1. Microprocessor-based Systems
Course 10 Design of Input/Output interfaces (continuation)

2. USB – Universal Serial Bus
Goal:
a communication standard for a wide range of peripheral devices, connectable to a PC
Mouse, keyboard, joy-stick – input devices
Printer – output device
Scanner, Digital camera, Webcam – multimedia devices
Memory-stick – storage devices
replaces traditional serial and parallel interfaces
Solution:
multilayer protocol with master-slave functionality

3. Performance features Very high speed Wide bandwidth (12 Mbs)
PnP (Plag and Play) facilities
One interface handle many devices in the same time
Many communication channels on the same physical support
Transmission modes adapted to different dataflow types:
sporadic data
periodic data
high volume of data

4. Physical characteristics
Transmission on standard cable:
4 wires:
2 for power supply: Vcc (red), GND (brown) => 5V, 500mA
2 for data (blue and yellow ), twisted
Cable length:
maximum 5 m
extensible with HUBs until 30 m
Standard connectors:
type “A” – for the computer (host or master)
type “B” – for devices
Connectable during normal work (hot-swappable)

5. Physical topology of the USB (network)
Host (computer)
Root Hub
Device
Hub
Compound Device
Star topology

6. Device types: “Host” (master) – usually a computer
the device that controls the communication on the USB network
distributes network access rights to the other devices
moditors the USB network topology
“Device” – peripheral device connected to the computer
offers a given “functionality” for the host (ex: mouse, printer, scanner, etc.)
when it is connected the device gets a unique address from the host
the format of data depends on the type of the device
information exchanged between the host and the device may be:
general information
information specific for a given type of devices

7. Device types (cont.): “Hub” – amplifying and branching device
“Compound Device” –
contains a HUB and some devices
every device has its own address (including the HUB)
“Composite Device” –
contains a number of functionalities but under a single address

8. The USB protocol model
the protocol is organized on three hierarchical layers:
USB bus interface layer – the physical layer
handles the physical transmission of data (on the cable)
USB device layer –
assures the logical connection between the host and the devices
it is the operating system’s vision over the USB interface
Function layer –
assures the connection between the “client software” in the host and the “function” in the device

9. The USB protocol model Host Device Physical connection
Client software
Function
System software
Logical device
Bus interface
USB controller
Physical connection
Logical connection

10. Logical topology (layer 2)
Host
Logical Device
Logical Device
Logical Device
Logical Device
Direct connections between the host and the logical devices
the physical connections (including HUBs) are ignored

11. Application level topology (layer 3)
Host
Client software
Client software
Client software
Client software
Dispozitiv
Function
Function
Function
Function

12. Communication channels
Host
Client software
Buffers
Communication flows
Channels (Pipes)
Device
Endpoints
Function

13. Communication channels
communication channel: a logical connection between a device and a software that runs on the host
a device may have a number of communication channels
the host allocates a given bandwidth for every channel according with:
the type of the data transmission and
the available bandwidth
Every channel ends with an endpoint, used to:
configure the data channel
address the channel

14. Endpoints Endpoint: a part of a USB device: bus access frequency
it is uniquely identified through a number
ends a communication channel
stores the parameters of a channel:
bus access frequency
required bandwidth
endpoint’s number
behavior in case of an error
maximum length of accepted packages
transfer type
transfer direction

15. Endpoints an USB device is a collection of endpoints
every device have two predefined endpoints, one for input and one for output, both on address 0; the other endpoints are not defined
the other endpoints are configured by the host, when the device is connected
the endpoints are for input or for output
a communication channel is identified through:
device address,
the endpoint number and
the dataflow direction

16. Communication on the USB bus
communication is made through frames of 1 millisecond (1000 cadre/s)
a frame contains a number of transactions
a given transfer may be performed through a number of transactions
Transfer types on the USB bus:
control transfer
isochronous transfer, or periodic
interrupt transfer or a-periodic
bulk transfer or on blocks

17. Control transfer initiated by the host, non-periodic
used for configuring and dialog with the device
a control transfer contains:
a “setup” transaction – through which the device is interrogated
zero ore more data transfers transactions
a status transaction – to find the state of the device
the control transfer is performed on a standard channel present in every USB device
the standard channel is used for configuration and for device interrogation
through this channel the host configures all the other channels (including endpoints)

18. Control transfer
maximum dimension of the control packet may be 8, 16, 32 or 64 bytes
the “setup” packet has a fixed dimension of de 8 bytes
the device descriptor specifies the maximum accepted length for the data packets
the control transfers have lower priority than isochronous or interrupt transfers
at most 10% of a frame is allocated for control transfers, the rest is allocated for isochronous or interrupt transfers
the control transfer used “handshake” in order to guarantee the correctness of the transfer
the erroneous packets are re-transmited

19. Isochronous transfer (periodical)
assures a guaranteed bandwidth for periodic transfers
the format is not specified by the USB standard; it depends on the type of the device, the content is interpreted only at the client software level
the transfer is continuous, it ends only on an explicit request
a reduced error detection mechanism, the erroneous frames are not re-transmitted
the transfer through an endpoint is made in a single direction; for bidirectional transfer two endpoints must be defined

20. Isochronous transfer (periodical)
the maximum dimension of a packet is 1023 bytes
if more isochronous devices are used in the same time the the packet dimension is divided accordingly
the system allocates 90% of the bandwidth for isochronous and interrupt transfers
every device will specify the required transfer rate, which may be between 1 and 216 , (one packet per several frames)

21. Interrupt transfers used for sporadic, small data transfers
examples: mouse, keyboard, multiple channel devices
maximum packet dimension: 64 bytes
a channel has a guaranteed bandwidth assured through the allocation of a frame portion
during configuration, a minimum appearance period is specified between microseconds
during configuration the system verifies if the required bandwidth necessary for a given frequency can be assured

22. Bulk transfer
used for transferring blocks of data without time restrictions
transfers are non-periodical
examples: printer, digital camera
the transfer will use the maximum available bandwidth
the erroneous packets are re-transmitted
maximum packet dimensions: 8,16, 32, or 64 pt. USB1.1 or 512 for USB 2.0
the bandwidth is not guaranteed
the priority is minimum

23. Host Controller Driver
Communication on USB
Client software
Data
USBD interface
USB Driver
HCD interface
Host Controller Driver
Transfers
Transaction list
Transactions
Transactions
Transaction
Hardware interface
Host Controller
Packets
USB

24. Components involved in the communication
Client software
determines the type of the transfer for each device
it does not handle communication channels
for communication uses the drivers offered by the operating system
USB driver (USBD)
handles the configuration and normal transfer requests received from the client software
establish and configure communication channels based on the received requests; some requests may be rejected because of bandwidth limitations

25. Components involved in the communication
Host controller’s driver (HCD)
receives transfer requests and handles their transmission
puts the received data in the buffer
checks the transmission limits (requests v.s bandwidth)
announce the end of the transmission
Transaction list
contains the list of transactions in progress
Host controller
transforms the waiting transactions in activities on the bus
handles the frames and send and receive packets

26. Parallel interface Transfer is made through a number of lines:
Data lines (ex: 8) – for data transmission
Control lines – for synchronization and dataflow control
Features:
High speed
Small distances
Usually unidirectional transfers
Types of parallel transfers:
a) without control signals
b) asynchronous protocol without confirmation
c) asynchronous protocol with confirmation (hand - shaking)

27. Transfer types a) Without control signals - only data lines are used
- useful for reading status lines or generating command signals
- cannot be used for sequential data transfer – there are no signals for synchronization or flow control
- examples: testing digital sensors, control of actuators
b) asynchronous protocol without confirmation
- an extra control signal is used (besides data lines); it may be generated by the emitter or by the receiver
- a sequence of data may be transmitted
- the transmission speed is controlled by the device generating the control signal
- there is no feedback signal (confirmation) from the other partner; the speed should be enough small to cover any possible delays; so the transmission is slow

28. Parallel transfer
c) Asynchronous protocol with confirmation (hand - shaking)
- two control signals are used: one from the emitter and one from the receiver
- transmission with “feed-back”
- it may be adapted to the variable speed of the emitter or receiver devices; the result is a higher speed
- if the correlation between control signals is not good errors may occur
Correct
Error
Error

29. Parallel transfer
c) asynchronous protocol (hand - shaking), with inter-conditioning between control signals
- solves the correct transmission of data
- the control signals are inter-conditioned

30. Parallel transfer
The I8255 circuit – controller for parallel transfers
3 data ports of 8 bits (A, B and C)
Working modes:
Inputs
Outputs
Bidirectional transfer
Port C divided in 2 in order to serve as control signals for ports A and B A si B

31. Counter/Timer circuits
Used for:
counting external events
generate different frequencies by dividing a base clock frequency
delaying signals (mono-stable)
generate periodic interrupts
Specialized controller: I8253
3 counters of 16 bits
different working modes
Use in a PC:
to generate interrupts in order to measure the time
periodical memory refresh cycles
to generate simple sounds

32. Design of an Input/Output interface
Steps
Define the functional block scheme
select circuits for special interface functionalities (controllers)
allocate addresses for registers from the I/O space
establish the selection/decoding mode
design circuits for selection and control.
Functional blocks:
selection/decoding block
Input/Output registers (ports)
Command device
Command register
Status register
Adaptation circuit for the peripheral device

33. Design of an Input/Output interface
Selection block:
Role: Select the interface and the registers inside it
Selection mode:
Total – all the address lines are considered (rarely used)
Partial – a limited number of address lines are used (most often used)
Linear – every address line is considered a selection lie (used for small, dedicated systems)

34. Design of an Input/Output interface
Register block:
Data registers/ports – for data transmission:
for Input
for Output
Control register(s)
generate control signals to the peripheral device or determine the working mode of the interface
Status register(s):
show the state of the interface or of the device
Data amplifier:
assures the proper fan-out

35. Circuits specialized for different interfaces
Parallel interface controller
PIO – parallel I/O: I8255
Serial interface controller
SIO – serial I/O, or USART – universal serial asynchronous receiver and transmitters: I8251
Counter/Timer controller:
CTC – counter/timer controller: I8253
Floppy disk controller: I8272
DMA controller: I8237
Interrupt controller: I8259A