e015030.PDF

GENERAL INTEREST

Major-Domo

a 16-channel microcontroller-driven switching clock

Design by H. Vos

The switching clock described here can control up to 16 channels and has

20 memories for individual switching times, each of which may apply to

multiple days of the week. This clock has a very easy to operate, 4-button

user interface. Monitoring of the outputs via an RS232 port is also possi-

ble. The ‘brains’ consist of an AT90S8515 from Atmel.

The circuit

An initial quick glance at Figure 1

makes it clear that a central place

has been reserved for the AT90S8515

from Atmel (IC1). The entire program

resides within the microcontroller.

The frequency of the microcontroller

clock is set to 3.6864 MHz, using X1.

This frequency permits exact baud

rates to be generated for the UART,

which is used for the serial commu-

nication with a terminal or PC.

The four control buttons S1-S4 for

the switching clock (a detailed

description of the operation follows

later) are directly connected to port

B of the controller. R5-R8 provide a

defined input level at the inputs of

port B when the pushbuttons are not

activated. The software takes care of

debouncing the pushbuttons.

The display consists of a standard

2 × 16 character LC-display, which is

connected directly to PA0-PA7 and

PC4, PC5, PC6. Port PA is used as a

bi-directional data bus. PC4, PC5

and PC6 are the control signals for

the display (RS, WR and E respec-

tively). Potentiometer R11 and resis-

tor R12 form a potential divider that

provides a bias voltage for the con-

trast control of the display. The

There exists no shortage of switching clocks.

Both ‘analogue’ and ‘digital’ (with LCD) mod-

els are available with widely varying fea-

tures. So, what are the merits of this design

that makes it deserve the light of day? Well,

the particular feature that separates this

clock from the masses, is that it does not con-

trol just one channel, but can be

expanded to no fewer than 16 chan-

nels. In addition, this clock has a ser-

ial interface, which enables every on

and off switching event for each

channel to be monitored on a PC

using, for example, HyperTerminal.

Elektor Electronics

5/2001

GENERAL INTEREST

+5V

IC6

+5V

LM7805

+5V

C12

C16

C10

C11

100n

10µ

25V

100n

10µ

25V

100n

PLUS

MIN

SELECT

RETURN

1 N4001

PB0(T0)

PB1(T1)

PC0(A8)

PC1(A9)

+5V

PB2(AIN0)

PC2(A10)

PB3

PC3(A11)

+5V

PB4(SS)

PC4(A12)

PC5(A13)

PC6(A14)

PC7(A15)

R12

PB5(MOSI)

PB6(MISO)

PB7(SCK)

R16

R11

IC1

+5V

RE1

+5V

ALE

RESET

AT90S8515

10µ

5 00 Ω

PD0(RXD)

PD1(TXD)

PA0(AD0)

PA1(AD1)

PA2(AD2)

PA3(AD3)

PA4(AD4)

PA5(AD5)

PA6(AD6)

PA7(AD7)

1N4001

C13

25V

PD2(INT0)

PD3(INT1)

100n

V+

R13

1k5

C1+

PD4

PD5

IC4

10µ

25V

C1–

BC547B

PD6(WR)

R1IN

R1OUT

R2OUT

PD7(RD)

R2IN

HC16202NY

TP1

TP2

TP3

TP4

TP5

TP6

IC5

T1OUT

T2OUT

T1IN

T2IN

C17

C2+

INT

10µ

25V

PCF

MAX232

8574P

10µ

25V

C2–

SCL

SDA

V-

15p

3.6864MHz

10µ

25V

120 Ω

R10

220 Ω

+5V

1N4148

BT1

1N4148

3V6

NiCd

RESET

SCL

SDA

SCL

SDA

+5V

3k3

R17

BC547B

18p

SDA

OSC

3k3

IC7

SCL

SDA

SCL

IC2

IC3

RESET

RESIN

SENSE

PCF8583

X24C04

TL7705

OSC

R18

32,768kHz

REF

INT

C14

C15

120n

150n

000184 - 11

Figure 1. The switching clock is designed around the AT90S8515 microcontroller.

I 2 C

The common thread that runs through the

entire switching clock is the I 2 C bus. The I 2 C

bus requires three controller pins (the SDA

line is bidirectional). These pins are PD4, PD5

and PD6, where PD4 is used as SCL, and PD5

and PD6 are SDA-send and SDA-receive

respectively. Transistors T1 and T2 function

as (inverting) buffers for what ultimately

becomes the I 2 C bus. The controller software

generates the correct I 2 C bus logic levels.

This implementation does not permit the use

of the INT signal that some I 2 C devices pro-

desired contrast is adjusted with

potentiometer R11.

An integrated ‘supply voltage

supervisor’ (IC7) from Texas Instru-

ments was chosen to be the reset

signal source for the controller. This

TL7705 contains a ‘power-on-reset’

and a ‘brown-out’ circ uit. Th e IC has

a RESET as well as a RESET-output,

both of which are of the open collec-

tor type. R17 and R18 serve as pull-

up and pull-down resistors respec-

tively.

Capacitor C15 is the time deﬁning

element for the reset pulse. Because

the AT90S8515 has a negative reset

input, pin 5 of IC7 is used as the

reset source.

The communications interface for

a PC or terminal is the well-known

MAX232 (IC4). Capacitors C4-C7 are

the external components that are

necessary for the internal DC/DC-

converter, which generates the

10 V power supply.

5/2001

Elektor Electronics

GENERAL INTEREST

R13

IC2

IC6

IC3

C15

C14

IC7

R12

R17

Figure 2. The printed circuit board houses the entire circuit, including connectors, display and pushbuttons.

COMPONENTS LIST

Semiconductors:

D1,D2,D3 = 1N4148

D4 = LED, 3 mm, low current

D5,D6 = 1N4001

T1,T2,T3 = BC547B

IC1 = AT90S8515-8PC, programmed,

order code 000184-41

IC2 = PCF8583P

IC3 = 24C02

IC4 = MAX232

IC5 = PCF8574P

IC6 = 7805

IC7 = TL7705CP

S1-S4 = pushbutton, 1 make contact, e.g.,

D6-0 (ITT/Schadow)

Re1 = 5V reed relay, DIL case, 1 make

contact, e.g., Clare MSS21A05B

K1 = 6-way mini DIN socket, PCB mount

K2 = 9-way sub-D socket (female), angled

pins, PCB mount

K3 = 14-way SIL pinheader for connecting

LCD module

K4 = 2-way PCB terminal block, lead

pitch 5mm

K5 = mains adapter socket, PCB mount

LCD module, 2x16 characters, e.g., Sharp

type LM16A211

PCB, order code 000184-1 (see Readers

Services page and website)

Disk, contains hex ﬁle and source-code:

order code 000184-11

Resistors:

R1,R2 = 3k

R3,R4,R12,R13 = 1k Ω

R5-R8 = 2k

R9 = 120 Ω

R10 = 220

Ω

R11 = 500 Ω

preset H

R16 = 390

Ω

R17,R18 = 10k Ω

Capacitors:

C1,C2 = 15pF

C3 = 18pF

C4-C9,C17 = 10 µ F 25V radial

C10-C13,C16 = 100nF

C14 = 120nF

C15 = 150nF

Miscellaneous:

Bt1 = 3.6V NiCd battery with solder tags

X1 = 3.6864MHz quartz crystal

X2 = 32.768kHz quartz crystal

Elektor Electronics

5/2001

GENERAL INTEREST

vide. Also, no other masters may be

connected to this I 2 C-bus. The con-

troller software supports single-mas-

ter operation only!

The I 2 C bus is available externally

through connector K1. This allows

additional switching modules to be

connected. The real-time clock (RTC)

functionality is provided by a

PCF8583P (IC2). This IC contains all

the circuitry for a complete clock and

calendar. X2 and C3 are required for

the correct functioning of the oscilla-

tor.

While mains power is switched

on, the RTC is powered via D1.

Simultaneously, NiCd battery Bt1 is

then charged via D3 and R10. During

periods when the mains is not avail-

able, Bt1 provides the power supply

to the RTC via D2 and R9. This

ensures that the clock continues to

run and that the date and time are

properly maintained. Diode D1 pre-

vents the remainder of the circuitry

from being inadvertently powered

by the NiCd battery.

IC3 is a 256-byte EEPROM, which

is used to store the switching times.

IC5 is a switching unit whose

address lines A0, A1 and A2 are con-

nected to ground, which causes this

unit to have the lowest address. LED

D4 indicates the switch state of

Relay Re1. The relay contact is

closed when D4 is on. The contact

itself may be used freely.

Software

The software for the switching clock is written entirely in AVR Studio, which can be

obtained from the Atmel website. The software consists of seven modules: SKLOK.ASM,

DATADEF.ASM, DIVFUNC.ASM, I2CFUNC.ASM, MENU.ASM, 8583SET.ASM and SWK-

LOK.ASM. The latter is the main module. This has to be indicated as such in the project-

manager of AVR-Studio. The source code has been liberally commented and has been kept

as simple as possible. It is, however, impossible to discuss the details of the software in its

entirety.

On start up, the list of switching times is read from the EEPROM. Every switching event

consists of ﬁve bytes (hours, minutes, days, status/channel, 2’s complement of the previous

four bytes). When the ﬁfth byte is correct it is assumed the previous bytes contain valid

data. Reading from the EEPROM continues until an invalid 2’s complement value is

retrieved. All these values are copied to an array and stored in the SRAM of the controller.

(To reduce the I 2 C activity on the bus.) The main loop of the software compares the times

in the array with the time in the RTC. If one or more array entries match, then the corre-

sponding I/O module is set to the desired state. These status changes are also passed on to

the RS232 port of the microcontroller. Whenever a change is made to the switching time,

both the array, as well as the contents of the EEPROM, are updated.

Finally, a brief description of the functionality of each module:

SKLOK.ASM is the main module. This details all the interrupt vectors, program variables

and data deﬁnitions. It also contains the main loop of the program.

DATADEF.ASM contains all the text strings, which may be shown on the display. These

strings may be translated to another language. The length of the text must remain

exactly the same (adding extra spaces if necessary), and every string has to be termi-

nated with a ‘00’. The software uses this to test for ‘end of string’.

DIVFUNC.ASM contains the routines that are called from various modules.

The ﬁle I2CFUNC.ASM contains all the routines that are required for controlling the I 2 C-

bus. This software does not support the multi-master protocol. The microcontroller is

in this case the only master. This version of software does not support interrupts either.

The software is, however, perfectly adequate for most applications.

The ﬁle MENU.ASM is self- explanatory. This module is called from the main module and

it in turn calls the underlying program modules. The design is such that is easy to add

menu options.

8583SET.ASM is one of the modules that is called from within the menu. This controls all

the programming of the RTC. For details refer to the data sheet of the PCF8583.

SWKLOK.ASM is also called from the menu. This module takes care of the initial entering

or changing of the switching times.

Practical matters

An ordinary 9 V mains adapter con-

nected to K5 will suffice to power the

switching clock. There are no partic-

ular requirements since voltage reg-

ulator IC6 will provide a stabilised

5V.

Figure 2 shows the printed circuit

board that has been designed for the

switching clock. Although the circuit

contains rather a large number of

ICs, we have been able to keep the

size of the PCB down, as you can

see. There are no difficult details

associated with the construction of

this circuit. If you build it according

to the parts list everything will, in all

likelihood, go smoothly. Because the

various connectors and four push-

buttons are ﬁtted directly on the PCB

there is almost nothing required

externally.

As already indicated on the

schematic, the PCB contains only a

single relay stage (T3, Re1). Of

course, the other outputs of IC5

(TP1-TP7) can each be connected to

an external relay stage. For details

regarding the operation of the I 2 C-

bus and the various components you

are referred to the ‘I 2 C Peripherals’

data book from Philips (order number

9397 750 00306). When connecting

I/O modules a considerable distance

away from the switching clock, it is

necessary to buffer the I 2 C bus using

an 82B715 I 2 C Bus Booster (refer to

the June 1994 issue of Elektor Elec-

tronics , for example).

The programmed controller, the

printed circuit board as well as a

diskette with both the hex file and

source code may be purchased from

Elektor Electronics . The latter two may also

be downloaded from our web site at

www.elektor-electronics.co.uk .

Operation

The four pushbuttons S1-S4 have the follow-

ing functions: Return, Select, Down and Up.

After switching the clock on, the display

always indicates the time and date which is

read from the RTC:

Time: 14:44:59

We 11 Oct 2000

By pushing the Select button, the switching

clock menu becomes available. This menu

offers the following choices:

- ‘RTC-setting’

5/2001

Elektor Electronics

GENERAL INTEREST

day). By pressing the +- and –-but-

tons you can choose to switch on a

particular day, every day of the

week, working days only or week-

end only. An underscore (‘_’) indi-

cates that the switching event is

inactive on that particular day.

Once the correct days have been

selected we have to choose which

output (Channel) has to be switched

and whether it has to be turned on

or off. Channel 0 means relay Re1,

Channels 1-7 are found on P1-P7 of

IC5. Channels 8-15 are only available

if an additional PCF8574P is con-

nected to the I 2 C bus.

fied or whether there is a problem

with the I 2 C bus, for example.

- ‘Switch clock’

- ‘Module list’

- ‘Manual’

Manual

This makes it possible to switch a

channel on or off manually, bypass-

ing the timer.

The display indicates the same

thing as the switching clock chan-

nel/status setting. An asterisk indi-

cates whether the module is present

or not.

Using the Up- and Down-buttons one of the

four options can be selected. The chosen

option is entered by pressing the Select

button.

Setting the RTC

After selecting the RTC setting, you have to

successively enter the hours (24-hour clock),

minutes, month, day of the month, day of the

week, and year.

Use the Up and Down buttons to set the

correct value. Pushing the Select button takes

you to the next ﬁeld. Once all the values have

been entered you are returned to the menu.

Serial connection

The switching clock can be con-

nected to a PC using a standard 9-

way cable; a terminal program such

as HyperTerminal in Windows may

then be used to keep a log of switch-

ing activity. Whenever an output

changes state, the clock will send a

line of text to the PC, indicating the

time of day, name of the day (2 char-

acters), the date, channel number and

the new state of the channel. The ser-

ial port on the PC has to be set as fol-

lows: 9600 baud, 8 bits, no parity, 1

stop bit, no ﬂow control.

Module list

The main intention for this menu

option is to display which (switch-

ing) modules are present on the I 2 C

bus. The display may indicate the

following:

Switching Times

The clock can store up to 20 different switch-

ing times. When adjusting the switching

times, the number of the event (0-19) is dis-

played in the top right hand corner.

Shown below ‘Time:’ is the actual time

when the switching action will occur. Fol-

lowing that, indicated with asterisks, is/are

the day(s) for which this action is planned

(MTWTFSS = Monday, Tuesday through Sun-

8574 *———

8574A ————

This may come in useful to check if

all the modules are properly identi-

(000184-1)

Elektor Electronics

5/2001

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