e011012.PDF

MICRO PROCESSOR

Upgrading to the

68HC12 16-bit

microcontroller (1)

an introduction

By Oliver Thamm

The application of microcontrollers on the other side of the 8-bit bound-

ary often seems like a closed book. Here we show that it can actually be

fairly easy to start working with such microcontrollers — it’s not even

necessary to be an accomplished ‘HC11 programmer!

Circuits using microcontrollers are

very ﬂexible, and they enjoy a corre-

spondingly high level of popularity.

The Motorola 68HC11 family has for

years been one of the favourites for

hobby applications. These 8-bit

microcontrollers have an easily

understood internal structure and

can be readily programmed via an

RS232 link. The HC11 fan club has

grown over the years, and the num-

ber of published designs has also

increased (see Literature). However,

these days the HC11 has become

somewhat technically obsolescent.

New application areas demand con-

trollers that are more flexible and

that have more processing power.

Motorola, the manufacturer of the

HC11, ﬁnally decided to make a rad-

ical change in their manufacturing

technology, which however could

not easily be carried out with the

HC11 types. The result was the

68HC812A4, the ﬁrst member of the

new, powerful HC12 family.

Many HC11 users do not realise

that they can easily change over to

Elektor Electronics

1/2001

MICRO PROCESSOR

the faster HC12. This is because the new

models have inherited the complete HC11

programming model. That means that the

structure of the CPU registers (see Figure 1 ),

the assembler instructions and the address-

ing modes have remained the same. Pro-

grammers will thus find it very easy to

change to the new family, since existing

source code only has to be retranslated. If

you’re only starting, rather than switching

over, you can still enjoy certain advantages.

In brief, the HC12 masters an important bal-

ancing act, combining high performance with

a relatively low level of complexity.

Practical circuits

Enough of the introductory remarks, now it’s

time to look at a circuit diagram for the HC12.

Figure 2 shows an HC812A4 IC and the

external circuitry that the microcontroller

needs to run. Is that all it takes? Yes, indeed,

and this is one of the main advantages of this

microcontroller unit (MCU) – almost all the

important components are integrated into the

controller.

IC3 is the well-proven Maxim TTL/RS232

signal-level converter. The HC812A4 has two

asynchronous serial interfaces, which are

internally designated SCI0 and SCI1. Their

operation is mutually independent, and they

easily manage a data rate of 38,400 baud

with the clock crystal used here. A second

serial interface has been on the wish list of a

lot of HC11 users for a long time. Now you

finally have all the resources you need, for

example to exchange messages with the

application program via the first channel

while sending debugging data via the second

channel. The two connectors X2 and X3 are

implemented as pin headers, to which one-

to-one crimped ﬂatcables can be connected,

with 9-pin sub-D connectors (also crimped) at

the PC end.

Figure 1. HC12 CPU registers.

HC12 Operating Modes

The HC12 can work in several different operating modes that are selected via

pins MODA and MODB. The levels on these pins are read in by the processor on

the rising edge of the reset signal, to determine the operating mode. Normally,

the BKGD pin must maintain a steady High level at this moment. The following

operating modes, which are the most important, are frequently encountered in

HC12 circuits.

Normal Single Chip Mode (MODA=L, MODB=L)

The HC12 operates without an external bus interface, using only the internal

memory (RAM, EEPROM and Flash EEPROM if available). The bus leads that are

thereby made free are made available as general-purpose I/O leads.

Normal Expanded Wide Mode (MODA=H, MODB=H)

The HC812A4 operates with a 16-bit-wide external bus interface. Ports A and B

provide the address bus, while the data bus is found at ports C and D. Bus con-

trol is provided by the signals ECLK, R/W and /LSTRB (port E). The HC12 gener-

ates up to seven chip select signals on port F, as necessary. In addition, it is possi-

ble to use Memory Banking to drive memory regions beyond the 64-kB bound-

ary. Up to six leads of port G are responsible for this.

A quick tour of the MCU

The clock is generated using the usual com-

bination of a quartz crystal, a pair of ceramic

capacitors and a resistor. HC11 users will

recognise the circuit, but in this case a 16-

Mhz crystal provides the time standard. Com-

pared to standard HC11 types, this provides

four times the internal processing speed

(8 MHz E clock).

In order to reliably reset the CPU when the

circuit is switched on (or restarted), an exter-

nal reset IC is prescribed. In our circuit, this

service is provided by IC2, an MC34064P5. In

addition, it is possible to connect a manual

reset pushbutton via jumper JP3.

Jumpers JP4 and JP5 are normally closed

Normal Expanded Narrow Mode (MODA=H, MODB=L)

In this mode, the HC812A4 behaves the same as in the Expanded Wide mode,

except that the data bus is only eight bits wide (Port C). The MCU automatically

splits every 16-bit access into two sequential 8-bit accesses.

Special Single Chip Mode (MODA=L, MODB=L)

This mode is specially provided for putting the IC into operation with the Back-

ground Debug Interface Mode interface. When the reset signal is released, the

connected BCM pod must hold the BKGD lead Low for a short time. The HC12

system and the pod subsequently communicate via this lead. The CPU does not

start with processing an application program, as usual, but initially executes only

BDM instructions. In this software-development operating mode, the restrictions

on the use of some of the control registers are relaxed, since some of the protec-

tive provisions of the MCU are disabled.

1/2001

Elektor Electronics

MICRO PROCESSOR

V CC

JP4

JP5

V CC

42 79

JP1

4k7

V CC

VDDPLL

VRH

MODA

XIRQ

100n

IRQ

IC1

JP2

MODA

MODB

10µ

16V

4k7

V+

C1+

MODB

68HC812A4

IC3

10µ

16V

C1–

RX0

RS232 #0

RXD0

TXD0

RXD1

TXD1

R1OUT

R1IN

V CC

TX0

T1IN

T1OUT

RX1

TX1

R2OUT

R2IN

100

T2IN

T2OUT

VDD

RES

GND

VPP

C1F

C2+

MAX232A

RESET

BKGD

100n

BKGND

10µ

16V

C2–

V-

BDM12

VSSPLL

VRL

V CC

10µ

16V

EXTAL

XTAL

RS232 #1

IC2

41 80 46

47 96

10M

V CC

OUT

MC34064

C1A

C1B

C1C

C1D

C1E

JP3

RESET

GND

47µ

16V

100n

18p

Q1 = 16MHz

000077 - 11

Figure 2. The circuit diagram of the HCA812A4 component set corresponds to that of the HC12 Welcome Kit.

(installed), so that the operating voltage (Vcc,

+5 V) is applied to the power supply and ref-

erence voltage inputs of the integrated A/D

converter. These inputs can be fed from exter-

nal voltages by removing the jumpers, for

example if it is necessary to use a more pre-

cise reference voltage.

The internal EEPROM serves as the

program store. With a size of 4 kB, it

is larger than the internal program

store of the HC11. In addition, there

is 1 kB of RAM for variable data.

Since the instruction set of the HC12

is very efficient, it is possible to store

a fairly sizeable program in the inter-

nal memory. Some examples will be

provided in the second part of this

article, in the December 2000 issue.

the microcontroller works via this

lead, which is the interface for the

Background Debug Mode (BDM). As

the name indicates, the BDM inter-

face is used for more than just down-

loading software into the RAM or

EEPROM. The BKGD pin also pro-

vides a ‘back door’ to the microcon-

troller, which allows any desired

memory addresses to be read and

written, ranging from the RAM and

EEPROM to the IC control registers.

This means that, via this pin and

BDM, you can start and stop appli-

cation programs as desired, set

breakpoints, run software in single-

step mode and read or modify the

contents of the processor registers.

The most important details of

BDM12 operation are explained in

the box ‘How does BDM work?’.

This all sounds like we have a

complete hardware emulator on

board. Normally, you can expect to

pay the cost of a small car for such a

Mode selection

Two additional jumpers are needed to set the

operating mode of the HC12. JP1 is con-

nected to the MODA pin, and JP2 to the

MODB pin. The manner in which these sig-

nals affect the behaviour of the HC12 is

explained in the ‘HC12 Operating Modes’

box. In our case, both jumpers are set to the

1–2 positions and thus select the Single Chip

operating mode. Single Chip mode does not

mean that there are not any other ICs on the

board. Instead, in this mode the HC12 dis-

ables all bus signals for external memory ICs.

The ports that are made free as a conse-

quence can be used as general-purpose I/O

lines, and this means a gain of up to 40 lines!

In the background

Apart from a few blocking capacitors

and pull-up resistors, we would be

finished with the description of the

schematic diagram, except for the

six-pin connector X1. Only two ‘real’

signals are tied to this connector,

namely /RES and BKGD. The signal

/RES is an extension of the MCU

/RESET signal, while BKGD is con-

nected directly to the microcontroller.

The built-in debugging function of

Elektor Electronics

1/2001

MICRO PROCESSOR

piece of equipment, but the HC12

includes the most important features

of such a debugging aid at no extra

charge. This sounds almost too good

to be true, and in fact there is a ﬂy in

the ointment.

The connection to a PC that is

used for developing and download-

ing the software is not entirely with-

out problems. The BDM interface is

simply too fast to work with a nor-

mal serial or parallel PC interface.

This means that you need a bit of

hardware and software to act as an

interpreter between the PC and the

HC12. This is called a BDM pod, and

it is not particularly suitable for DIY

construction. Fortunately, there are

alternative solutions, such as the

NoICE debugger, that will not soak

up the annual budget of an ambi-

tious HC12 developer in one blow. A

large collection of links relating to

BDM and the general subject of

HC12 microcontrollers can be found

on the author’s HC12 Internet site at

http://hc12web.de .

of the HC812A4 are expensive and

hard to come by. In fact, they are so

expensive that it is worth taking a

look at the second option. This con-

sists of a pre-assembled controller

module, as shown in the photo-

graph. It not only includes the

peripheral circuitry shown in Fig-

ure 2 , but also makes all processor

signals available on lateral pin

strips, where they can be easily

accessed (even using non-profes-

sional means).

The German firm Elektronikladen

(see http://www.elektronikladen.de

on the Internet) offers such modules

in credit-card format under the des-

ignation ‘HC12 Welcome Kit’ (see

http://elektronikladen.de/kit12.html )

The MCU is soldered directly to the

circuit board in an SMD production step,

without a socket. This makes the whole thing

affordable, and for around £75 you receive a

complete starter kit, including a monitor pro-

gram for easy downloading of application

programs via the serial interface.

(000077-1)

Part 2 will discuss the programming of the internal

functional modules of the HC12 and the tools that

are needed for this.

Literature:

68HC11 emulator,

Elektor Electronics February 1997.

How does BDM work?

The Background Debug Mode (BDM) interface of the HC12 is basically supported by a sin-

gle lead (BKGD) of the microcontroller. In the idle state, this lead is held High by a pull-up

resistor. A special asynchronous serial protocol is used for communications. The underlying

clock is based on the clock speed of the HC12. For example, if a 16-MHz crystal is used,

the basic time unit of the BCM system is 125 ns.

The timing diagram shows how an individual bit is transferred. This requires 16 clock

cycles, which means that it takes 2 µs to transfer one bit at 125 ns per clock, and a full byte

thus takes 16 µs. A typical BDM command sequence consists of ﬁve bytes, and thus takes

around 80 µs. In practice, you can expect to see around 5 to 10 thousand BDM instructions

per second – and that’s already pretty fast.

A BDM instruction consists of a command byte and any necessary parameters. For

example, the instruction sequence for a 16-bit write access to address $0800/1 appears

as follows:

I/O leads

It’s a fortunate person who has

enough I/O heads to connect all of

his LCDs, pushbuttons, lamps and

relays. With an HC812A4, you won’t

ﬁnd your self in a predicament quite

so quickly, at least if it is working in

Single Chip mode. More than 80 I/O

connections are available to the user,

each of which can be addressed as

an input and/or output. With regard

to the internal pull-up resistors, there

are several conﬁguration options. A

substantial number of input signals

(roughly one third) can function as

interrupt triggers.

C8 08 00 1A 2F

1 Bit

E-Clock

(Target)

28 Pins

case 1: pod sends "0"

HC12 reads "0"

BKGND

If you’re already getting curious and

would like to take a closer look at the

HC12, you have basically two

options for getting started quickly.

The first option is to obtain the

microcontroller IC and mount it in a

socket on a universal printed circuit

board. The relatively few external

connections will not present that

much of a problem. However, the

catch is that it is very difficult to

manually solder the IC, so that a

socket must be used – and suitable

sockets for the 112-pin QFP package

HC12 reads "1 "

case 2: pod sends "1"

BKGND

000077 - 13

The individual components of the message have the following meanings:

$C8 BDM command ‘WRITE_WORD’

$0800 address

$1A2F data word

1/2001

Elektor Electronics

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: