ST AN431 Application note

INTRODUCTION

AN431

Application note

Using ST6 analog inputs for multiple key decoding

The ST6 on-chip Analog to Digital Converter (ADC) is a useful peripheral integrated into the sili con of the ST6 family members.The flexibility of the I/O port structure allows the multiplexing of up to 13/8 Analog Inputs into the converter in a 28/20 pin device for the ST6210/15 2k ROM and ST6220/25 4k ROM families, enabling full freedom in circuit layout. Many other members of the ST6 family also offer the Analog to Digital converter.

One of the more novel and practical applications of this converter, is to decode a number ofkeys. The technique is to connect the keys by resistive voltage dividers to the converter inputs.An example of key detection using 10 keys is illustrated in this note.

Using the Analog to Digital converter in this fashion does not require a static current and avoids false key detection.

BASIC CIRCUIT

The basic circuit of the key decoder consists of a pull-up resistor connected to the ST6 Analog to Digital converter input with the first key directly switching to ground. The following keys are then connected in sequenceto the ADCinput through serial resistors.The number of keys which may be detected depends on the tolerance of the resistors used. It can be seen that if more than one key is pressed at the same time, the key detected will be the next key in the chain closest to the ADC input. This also allows the keys in the keyboard to be prioritized.

June 2008

Rev 2 1/15

ANALOG KEYBOARD

PRINCIPLE OF OPERATION

The combination of the pull-up resistor, the serial resistors and the pressed key form a resistive voltage divider, generating a different voltage at the ADC input for each key pressed. The serial resistors are selected in order to give an equal distribution of voltage between V

and VSSfor

each switch combination to give the best noise margin between keys. When a key is pressed, the voltage at the ADC input is given by the activated voltage divider.

This analog voltage is converted by the ADC and the digital value is used to determine which switch is closed. Two successive conversions may be madeto avoid the influence of key bounce.

If the top key is pressed, the voltage measured is always zero.For n keys, the resistor values should be selected such that the voltage for the second key from top is V

/n, for the 3rd - 2xVDD/n, for the

Figure 1. Analog Keyboard resistor key matrix

Figure 2. Multiple key press

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Table 1. Key code ranges

Key Nr

10 E5-E6 25

Valid Code

Range

10 24

2 18-1A 22

3 30-33 22

5 63-68 20

6 7C-81 22

7 97-9B 21

8 B0-B4 22

9 CA-CD 24

49-4E 21

Distance to

next key

Table 2. Used resistors and Tolerance

ANALOG KEYBOARD

4th - 3xV

/n and for the nth - (n-1)xVDD/n. Re

sistor values from the tolerance set used must be selected to meet this requirement.

The recommended resistor values for a 10-key keyboard with 2% resistors from the E24 series, used with a 10kΩ pull-up resistor, are shown intable 2. If more current can be al lowed, then a 1kΩ resistor can be used in which case the serial resistor values should be divided by 10.

Resistor

Rp 10000 9800

R1 1100 1078

R2 1300 1274

R3 1800 1764

R4 2400 2352

R5 3300 3234

R6 5100 4998

R7 8200 8036

R8 16000 15680

Active Key R Error Range (LSB) Distance to next Key

S0 0 24

Value ( ) -2% ( )

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ANALOG KEYBOARD

PRACTICAL LIMITATIONS

Theoretically, for an ideal power supply, ADC and resistors, 255 keys could be detected. Practi cally however, it is necessary to take into account potential errors coming from:

- the power supply - the key resistivity - the resistor tolerance - the ADC error The power supply tolerance can normally be neglected providing noise is not present at a fre

quency within or above the frequency range of the RC delay of the resistive divider, as the ADC reference is normally provided by the powersupply of the ST6. For ST6 family members with ex ternal ADC reference voltage inputs, AV

and AVSSmay be used instead of VDDand VSS.

The sensitivity of the key can normally be neglected, as the resistance of the divider is high in comparison to it. If the key resistivity is significant, it should be added to the “serial” pull-down re sistance of the different dividers. The key resistivity variation must also be added to the toler ance of the serial pull-down resistor (see resistor tolerance following).

The resistor tolerance affects the tolerance of the dividers. Two situations must be taken into ac count:

a) minimum value of pull-up combined withmaximum values of pull-down = maximum voltage of the divider at the ADC input.

b) maximum value of the pull-up combined with the minimum values of pull-down = minimum voltage at the ADC input. These two cases give the maximum voltage variation of each divider (see Table 3). The voltage variation ranges of two dividers must not overlap otherwise the key cannot be decoded, even with an ideal converter.

Table 3. Effective Divider Resistors RX

Active

Key

R -2% ( ) R +2% ( )

1078 1122

2352 2448

4116 4284

6468 6732

9702 10098

14700 15300

22736 23664

38416 39984

88396 92004

Realistic converters require a margin between the range of variation. In the case of a significant variation in the key resistivity, the maximum resistivity of the key has to be added to the value of the pull-down resistor in case a). For case b) no error needs to be added as the resistivity cannot be less than 0 Ω.

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ANALOG KEYBOARD

The linearity of the ADC converter of the ST6 is normally specified for ⎛2 LSB, therefore a minimum distance of 4 LSB is needed between the edges of the resistance tolerance ranges.For the best re sults, a minimum of 8 LSB should be used (see Table 4).

Table 4. Voltage at the ADC-Input,Converter Results (5V supply)

V (Rxmin-Rpmax) V (Rxmax-Rpmin)

Active Key

V hex. dec. V hex. dec.

S0 0.00 00 0 0.00 00 0

S1 0.48 18 24 0.51 1A 26

S2 0.94 30 48 1.00 33 51

S3 1.44 49 73 1.52 4E 78

S4 1.94 63 99 2.04 68 104

S5 2.44 7C 124 2.54 81 129

S6 2.95 97 151 3.05 9B 155

S7 3.45 B0 176 3.54 B4 180

S8 3.95 C9 201 4.02

Table 5. AD-Converter Results

Active Key

S0 0 24 0-0

S1 2 22 18-1A

S2 3 22 30-33

S3 4 21 49-4E

S4 5 20 63-68

S5 5 22 7C-81

S6 5 21 97-9B

S7 4 22 B0-B4

S8 3 24 C9-CD

S9 2 25 E5-E6

R Error Range

(LSB)

Distance to next

Key

Valid Key Range

205

5/15

ANALOG KEYBOARD

EXTENSION FOR WAKE UP

ST6 family members with the Analog input capacity can also generate a wake-up opera tion (from WAIT or STOP modes) on the pressing of a key. This can be achieved by a modification of the circuit shown in figure 1. The pull-up resistor is not connected to V

but to an additional I/O port bit. During key polling, this additional port bit is set to output mode active high, thus effectively switching

to the pull-up resistor. The resistance of

the pull-up resistor must be high enough to give no significant voltage drop, or the result ing error must be calculated and taken into account. The other I/O bit is used as the Ana log input tothe ADC as in theoriginal circuit. During the wait for the key press, the first I/O pin, used to pull the pull-up resistor high to

while polling, is switched into a high im-

pedance state (e.g. open drain output mode). The second I/O pin, used as the ADC input while polling, is switched to the interrupt input with pull-up mode. The internal pull-up is in the range of 100k, in comparison to the 1k 10k of the external resistor used during polling. If any key is now pressed an interrupt will be generated if the voltage at the second I/O pin is below the Schmitt trigger low level threshold.The serial resistors in the keyboard chain must not be too high in this case, therefore the maximum number of keys is reduced in comparison to the normal mode.

Figure 3. Keyboard wake-up circuit

Figure 4. Keyboard reading

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Figure 5. Interrupt configuration

ANALOG KEYBOARD

APPENDIX A: Key Input by Polling

;************************************************************************** ;* *

;* SGS-THOMSON GRAFING * ;* * ;* APPLICATION NOTE 431 - ST6 *

* ;* * ;* Use of ADC inputs for multiple key decoding * ;* *

* ;* With the inbuilt A/D converter of any ST6 it is easy to * ;* implement a small routine which enables ONE port pin, con- * ;* figured as an ADC input, to decode up to ten different switches*

;* All that is necessary is to set one port pin as an ADC input * ;* Then the program runs in an endless loop until one of the *

* ;* connected keys is pushed. *

* ;* The value from the ADC data register is then used to decide * ;* how the program will continue,on reaction to the key-push. *

* ;* * ;**************************************************************************

;***REGISTERS***

ddrpb .def 0c5h ;port B data direction register orpb .def 0cdh ;port B option register drpb .def 0c1h ;port B data register adr .def 0d0h ;A/D data register adcr .def 0d1h ;A/D control register a .def 0ffh ;accumulator

;***CONSTANTS***

inpall .equ 000h ;used for setting all pins input peg1_2 .equ 00ch ;border to distinguish between switch1 and switch2

peg2_3 .equ 025h ;border to distinguish between switch2 and switch3 peg3_4 .equ 03eh ;border to distinguish between switch3 and switch4 peg4_5 .equ 058h ;border to distinguish between switch4 and switch5 peg5_6 .equ 072h ;border to distinguish between switch5 and switch6 peg6_7 .equ 08ch ;border to distinguish between switch6 and switch7 peg7_8 .equ 0a5h ;border to distinguish between switch7 and switch8 peg8_9 .equ 0beh ;border to distinguish between switch8 and switch9

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ANALOG KEYBOARD

peg9_10 .equ 0d9h ;border to distinguish between switch9 and switch10

ldi ddrpb,inpall ;sets all port B pins low — all input ldi orpb,01h ;option register:

;sets bit b0 high, the rest low

ldi drpb,01h ;direction register:

;sets bit b0 high, the rest low ;— pb0 becomes analog input ; pb1-7 become input with pull-up, but ; are not used here (only one pin may be ; analog input for A/D at the same time)

ldi adcr,30h ;A/D control register:

; 0011 0000 — -activate A/D converter ; -start conversion ; -disable A/D interrupt

loop: jrr 6,adcr,loop ;loop until the End Of Conversion bit is

;set (indicator that a conversion has ;been completed)

ld a,adr ;load acc with the result of the A/D

;conversion

;switches

; values which represent the different

sw1: cpi a,peg1_2 ;compare with peg1_2

jrnz sw2 ;A/D result was smaller than peg1_2 jp s1 ; — switch1 was pressed: jump to s1

;now the result is compared with the

sw2: cpi a,peg2_3 ;compare with peg2_3

jrnz sw3 ;A/D result was smaller than peg2_3 jp s2 ; — switch2 was pressed: jump to s2

sw3: cpi a,peg3_4 ;compare with peg3_4

jrnz sw4 ;A/D result was smaller than peg3_4 jp s3 ; — switch3 was pressed: jump to s3

sw4: cpi a,peg4_5 ;compare with peg4_5

jrnz sw5 ;A/D result was smaller than peg4_5 jp s4 ; — switch4 was pressed: jump to s4

8/15

sw5: cpi a,peg5_6 ;compare with peg5_6

jrnz sw6 ;A/D result was smaller than peg5_6 jp s5 ; — switch5 was pressed: jump to s5

sw6: cpi a,peg6_7 ;compare with peg6_7

jrnz sw7 ;A/D result was smaller than peg6_7 jp s6 ; — switch6 was pressed: jump to s6

sw7: cpi a,peg7_8 ;compare with peg7_8

jrnz sw8 ;A/D result was smaller than peg7_8 jp s7 ; — switch7 was pressed: jump to s7

sw8: cpi a,peg8_9 ;compare with peg8_9

jrnz sw9 ;A/D result was smaller than peg8_9 jp s8 ; — switch8 was pressed: jump to s8

sw9: cpi a,peg9_10 ;compare with peg9_10

jrnz sw10 ;A/D result was smaller than peg9_10 jp s9 ; —> switch9 was pressed: jump to s9

ANALOG KEYBOARD

sw10: jp s10 ;A/D result was greater than peg9_10

; — switch10 was pressed: 0

; —> switch10 was pressed: s10

;

;*** the routines handling to the reaction to the individual key presses ;*** are to be included here.

s1: s2: s3: s4: s5: s6: s7: s8: s9: s10:

9/15

ANALOG KEYBOARD

APPENDIX B: Key Input by Interrupt

;************************************************************************** ;* *

;* SGS-THOMSON GRAFING * ;* * ;* APPLICATION NOTE 431 - ST6 * ;* * ;* Use of ADC inputs for multiple key decoding * ;* * ;* With the inbuilt A/D converter of any ST6 it is easy to * ;* implement a small routine with which you can recognize * ;* if one of nine connected keys is pushed by creating an * ;* interrupt. The program can then decide how it will react * ;* to the key pushed. * ;* * ;* *

;**************************************************************************

;***REGISTERS***

ddrpb .def 0c5h ;port B data direction register orpb .def 0cdh ;port B option register drpb .def 0c1h ;port B data register ior .def 0c8h ;interrupt option register adr .def 0d0h ;A/D data register adcr .def 0d1h ;A/D control register a .def 0ffh ;accumulator

;***CONSTANTS***

inpall .equ 000h ;used for setting all pins input peg1_2 .equ 00ch ;border to distinguish between switch1 and switch2

; en_kint (enable key-interrupt) sets the registers in a way that pushing ; any key will cause an interrupt. This subroutine must be called to ; re-enable the key interrupt (e.g. after handling the key service routine)

10/15

ANALOG KEYBOARD

en_kint:

ldi ddrpb,inpall ;sets all port B pins low — all input ldi orpb,02h ;option register:

; sets bit b1 high, the rest low

ldi drpb,01h ;data register:

; sets bit b0 high, the rest low ;— pb0 becomes input, no pull-up, no int ; pb1 becomes input with pull-up and int. ; pb2-7 become input with pull-up, but ; are not used here

ldi ior,10h ;interrupt option register:

;— set D4: enable all interrupts ; reset D5: falling edge on int.input(#2)

ret ;return to the calling address

;*** hd_kint (handle key interrupt) interrupt service routine ;*** evaluates the data resulting in pushing a key. ;*** Interrupt vector #2 (0ff4h and 0ff5h) must point (jump) to hd_kint.

hd_kint: ldi drpb,03h ;data register:

; 0000 0011

ldi ddrpb,01h ;data direction register:

; 0000 0001 ; — pb0 becomes output

ldi orpb,03h ;option register:

; 0000 0011 ; — pb0: push-pull output ; — pb1: ADC-input ; pb2-7 become input with pull-up, but ; are not used here

ldi adcr,30h ;A/D control register:

; 0011 0000 — -activate A/D converter ; -start conversion ; -disable A/D interrupt

loop: jrr 6,adcr,loop ;waits until the End Of Conversion

; bit is set (indicator that a conversion ; has been completed)

ld a,adr ;load acc with the result of the A/D

; conversion ;now the result is compared with the ; values which represent the different ; switches

11/15

ANALOG KEYBOARD

sw1: cpi a,peg1_2 ;compare with peg1_2

jrnz sw2 ;A/D result was smaller than peg1_2 jp s1 ; — switch1 was pressed: jump to s1

sw2: cpi a,peg2_3 ;compare with peg2_3

jrnz sw3 ;A/D result was smaller than peg2_3 jp s2 ; — switch2 was pressed: jump to s2

sw3: cpi a,peg3_4 ;compare with peg3_4

jrnz sw4 ;A/D result was smaller than peg3_4 jp s3 ; — switch3 was pressed: jump to s3

sw4: cpi a,peg4_5 ;compare with peg4_5

jrnz sw5 ;A/D result was smaller than peg4_5 jp s4 ; — switch4 was pressed: jump to s4

sw5: cpi a,peg5_6 ;compare with peg5_6

jrnz sw6 ;A/D result was smaller than peg5_6 jp s5 ; — switch5 was pressed: jump to s5

sw6: cpi a,peg6_7 ;compare with peg6_7

jrnz sw7 ;A/D result was smaller than peg6_7 jp s6 ; — switch6 was pressed: jump to s6

sw7: cpi a,peg7_8 ;compare with peg7_8

jrnz sw8 ;A/D result was smaller than peg7_8 jp s7 ; — switch7 was pressed: jump to s7

sw8: cpi a,peg8_9 ;compare with peg8_9

jrnz sw9 ;A/D result was smaller than peg8_9 jp s8 ; — switch8 was pressed: jump to s8

sw9: jp s9 ;A/D result was bigger than peg8_9

; — switch9 was pressed: jump to s9 ;

;*** The routines handling the reaction to the individual key presses ;*** are to be included here

12/15

s1: s2: s3: s4: s5: s6: s7: s8: s9:

;*** Each routine must end with the following lines in order to enable ;*** another interrupt when the next key is pressed.

call en_kint ; enable another interrupt

return: reti

ANALOG KEYBOARD

13/15

ANALOG KEYBOARD

Table 6. Revision history

Date Revision Description of changes

September 1992 1 Initial release

19-June-2008 2 Logo modified

14/15

ANALOG KEYBOARD

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ST AN431 Application note

Specifications and Main Features

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