Hbusout Control, Address, ["Hello World", MyByte, MyWord]Using the second variation of the Hbusout command, necessitates using the low level commands i.e. HbStart, HbRestart, HbusAck, or HbStop.
Using the Hbusout command with only one value after it, sends a byte of data to the I2C bus, and returns holding the Acknowledge reception. This acknowledge indicates whether the data has been received by the slave device.
The Ack reception is returned in the microcontroller's CARRY flag, which is SR.0, and also System variable PP4.0. A value of zero indicates that the data was received correctly, while a one indicates that the data was not received, or that the slave device has sent a NAck return. You must read and understand the datasheet for the device being interfacing to, before the Ack return can be used successfully. An code snippet is shown below: -
' Transmit a byte to a 24LC32 serial EEPROM
HbStart ' Send a Start condition
Hbusout %10100000 ' Target an EEPROM, and send a Write command
Hbusout 0 ' Send the HighByte of the address
Hbusout 0 ' Send the LowByte of the address
Hbusout "A" ' Send the value 65 to the bus
If SRbits_C = 1 Then GoTo Not_Received ' Has Ack been received OK?
HbStop ' Send a Stop condition
DelayMs 10 ' Wait for the data to be entered into EEPROM matrix
Hbusout Declares
Declare HSDA_Pin Port.Pin Declares the port and pin used for the MSSP1 module’s data line (SDA). If the declare is not used in the program, it may not always default to the standard pin configuration, so make sure they are in the BASIC listing.
Declare HSCL_Pin Port.Pin Declares the port and pin used for the MSSP1 module’s clock line (SCL). If the declare is not used in the program, it may not always default to the standard pin configuration, so make sure they are in the BASIC listing.
Declare Hbus_Bitrate Constant 100, 400, 1000 etc. The standard speed for the I2C bus is 100KHz. Some devices use a higher bus speed of 400KHz. The above Declare allows the I2C bus speed to be increased or decreased. Use this Declare with caution, as too high a bit rate may exceed the device's specs, which will result in intermittent transactions, or in some cases, no transactions at all. The datasheet for the device used will inform you of its bus speed. The default bit rate is the standard 100KHz.
Notes.
When the Hbusout command is used, the appropriate SDA and SCL Port and Pin are automatically setup as inputs if they have been previously declared. Because the I2C protocol calls for an open-collector interface, pull-up resistors are required on both the SDA and SCL lines. Values of 1KΩ to 4.7KΩ will suffice.
