Communications Interface Read Almy Chapter 24 Homework 10 and Lab 10 due next week Exam 2 next week Weve used the HCS12s generalpurpose IO ports Ports A B E to communicate with simple devices such as switches and LEDs that use standard TTLlevel signals ID: 760166
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Slide1
EET 2261 Unit 10Serial Communications Interface
Read
Almy
,
Chapter 24.
Homework #10 and Lab #10 due next week.
Exam #2 next week.
Slide2We’ve used the HCS12’s general-purpose I/O ports (Ports A, B, E,…) to communicate with simple devices, such as switches and LEDs, that use standard TTL-level signals (0 V for LOW and 5 V for HIGH).In addition to this low-tech form of communication, our HCS12 contains several blocks that implement more complicated communication bus standards.
Communicating with External Devices
Slide3The HCS12’s communication
blocks include:Two serial communications interface (SCI) blocks.Three serial peripheral interface (SPI) blocks.Two controller area network (CAN) blocks.One inter-integrated circuit (IIC) block. Figure from p. 6 of textbook or page 23 of Device User Guide).
Communication Blocks in the HCS12
Slide4Each SCI block has two I/O pins: RXD for receiving data, TXD for transmitting data. These pins are shared with four of the pins on general-purpose I/O port S.Detailed description of the SCI blocks is found in the SCI Block User Guide.
Two SCI Blocks on the HCS12
Slide5Block Diagram of SCI Block
Diagram from page 12 of the SCI Block User Guide.
Remember: there are two copies of this circuit on our HCS12 chip. One is called SCI0, and the other is SCI1. We’ll use SCI1.
Slide6The board has movable jumpers that let us reconfigure the board’s communication resources for different needs. But we’ll leave these jumpers alone.
SCI0 and SCI1 on the Dragon12
On the Dragon12 board:The HCS12’s SCI0 is normally connected to the board’s USB port, through which the chip communicates with CodeWarrior on your personal computer.The HCS12’s SCI1 is normally connected to the RS232 interface.
Slide7RS232 Ports On Other Instruments
The RS232 interface is one of the oldest and most widely used ways of communicating between test instruments and other devices.You’ll often find RS232 ports on older equipment such as:ComputersMultimetersOscilloscopes
Slide8Establishing Communication
For
two RS232 devices
to successfully communicate with each other, they must first be configured so that they agree on the settings of several parameters. These parameters include:
Baud rate, which is a measure of how many bits are sent or received per second.
The number of data bits (typically 8 or 9) transmitted as a group.
Whether or not the data bits are accompanied by parity bits for error detection; and if so, whether we are using even parity or odd parity.
Slide9Special-Function Registers Associated with the SCI Blocks
Eight special-function registers are dedicated to the SCI1 block. See p. 37 of Device User Guide.(Eight others are for the SCI0 block, which we won’t use.)
Slide10Special-Function Registers Associated with the SCI Blocks (Cont’d.)
Recall that
you, as the programmer, write to control registers, read from status registers, and may either write to or read from data registers (depending on whether the block is transmitting or receiving data).
4 control registers
2 status registers
2 data registers
Slide11SCI Baud Rate Registers (SCInBDH and SCnIBDL)
SCInBDH and SCInBDL combined hold 16 bits, of which 13 form a number called SBR. The SCI’s baud rate depends on SBR and the system’s bus clock frequency, as follows: Baud rate = Bus clock freq / (16 × SBR)
Figure
from p.
5
of
SCI
Block
User
Guide
.
Slide12Possible Baud Rates
As stated on the previous slide, the SCI’s baud rate is given by
Baud rate = Bus clock
freq
/ (16 × SBR)
where SBR is a 13-bit number.
It’s not always possible to set the baud rate exactly to one of the standard values used by other communications devices (such as 300,
600, 1200, 2400, 4800, 9600,
19200).
But usually we can get close enough for communication to take place successfully.
Slide13SCIn Control Register 1 (SCInCR1)
The bits we care most about in this control register are:M, which we use to specify whether we’re working with 8-bit or 9-bit data.PE, which we use to enable or disable parity generation/checking.PT, which (if parity is enabled) we use to specify whether even or odd parity is used.Figure from p. 6 of SCI Block User Guide.
Slide14SCIn Control Register 2 (SCInCR2)
The bits we care most about in this control register are:TE, which we use to enable or disable data transmission.RE, which we use to enable or disable data reception.Figure from p. 7 of SCI Block User Guide.
Slide15SCI Data Registers (SCInDRH and SCInDRL)
The SCInDRH and SCInDRL registers hold the data that is being either transmitted or received.For 8-bit data (which is what we’ll use), only SCInDRL is used.
Figure from p. 11 of SCI Block User Guide.
Slide16SCIn Status Register 1 (SCInSR1)
The bits we care most about in this status register are:TDRE, which tells us whether the transmission data register is empty.RDRF, which tells us whether the receive data register is full.Figure from p. 8 of SCI Block User Guide.
Slide17Steps for Transmitting Data
Programming
the SCI to transmit data (
without
using interrupts):
Set
baud rate
using
SCI
n
BDH:SCI
n
BDL
registers.
Write $00 to SCI
n
CR1 register,
indicating 8-bit
data,
no parity
bit.
Write $08 to SCI
n
CR2 register to
enable
transmission, also disabling interrupts.
Monitor the TDRE
bit of the
SCI
n
SR1
register to make sure
data register is empty before sending a
byte to
SCI
n
DRL
. If TDRE = 1, then go to the next step.
Write the byte
to be transmitted
to
SCI
n
DRL
.
To
transfer
another byte,
go to Step
4.
Slide18Simple Code for Transmitting Data (File named Lab10SerialTransmit.txt on website)
Slide19Steps for Receiving Data
Programming
the SCI to receive
data (
without
using interrupts):
Set
baud rate
using
SCI
n
BDH:SCI
n
BDL
registers.
Write $00 to SCI
n
CR1 register,
indicating 8-bit
data,
no parity
bit.
Write $04 to SCI
n
CR2 register to
enable
reception, also disabling interrupts.
Monitor the RDRF
bit of the
SCI
n
SR1
register to
see if an entire byte has been received.
If
RDRF
= 1, then go to the next step.
Read the received byte from
SCI
n
DRL
.
To receive another byte,
go to Step
4.
Slide20Simple Code for Receiving Data(File named Lab10SerialReceive.txt on website)
Slide21A Microsoft Windows accessory program named HyperTerminal lets you send or receive data over your computer’s RS-232 port. Shown here is a dialog box for configuring HyperTerminal. Lab 9 contains directions for configuring HyperTerminal.
Microsoft HyperTerminal
Slide22The two previous slides assumed that we’re not using interrupts. Without interrupts, our program will sit in a loop, polling the TDRE bit (or the RDRF bit) repeatedly until it is set, at which point the program proceeds to take some other action:Over: BRCLR SCI1SR1, %10000000, OverAnother way is to use interrupts instead of repeatedly polling the TDRE bit (or the RDRF bit).
Polling Versus Interrupts for Serial Communications
Slide23Each SCI module has its own interrupt, which can be caused by either the receive data register being full or the transmit data register being empty: Enabled or disabled by TIE and RIE in SCInCR2:We’re familiar with the TDRE and RDRF flag bits in SCInSR1:
Serial Communications Interface (SCI) Interrupt
Slide24In the interrupt vector table, the two words starting at $FFD6 and $FFD4 are reserved for the starting addresses of the service routines for SCI0 and SCI1 interrupts, respectively.Remember: the programmer is responsible for setting up correct addresses in the vector table.
Interrupt Vectors for SCIn Interrupts
From table on page 75 of the Device User Guide.
Slide25In previous weeks we used the HCS12’s general-purpose I/O ports (Ports A, B, E,…) to communicate with simple devices, such as switches and LEDs, that use standard TTL-level signals (0 V for LOW and 5 V for HIGH).In addition to this low-tech form of communication, our HCS12 contains many blocks that implement more complicated communication bus standards.
Review: Communicating with External Devices
Slide26The HCS12’s communication
blocks include:Two serial communications interface (SCI) blocks.Three serial peripheral interface (SPI) blocks.Two controller area network (CAN) blocks.One inter-integrated circuit (IIC) block. Figure from p. 6 of textbook or page 23 of Device User Guide).
Review: Communication Blocks in the HCS12
Slide27Sometimes the term bus simply refers to a group of conductors (wires or circuit board traces) that carry signals from one device to another.Examples: address bus, data busAt other times it refers to a standard set of specifications (voltage levels, timing specs, connectors, etc.) used for communication between devices.Examples: RS-232, SPI, USB
Two Meanings of “Bus”
Slide28There are dozens of bus standards in common use. From Wikipedia’s article on the USB bus:
Many Bus Standards
Slide29We’ll Focus on the SCI Blocks
With all of these HCS12 blocks and bus standards, this is a large and complex topic.
We’ll restrict our attention the HCS12’s SCI blocks (Chapter 24).
If you’re interested in the SPI blocks, see Chapter 25. For the IIC block, see Chapter 26.
Slide30Some bus standards apply to serial communication (1 data bit transferred at a time).Others apply to parallel communication (several data bits—usually 8—transferred at a time).
Terminology: Serial
vs. Parallel
Slide31These are two common measures of speed in communications. Many writers loosely treat these as being synonyms, but this is not strictly correct.Bits per second (bps) is the easier to understand. Often expressed as kbps or Mbps.In the simplest cases, baud rate equals bps. In more sophisticated schemes, the two are related but not equal. Traditional baud rates are 300, 600, 1200, 2400, 4800, 9600, 19200.
Terminology: Bits per Second and Baud Rate
Slide32Simplex: Information flows in one direction only. Example: a temperature sensor sending data to a personal computer.Half-duplex: Information can flow in both directions, but only one at a time.Example: walkie-talkie.Full-duplex: Information can flow in both directions at the same time. Example: telephone.
Terminology: Simplex vs. Duplex
Slide33In asynchronous communication, the two devices do not share clock signals, so they use “handshaking” signals or some other method to coordinate their activity.Widely used serial asynchronous standards:RS-232RS-423RS-422RS-485USB (Universal Serial Bus): can operate synchronously or asynchronously.
Communications Terminology: Asynchronous
vs
.
Synchronous
Slide34In synchronous communication, the sender and receiver share a clock signal. Widely used serial synchronous standards:SPI (“four-wire,” or “three-wire” variant)IIC or I2C (also called “two-wire”)1-wire
Communications Terminology: Asynchronous
vs
.
Synchronous
Slide35The terms “mark” and “space” are old terms from the days of telegraphs. These terms are still often used when discussing serial communication.Mark simply means a binary 1 level on the serial line. (When no data is being transmitted, the line sits high, and we are “marking time.”)Space simply means a binary 0 level on the line.
Communications Terminology: Mark and Space
Slide36Asynchronous communication standards such as RS232 generally use start bits and stop bits at the beginning and end of a transmitted byte.The start bit is a binary 0. The stop bits—there may be one or two—are binary 1.These bits are not part of the data being transmitted. They form a “frame” around the data.
Communications Terminology: Start Bits, Stop Bits
Figure from page 229
of the textbook.
Slide37Parity is an error-checking system that attaches an extra bit (called the parity bit) to a byte when the byte is transmitted.When we configure a serial device, we often have the choice of using odd parity, even parity, or no parity.
Communications Terminology: Parity Bits
Figure from page 229
of the textbook.
As shown here,
we transmit
the data byte’s LSB first.
Slide38First version created in early 1960’s.Obsolete in some respects, but still very widely used. In recent years, has been applied in ways that its original creators never imagined, sometimes leading to problems.Original spec defined 25 pins (signals), but often only 9 or fewer are used.
RS-232 Standard
Slide39In any RS-232 application, each device is designated as either Data Terminal Equipment (DTE) or Data Communications Equipment (DCE).Simple case: When you connect a personal computer to a modem, the computer is the DTE and the modem is the DCE.
Terminology:
DCE
vs
DTE
Slide40Original RS-232 standard called for a DB-25 connector. Since many later applications didn’t use most of the pins, it became common to use DE-9 connectors (often incorrectly referred to as DB-9).
Connectors
Slide41The nine most important signals:
RS-232 Signals
Description
Abbrev.
Direction
DTE
-
DCE
DB-25 Pin #
DE-9 Pin #
Transmitted data
TxD
2
3
Received data
RxD
3
2
Request to send
RTS
4
7
Clear to send
CTS
5
8
Signal Ground
7
5
Protective Ground
1
Data set Ready
DSR
6
6
Data carrier detect
DCD
8
1
Data terminal ready
DTR
20
4
Slide42As we’ve seen, the original RS-232 standard defines 25 lines, but most applications make do with far fewer than 25—usually 9 or even less.With the HCS12’s Serial Communications Interface, we use only three lines: RxD, TxD, and Signal Ground.So we’re not using any of the handshaking lines (such as CTS, RTS, DSR, DTR). As a result, data can be lost if the sender transmits data when the receiver is not ready to receive it.
HCS12 Uses a Scaled-Down RS-232 Interface
Slide43TTL voltage levels are: 0 V for a binary 0. +5 V for a binary 1. This scheme is “unipolar” because it doesn’t use negative voltages.For transmission over a cable, it’s undesirable to have either logic level close to 0 V.So RS-232 uses a “bipolar” scheme, with: +3 V to +25 V for a binary 0 (“space”) -3 V to -25 V for a binary 1 (“mark”)
RS-232 Voltage Levels
Slide44Since TTL voltage levels are incompatible with RS-232 voltage levels, many digital systems need to translate from one to the other
.A popular chip for this purpose is Maxim’s MAX232A.
MAX232 Chip
Slide45The Dragon12 board contains an RS-232 interface, which consists of a MAX232A chip and a DE9 connector.See the Dragon12 Schematic Diagram 3.
RS-232 on the Dragon12