Digital to Analogue Conversion:

the mcp4275

Lecture 10

Uli Raich

UCC semester 2017/2018

• on/off for the LEDs
• on/off to read the LED state
• Powering or not powering coils to generate magnetic fields in a stepping motor

• Temperatures are changing continuously and not in steps
• Pressure is an analogue value
• Distance, time, current, resistance … are all analogue values

• Convert digital values to analogue voltage levels
  Digital to Analogue Conversion (DAC)
  
  
• … and we must convert external analogue values to digital
  Analogue to Digital Conversion (ADC)

We need a low-pass filter, which filters out high frequencies

(the abrupt steps we have in the output signal) and lets pass only slower transitions.

When looking carefully at the output of our sine generator you will

also see these steps. In this case however they come from the

limited number of sine values (100) we calculate. To improve the resolution

we would have to increase the number of samples and the frequency

with which we send these values to the DAC.

If you want to know more about DAC technology (and you should!)

have a look at this excellent tutorial, from which I have copied the above illustration.

data sheet

and here a photo of the device

Adding Vcc and Gnd we need a mere 4 wires to connect

a I2C device to the Raspberry Pi cobbler

The I2C bus was invented by Philips

but you cannot set it to a high level.

If nobody pulls the line down, then it is at Vcc level,

pulled up by a pull-up register.

Like this the contention problem is solved where one device tries

to set a line to a high level, while another sets it to low,

thus creating a short circuit.

Raspberry Pi’s ARM processor) and several slaves

We have the following I2C slave devices:

• mcp4275 DAC
  
  
• bmp180 barometric pressure sensor
  
  
• pcf8581 8 bit ADC
  
  
• ads115 16 bit ADC
  
  
• at24c32 eeprom
  
  
• ds1307 real time clock
  
  
• mma845x accelerometer
  
  
• pcf8574 I/O expander used on the 2-line LCD display

be a means of distinguishing them through addressing:

Every I2C slave has a 7 bit address associated with it

Usually this address is determined by the manufacturer but

often there are address pins on the devices allowing the

user to have several devices of the same type on the bus

You can find out the addresses of the I2C slaves are currently

connected with the i2cdetect command

start condition:

• high to low transition on SDA while SCL is high

• stop condition: low to high transition on SDA while SCL is high4

First the master sends the slave address with R/W set to write

Then it sends the register information.

After that another address byte is sent, this time with RW set to read

And finally the slave sends the data. The master still sends the clock but

releases the SDA line allowing the slave to control it

• Standard: 100 kbps
  
  
• Fast: 400 kbps
  
  
• High speed: 3.4 Mbps

while the Raspberry Pi can run I2C transfers of up tp 1.66 Mbps

However: you need more low level libraries or direct register

access to accomplish these high speeds

The MCP4275 fast write cycle write only the DAC register and not the EEPROM.

This is enough for what we want to do.

make a connection to the I2C driver:

• Address + R/W
• Register
• Data

The first byte is created and written within the library extracting the I2C address

for the I2C initialization call

This means we have to split our 12 bit DAC data into 2 parts:

The highest 4 bits go into i2c_reg, the lower 8 bits into bVal

• Its resolution or its least significant bit (lsb)
  
  
• Its settling time (maximum speed you can go)
• Its integral non-linearity or relative accuracy
• Its differential non-linearity
• Its offset error
• Its gain error ...