1. Embedded Bus – i2c David E. Culler Lab 2 Feb 2, 2015

2. What’s a bus ?
An efficient means of connecting together several devices so they can interact and transfer information.
The method of HW composition in computer systems
N devices, N2 interactions, 1 set of wires
Defined set of Bus Transactions P M M
i/o
i/o
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3. Modern Computer Systems
network

4. Mainstream Systems
Read / Write
wires
I/O
Controllers
Processor
Secondary Storage (Disk)
Core
L2 Cache
Registers
L1 Cache
interrupts
Secondary Storage (SSD)
Main
Memory
(DRAM)
Read / Write
DMA transfer
L3 Cache (shared)
Core
L2 Cache
Registers
L1 Cache
I/O devices you recognize are supported by I/O Controllers
Processors accesses them by reading and writing IO registers as if they were memory
Write commands and arguments, read status and results
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5. Example Device-Transfer Rates in Mb/s (Sun Enterprise 6000)
Device Rates vary over 12 orders of magnitude !!!
System better be able to handle this wide range
Better not have high overhead/byte for fast devices!
Better not waste time waiting for slow devices

6. Bus Transaction Master => Slave Arbitration
Master requests access to bus (if multi-master)
Command (Rd, Wt), Address
Slave identified by address ack’s request
Data transfer
Many variants
Synchronous/Asynchronous
Split-Phase …
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7. What about embedded systems?
Many simple devices
Extremely constrained interfaces
Serial busses => transaction over a wire or two
i2c
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8. i2C Invented by Philips as chip-chip interconnect for TV sets
2 signal lines, Vcc and Gnd
SDA (serial data) and SCL (serial clock)
Both bi-directional (actively pulled down)
7-bit slave address
Data in 8-bit bytes – 100 kbps – 2.4 mbps
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9. I2c transaction
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10. i2C Signaling Start/Stop closely tied to physical structure
SDA falling/rising when SCK is high
Data must remain stable while sck is high, may change while low
Slow slave and stretch clock
Multi-master arbitration by detection of failure to drive
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11. Transferring Data To/From Controller
Programmed I/O:
Each byte transferred via processor in/out or load/store
Pro: Simple hardware, easy to program
Con: Consumes processor cycles proportional to data size
Direct Memory Access:
Give controller access to memory bus
Ask it to transfer data blocks to/from memory directly
Sample interaction with DMA controller (from OSC):
addr
len

12. In embedded ? PIO => bit banging the pins to generate a waveform
DMA
Not just for high performance
Allows MCU to do other things during SLOOOOOW transfers
Even to go to sleep
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