Logic Families Voltage Translation

No Translation required for the following:

[TTL] to [TTL]

[TTL] to [CMOS with input switching at TTL levels]

[CMOS 5V] to [TTL]

Translation required for the following:

[TTL] to [CMOS*]

Pull up of 1k to 2k required

TTL outputs 2.4V to 3.3V for a high level

CMOS required 3.7 for high level.

The pull up resister increases the output voltage (of TTL driver).

[CMOS*] to [TTL]

*The translation depends on the VCC of CMOS device.

[TTL] to [ECL] / [ECL] to [TTL]

TTL to ECL (ECL to TTL) translator along with ECL termination resistors are needed.

[PECL] to [TTL] / [TTL] to [PECL]

PECL is Positive ECL

PECL to TTL (TTL to PECL) translator needed.

[PECL] to [CMOS] / [CMOS] to [PECL]

Same as above, but pull of 1k to 2k is required for CMOS.

[PECL 5V] to [LVDS 5V]

Pull up both +,- signals to 3V through 50E resistors.

[LVPECL 3.3V] to [LVDS 5V]

Pull up both +,- signals to 1.3V through 50E resistors

Signal Integrity Principles

Each interconnect is a transmission line

Forget the word ground; Think return path.

Bandwidth of a signal is the highest sine wave frequency component.

All SI formulas are definitions or approximations.

Even a two inch long trace on a PCB can affect SI.

SI problems: Timing, Noise, EMI

Noise sources:

Signal on a net: Reflections, Distortions from Impedance discontinuities

Crosstalk: Mutual Inductance, Mutual (parasitic) capacitance

Rail collapse: Voltage drop in power/ground.

EMI: A component or entire system

Impedance discontinuities: Cross section, topology, added components(via, connector, etc.)

Ways to minimize cross talk (cross talk should kept as minimum as 5%)

1) Use uniform plane as return path.

2) Spacing traces farther apart.

3) Use guard traces.

Rail-Collapse noise (Rail-bounce, Ground bounce):

Because of the impedance of the power and ground distribution, a voltage drop will occur as the IC current switches. This voltage drop means the power and ground rails have collapsed from their nominal values.

Simultaneous switching noise (SSN) or simultaneous switching output (SSO): Whenever an output pin changes state, the current it draws from its ICs 0V and power rails eventually flows through the interconnections to the PCB’s 0V and power rails. The inductance in these paths causes voltage drops that make the IC’s internal 0V and power rails ‘bounce’ with respect to the PCB rails, according to the activity of the output ports

Ways to minimize:

1) Closely placed power-ground planes with thin dielectric

2) Low inductance decaps, on-chip decaps

3) Multiple short power, ground pins in packages

Sources of EMI

1) Conversion of some differential signal into a common signal

2) Ground bounce

Ways to minimize:

1) Use shielded cables

2) Use low-impedance connections

Rise time: Time taken to go from 10% to 90% of the final value.

Mostly rise time is about 10% of clock period.

Bandwidth = 0.35*[Rise Time]

For most microprocessor, ASIC based systems rise time is 7% of clock period.

Approximately bandwidth is five times the clock frequency.

An electron travels (in a copper wire) at a speed of 1cm/s.

Time to travel down a 6” of interconnect in FR4 is about 1ns.

Ideal Square Wave

Duty cycle = 50%

Peak value = 1 V

Frequency = 1 GHz

Amplitude of nth harmonic An = 2/(n*pi)

All even harmonics are zero.

Self inductance of circular loop of wire

L = 32*R*ln(4R/D) nH

where R = radius of loop in inches

D = diameter of wire in inches

High speed digital buses

Bus	Clock Frequency
RapidIO	1 GHz
Hypertransport	800 MHz
3GIO	1.25 GHz
Serial ATA	3 GHz
SCID	1.6 GHz
Infiniband	1.25 GHz
Gigabit Ethernet	625 MHz

USB – Universal Serial Bus

USB is a polled cable bus with single-host-scheduled, token-based protocol.

Tiered-star topology: USB host is the root hub. A hub is center of a star. A star consists of point-to-point connections between [host and hub/function] or [hub and (another) hub/function].

Allowed topology: Maximum tiers = 7; Maximum non-root hubs in a tier = 5.

Data rates:

High-speed: 480 Mbps

Full-speed: 12 Mbps

Low-speed: 1.5Mbps

For effective utilization of bandwidth, full-speed and low-speed data can be transmitted at high-speed between host and hub, but transmitted between hub and device at full-speed or low-speed.

Operation: Clock is transmitted, encoded (NRZI encoding with bit stuffing) with differential data. A SYNC field precedes each packet for synchronization. The cable also carries VBUS (+5V at source) and GND to deliver power to devices. Cable with biased terminations at each end and up to seven metre length is allowed.

Most bus transactions involve the transmission of up to three packets. Each transaction begins when the Host Controller, on a scheduled basis, sends a USB packet describing the type and direction of transaction, the USB device address, and endpoint number. This packet is referred to as the “token packet.” The USB device that is addressed selects itself by decoding the appropriate address fields. In a given transaction, data is transferred either from the host to a device or from a device to the host. The direction of data transfer is specified in the token packet. The source of the transaction then sends a data packet or indicates it has no data to transfer. The destination, in general, responds with a handshake packet indicating whether the transfer was successful.

Some bus transactions between host controllers and hubs involve the transmission of four packets. These types of transactions are used to manage the data transfers between the host and full-/low- speed devices. The USB data transfer model between a source or destination on the host and an endpoint on a device is referred to as a pipe. There are two types of pipes: stream and message. Stream data has no USB-defined structure, while message data does. Additionally, pipes have associations of data bandwidth, transfer service type, and endpoint characteristics like directionality and buffer sizes. Most pipes come into existence when a USB device is configured. One message pipe, the Default Control Pipe, always exists once a device is powered, in order to provide access to the device’s configuration, status, and control information.

The transaction schedule allows flow control for some stream pipes. At the hardware level, this prevents buffers from under-run or overrun situations by using a NAK handshake to throttle the data rate. When NAKed, a transaction is retried when bus time is available. The flow control mechanism permits the construction of flexible schedules that accommodate concurrent servicing of a heterogeneous mix of stream pipes. Thus, multiple stream pipes can be serviced at different intervals and with packets of different sizes.

UART

Universal Asynchronous Receiver Transmitter: Used for serial communications via cable. UART generates signals of same timing as RS-232 used by Personal Computer’s COM ports.

Standard	Logic 0	Logic 1
UART	0V	5V
RS-232	+12V	-12V

Asynchronous communication requires clock recovery, where a known transition event in the data is used to synchronize transmitter/receiver.

Baud rate of UART: integer multiples or submultiples of 9600 Hz.

RS-232 frame:

1) Start bit (always logic 0)

2) Data bits (5, 6, 7, or 8 of them)

3) A parity bit (optional, even or odd parity)

4) A stop bit (always logic 1); may be 1, 1.5, 2 bit times in duration

The synchronization point is at the start of the frame (always a 1 to 0 transition).

• The 8 received data values are sampled 1.5BT, 2.5BT, … , 8.5BT after the synchronization point (BT = bit time).

• The stop bit is sampled 9.5BT after the synchronization point (if it is not a logic 1, this is a framing error).

Send, Receive data are buffered using Tx, Rx registers.