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MICRO PROCESSOR

MCS BASIC–52 V1.3

rejuvenating a popular interpreter

By H.-J. Böhling and D. Wulf

The MCS BASIC–52 interpreter, developed by

Intel, is still very popular, but in its 15 th

year it

needs a thoroughgoing facelift.

with its many derivative versions,

has become the most widely used

microcontroller family. Regrettably,

MCS BASIC 1.1 will not run directly

on the fastest members of this fam-

ily. The authors have made numer-

ous modiﬁcations to the source code

to arrive at version 1.3, with the fol-

lowing results:

– The interpreter no longer has any

known errors.

– BASIC–52 V1.3 can program EEP-

ROMs, even if they are located in

external memory.

– The new command ‘ERASE’ erases

an EEPROM.

– The maximum value for ‘XTAL’ can

now be set as high as 78 MHz, so

the interpreter can even run on a

Dallas 80C320 at 33 MHz!

– A new reset routine recognises the

baud rate and various types of

controllers. This means that

BASIC–52 V1.3 can run on many

8052-family members (80616,

517(A), 528, 535, 537, 575 etc.).

– The new OPCODE 43h reads a

value from the argument stack into

a variable.

– The size of the interpreter is still

only 8 kB, which means that it will

ﬁt into an 87C52 with 8 kB of inter-

nal EPROM.

Intel has now released the inter-

preter as freeware. It can thus be

legally obtained via the Internet as

commented source code. In spite of

the comments, the source code is not

The MCS BASIC–52 V1.0 interpreter was spe-

cially developed by Intel in 1985 for the 8052-

AH microcontroller. With its code size of only

8 kB, it ﬁtted into the internal ROM storage of

this controller. Unfortunately, the 8052-AH

has gone out of production. Nonethe-

less, the interpreter still enjoys con-

siderable popularity in its ﬁnal ‘offi-

cial’ version (1.1), in spite of a few

errors. Nowadays, the 8052 family,

Elektor Electronics

2/2001

MICRO PROCESSOR

easy to read, so the authors were

forced to use a simulator to unravel

the functions of the interpreter. How-

ever, this need not concern us any

further here.

rewrote the interpreter as version 1.2

for an 8031 controller, and in the

process he eliminated two known

errors, so that:

– a BASIC Autostart EPROM could

work together with command

extensions, and

– variable names with ‘F’ could be used with-

out any restrictions.

Dan Karmann was kind enough to provide

the authors with the source code for his

improvements.

Tailor-made

improvements

The authors’ work started with an

extensive Internet search for pro-

posed improvements and error

descriptions related to MCS

BASIC–52. Unfortunately, the avail-

able source code was not relocat-

able, which meant that its instruc-

tions could not be freely moved

around. The insertion of even a few

extra bytes of code immediately

resulted in incorrect operation. This

was primarily due to the fact that the

source code was contained in two

separate ﬁles, one for the interpreter

itself and another for floating-point

calculations. Consequently, the ﬁrst

task was to merge the two ﬁles into

a single, common file. Labels were

also added to all unlabelled absolute

and relative jumps and calls. Some

parts of the source code were coded

as literal data, because the assem-

bler used at the time did not support

the new 8052 instructions. The literal

data were translated into equivalent

mnemonics. One routine in the

source code differed from the origi-

nal Intel ROM code, so it was

replaced by the mnemonics corre-

sponding to the ROM code. On com-

pletion of these modifications, the

code was relocatable, so a number of

ORG directives, in particular those

linking the interpreter and floating-

point code, could be eliminated.

However, the primary benefit was

that memory space gained by

improvements to the code could now

be used for further error corrections

(see the October 1991 issue of Elek-

tor Electronics ). Still, the amount of

recovered space was insufficient to

allow all error corrections and

improvements to be fitted into the

code. Consequently, the ‘ego’ mes-

sage of the Intel programmer J.

Katausky had to be deleted (sorry!).

Bit 40h in the internal bit-address-

able memory, which was made free

as a result, was used to detect a

multiplication underrun. Previously,

this had not been handled properly.

As early as 1993, D. Karmann

Running BASIC–52 V1.3 on

Elektor Electronics boards

The hardware requirements for running BASIC–52 on the 80C537 or ‘537-Lite’ boards are

fully described in the article ‘Basic-537’, which appeared in the February 2000 issue of Elek-

tor Electronics . The information in that article is equally applicable to V1.3, with the follow-

ing exceptions:

–

It is no longer necessary to manually set the value of MTOP, since V1.3 automatically

detects the amount of available RAM.

–

BASIC–52 V1.3 automatically recognises the baud rate being used, so a space character

must be sent to the board after a restart!

The 80C32 Control Computer described in the February and March 1998 issues of Elektor

Electronics can be used with BASIC–52 V1.3, including using a Dallas 80C320 ‘Speed it µP’.

The Dallas controller is software and hardware compatible with the 8052. However, the

processor design is completely new, and it takes only four clock cycles to execute an

instruction instead of twelve. It can be run at clock frequencies up to 33 MHz. To revise the

board for this controller, you can replace the quartz crystal with a 22.118400 MHz crystal,

and the resulting processing speed will be approximately six times greater than that of a

standard 80C32. The controller can handle 33 MHz, but this is not feasible here, in part

due to the lack of a sufficiently fast EPROM (which would have to have an access time of

around 40 ns) and in part because a suitable crystal is not available. For technical reasons,

fundamental-mode crystals are only made for frequencies up to around 22 MHz. Above this

frequency, overtone crystals are used, with a supplementary resonant network to force the

crystal to oscillate at a harmonic frequency. Even at 22 MHz, sufficiently fast program mem-

ory (70 ns) must be used for the interpreter. The data memory can be populated with

devices a having a standard access time of 150 ns, since the controller automatically inserts

wait states when accessing data memory. The two TTL ICs (74HC573 and 74HC00) should

be replaced by fast 74AC or 74F types. A 28C64 can be used for the EEPROM (IC5 on the

circuit board). However, you must ﬁrst cut the connections between +5 V and pins 1 and

27 of the socket for the EEPROM. Pin 1 may be left open, but pin 27 must be connected to

the /WR signal from the controller (pin 27 of IC3).

Terminal MCS–51

If you order BASIC–52 V1.3 from the Publishers’ Readers Services on diskette (order num-

ber 000121-11 ), you will also receive a full edition of the latest version of Terminal

MCS–51. This is a terminal emulator program for DOS computers, which naturally can also

be run in a DOS window under Windows. Terminal MCS–51 was written to compensate

for the known weaknesses of BASIC–52. For example, BASIC–52 does not have a

RENUMBER command for renumbering BASIC command lines, and it has only very limited

functions for editing BASIC programs. The functions provided by Terminal MCS–51 include:

–

easy loading and storing of BASIC–52 programs,

–

renumbering BASIC–52 programs,

–

integrated line editor for BASIC–52 command lines,

–

any desired ASCII editor can be linked in.

A shareware version of Terminal MCS–51 can also be downloaded from the Free Down-

loads section on the Elektor Electronics website at http://www.elektor-electronics.co.uk . The

shareware version is fully functional and not time-limited, but it does not allow BASIC pro-

gram lines to be renumbered.

2/2001

Elektor Electronics

MICRO PROCESSOR

EPROM programming

4 REM ******************************************************

5 REM * Program for controlling A/D converter in 80535/537 *

6 REM ******************************************************

10 RANGE=5 : REM Controller reference voltage

20 ADCO=0D8H : REM Register to check A/D state etc.

30 ADDAT=0D9H : REM Register for measurement output

40 DAPR=0DAH : REM Register for meas. range and start

50 RDSFR (ADCO)BUSY : REM Read other function bits

60 BUSY=BUSY.AND.0C0H : REM Isolate function bits

70 WRSFR (ADCO)BUSY : REM Pick measurement input 0 (Port 7.0)

80 WRSFR (DAPR)0 : REM Start measurement in highest meas. range

90 RDSFR (ADCO)BUSY : REM Read A/D converter state

10 IF (BUSY.AND.020H)>0 THEN GOTO 90 : REM wait if busy

110 RDSFR (ADDAT)VOLTAGE : REM Read measurement value

120 PRINT ”Voltage on Input 0 equals:”,

130 PRINT VOLTAGE*RANGE/256,”Volt” : REM Adapt value to meas. range

140 PRINT ”Another measurement required in 1-V range (Y/N)?”,

150 I=GET.AND.0DFH

160 IF I=ASC(Y) THEN GOTO 190

170 IF I=ASC(N) THEN GOTO 260

180 GOTO 150

190 RANGE=1 : REM Measurement range = 1 Volt

200 WRSFR (DAPR)040H : REM Set range and launch measurement

210 RDSFR (ADCO)BUSY : REM Read A/D converter state

220 IF (BUSY.AND.020H)>0 THEN GOTO 210 : REM Wait if busy

230 RDSFR (ADDAT)VOLTAGE : REM Read measurement value

240 PRINT ”Voltage on Input 0 equals exactly:”,

250 PRINT VOLTAGE*RANGE/256,”Volt” : REM Adapt value to meas. range

260 END

Unfortunately, MCS BASIC–52 V1.1

cannot program an EPROM when it

is run from external memory. The

improvement made to remedy this

shortcoming is based on a sugges-

tion made by R. Skowronek in 1996.

BASIC–52 V1.3 can directly program

hardware EEPROMs using the

‘PROG(1–6)’ and ‘PGM’ commands,

even when it is run from external pro-

gram memory. The EEPROM is writ-

ten in the same way as RAM, but

with a suitably longer cycle time.

With thi s chan ge, port pins 1.3

( ALEDIS ), 1.4 (PRGPLS) and 1.5

(PRGEN), as well as bit 15h (INTELB)

in the internal memory and memory

locations 12Ah and 12Bh in external

RAM, are no longer needed and thus

can be freely utilised for other pur-

poses. The ‘PROG’ commands are

also redundant, so they have been

deleted. This yields additional space

savings, and the BASIC token 0F9h

that has been made free is used for

the new ‘ERASE’ command. This

command erases an EEPROM by

overwriting all memory locations

between 08000h and 0C000h with

0FFh. ‘ERASE’ is called in command

mode, and it takes around 2.75 min-

utes to erase this 16-kB region.

A special feature of BASIC–52 is the

possibility of extending the com-

mand set using up to 16 user-deﬁned

commands programmed in assembly

language. ‘OPCODEs’ are provided

in order to allow system routines to

be used in these extensions. Unfor-

tunately, there was no OPCODE

available to assign a value placed on

the argument stack to a variable

deﬁned in BASIC text. Consequently,

it was always necessary resort to

awkward routines using the POP

command. In BASIC–52 V1.3,

OPCODE 43h has been defined for

this purpose. It is used, for example,

in the command extension ‘RDSFR

(address) variable’.

Previously, the instruction ‘TIME =

0’ did not reset the millisecond

counter to zero, so that the value of

TIME was not reset to exactly zero.

This unfortunate error has been cor-

rected in V1.3.

The BASIC–52 instruction ‘ASC(x)’

always returned an incorrect value for

any character position occupied by a

BASIC operator (*, +, –, /, <, =, > or

The A/D converter in the 80517A has a resolution of 10 bits and increased measurement accuracy. How-

ever, with this type of controller it is not possible to modify the internal reference voltage via a control

measured.

4 REM ***************************************************

5 REM * Program for controlling A/D converter in 80517A *

6 REM ***************************************************

10 RANGE=5 : REM Controller reference voltage

20 ADCO=0D8H : REM Register for measurement input selection

30 ADDATL=0DAH : REM measurement value, LSB

40 ADDATH=0D9H : REM measurement value, MSB

50 PRINT ”Please enter number of measurement input (0-7):”,

60 INPUT CHANNEL

70 CHANNEL=CHANNEL.AND.7 : REM Mask superﬂuous bits

80 RDSFR (ADCO)BUSY : REM Read function bits

90 BUSY=(BUSY.AND.0C0H).OR.CHANNEL : REM add channel selection

100 NRSFR (ADCO)BUSY : REM Copy channel selection to controller

110 WRSFR (ADDATL)0 : REM Launch measurement

120 RDSFR (ADCO)BUSY : REM Monitor A/D Busy state

130 IF (BUSY.AND.020H)>0 THEN GOTO 120 : REM Wait if busy

140 RDSFR (ADDATL)UOLTPART : REM Read least signiﬁcant part

150 RDSFR (ADDATH)VOLTAGE : REM Read most signiﬁcant part

160 VOLTAGE=VOLTAGE+UOLTPART/256 : REM Compute maesurement values

170 PRINT ”Voltage at input:”,CHANNEL,”equals:”,

180 PRINT VOLTAGE*RANGE/256,”Volt” : REM Adapt value to meas. range

190 END

Elektor Electronics

2/2001

MICRO PROCESSOR

?). The interpreter translated these

characters into equivalent tokens dur-

ing the tokenisation process. For

example, a question mark was trans-

lated into the token for ‘PRINT’. Con-

sequently, the value of the token was

returned instead of the correctly com-

puted ASCII value of the character.

BASIC–52 V1.3 now supplies correct

values for all ASCII characters.

In order to allow BASIC–52 V1.3 to

run on as many 8052-family deriva-

tives as possible, and in particular

on the Dallas Semiconductor 80C320

‘Speed it µP’ controller, the reset rou-

tine has been extensively reworked,

as follows:

– A new baud rate detector utilises

Timer 2 instead of ‘code loop’ tim-

ing to measure the utilised baud

rate. This makes the measurement

fully independent of the controller

type and clock frequency.

– Timer 0, which is used for time

measurements (BASIC Software

Clock), is now operated in the 16-

bit mode instead of the 13-bit

mode that was previously used.

This means that the maximum

value for XTAL is now

78,627,473 Hz. The power-up

default value is still 11,059,200 Hz,

as before, but this value can be

modified as necessary using

‘XTAL = value’. Alternatively, the

default value can be changed by

patching the following locations in

the EPROM with suitable new val-

ues:

17F1h

Table 1. 80535/537 A/D converter SFRs

SFR

Address

Bit(s)

Name(s)

Function

ADCON

0D8h

2–0

MX2,MX1,MXO

Select analogue input

ADCON

0D8h

ADEX

Start via hardware/software

ADCON

0D8h

ADM

Single/continuous conversion

ADCON

0D8h

BSY

Conversion in progress

DAPR

0DAh

3–0

DAPR.3-.0

Lower reference voltage

DAPR

0DAh

7–4

DAPR.7-.4

Upper reference voltage

ADDAT

0D9h

7–0

8-bit measurement result

exactly 12 MHz, the available rates

are 4800 and 9600 baud. If the

BASIC–52 V1.3 set-up routine

detects this type of controller, it acti-

vates the baud rate generator with a

baud rate that corresponds to that of

the measured timing of the space

character (4800 or 9600 baud).

The 80C517A controller, which is

also frequently used, has an

enhanced special baud rate genera-

tor. This supports arbitrary data

transmission rates, but it still oper-

ates independently of Timer 2.

BASIC–52 V1.3 also automatically

recognises this processor type and

activates it with the appropriate

baud rate. With all of these special

types of controller, Timer 2 is freely

available to the user.

Two additional commands allow Special

Function Registers (SFRs) to be read and

written. The various derivative versions in the

controller family employ supplementary SFRs.

The following commands have been imple-

mented to allow these registers to be

accessed via BASIC:

WRSFR (address) value

Writes ‘value’ to the indicated SFR address.

RDSFR (address) variable

Reads the content of the indicated SFR

address and writes it to ‘variable’.

Using these commands, it is easy to address

internal extensions of the various processor

derivatives, such as the additional I/O ports

of an A/D converter. The two sample BASIC

programs in the text boxes (AD-535.LIS and

AD-517A.LIS) illustrate how the SFR com-

mands can be used.

Since BASIC–52 V1.3 also runs on the 80535,

537 and 517 controllers, as well as other

types, and since the external command

extensions allow extended SFRs to be used

without any problems to access new hard-

ware features, it is now easy to write pro-

grammable control and adjustment programs

in BASIC, instead of wrestling with the intri-

cacies of assembly language programming.

The sample programs illustrate the use of the

A/D converter of an 80535 and an 80517(A).

The A/D converter in an 80535/537 controller

has a resolution of 8 bits. The external refer-

ence voltage for the converter is normally set

to +5 V. This yields a minimum resolution of

20 mV. Since the internal reference voltage of

the converter can be programmed using the

‘DAPR’ SFR, it is possible to achieve a higher

resolution (for example, 4 mV) by making a

second measurement using a smaller mea-

surement range. The SFRs listed in Table 1

can be used for this purpose.

I 2 C and SFR

BASIC–52 can easily be extended in

assembly language with additional

BASIC commands. Naturally, these

extended commands cannot be

located within the 8-kB code region

of the BASIC interpreter. The authors

have implemented six new com-

mands. Four of these are related to

I 2 C communications and have

already been described in the article

“Implementing the I 2 C Bus” in the

July/August 2000 issue of Elektor

Electronics , as follows:

17F0h

17EFh

17EEh

For baud rate detection, the user still

has to send a space character to

BASIC–52 via the serial interface.

The character length is then mea-

sured by the detection routine using

Timer 2. In principle,

BASIC–52 utilises only Timer 2 as

the baud rate generator. This applies

to 8052 controllers and the Dallas

80C320. The 80535, 80515 and 80517

controllers cannot activate Timer 2

as a baud rate generator. They have

a special baud rate generator that is

fully independent of Timer 2, and

this allows only two different baud

rates to be generated with a speciﬁc

relationship to the processor clock

frequency. If the clock frequency is

I2CSTART

Transmits a Start condition on the

I2C bus.

I2CSTOP

Transmits a Stop condition on the

I2C bus.

I2CPUT (value)

Transmits a byte on the I2C bus.

I2CGET (value)

Reads a byte from the I2C bus into

a variable.

(000121-1)

2/2001

Elektor Electronics

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