ST AN979 APPLICATION NOTE
AN979
APPLICATION NOTE
DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC
By Microcontroller Division Applications
1 INTRODUCTION
The goal of this application note is to present a standard example of the us e of the Analog to
Digital Converter (ADC) of the ST7.
In this note, the ADC is used to convert a 16-key analog keyboard. The technique for the keyboard is to connect the keys by resistive dividers to one of the converter inputs. T he Application note is based on ST72F324 which has an embedded 10-bit ADC.
2 ST7 / KEYBOARD INTERFACE
Connect the analog keyboard to one of the analog inputs of the ST 7 and connect the high and
low voltage references as shown in Figure 1.
Figure 1. ST7 / keyboard interface set-up
KEYBOARD
ST7
ADC
V
DD
V
SS
analog
input
AINx
high voltage reference
low voltage reference
keyboard
output
3 ST72324 CONFIGURATION
The application has been tested with a ST72F324 configured as described below. Refer to the
datasheet for more details on configuring the ST72F324.
3.1 I/O CONTROL
Depending on the num ber of pins av ailable on the device p ackage , th e S T7 theor etically allows up to 16 multiplexed analog inputs to the ADC, but in practice, on the ST72F324 device,
there can be up to 12 analog inputs. They are alternate functions I/O Port D (PD0-PD5), Port
AN979/0303 1/14
1
DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC
F (PF0, PF4), and Port C (PC0, PC1, PC5, PC7). The I/O port pins used by the ADC must be
configured as floating inputs to avoid conflicts in alternate function mode.
Refer to the Data Sheet for information on configuring the I/O ports.
3.2 ANALOG TO DIGITAL CONVER TER
The ST7 ADC is a 10-bit successive approximation converter, with internal sample and holdcircuitry.
3.2.1 ADC control
You control the ADC using the ADC Control Status Register (ADCCSR).
CH0CH1CH2CH30ADONSPEEDEOC
The EOC bit is the end of conversion bit:
- When this bit is set, the conversion is done and result can be read from the ADC
Data Registers (ADCDRL & ADCDRH).
- When the bit is reset, the conversion is not complete.
The SPEED bit:
- When this bit is set, f
ADC
- When this bit is reset, f
= f
ADC
CPU
= f
/ 2.
CPU
/ 4.
The ADON bit:
- Enable ADC and start conversion when this bit is set.
- Disable ADC and stop conversion when this bit is reset.
CH3-CH0 bits:
- They are used to s elect which analog i nput to convert. In the ST72324 there are 12
analog pins.
3.2.2 Characteristics
The conversion time is 15 ADC cycles including a sampling time of 4 ADC cycles.
The ADC is linear and the digital result of the conversion is given by the formula:
1023*Input Voltage
Digital Result =
Reference Voltage
3.2.3 Process
First the analog input pins must be configured as floating inputs (see Section 3.1).
2/14
DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC
Then the analog channel to c onvert must be s elected using CH3-CH0 bits of ADCCSR regsister.
Then select the SPEED of conversion.
Setting the ADON bit will switch the converter on.
Figure 2. Flowchart: initialization of the ADC
KBD_init
I/O Initialization
Channel Selection
SPEED Selecti on
ADC ON
return
Once a conversion is done, the EOC bit is set by hardware. It will be r eset when the ADCDRH
register is read.
Once enabled, conversions will run continuously until the peripheral is disabled.
3/14
DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC
Figure 3. Flowchart: conversion process
Conversion Process
KBD_init
no
EOC = 1 ?
yes
read ADCDRL
read ADCDRH
4 ANALOG KE YBOARD
4.1 PRINCIPLE
The purpose is to recognize a key when pressed. In an analog keyboard each key is associated with a voltage. The description of an analog keyboard is given by Figure 4.
Figure 4. Hardware description of a keyboard with 16 keys
V
DD
V
key
(Keyboard Value)
R
up
R
0
key 0
key 1
R
1
Σ
with R
is a pull-up resistor. So, when no key is pressed, V
R
up
4/14
>> R
j
up
R
13
R
14
key 14
key 15
is equal to VDD.
key
V
SS
DRIVING AN ANALOG KEYBOARD WITH THE ST7 ADC
When ‘key i’ is pressed (where i is in the range 1 to 15), the resistor R
is connected to Vss.
i-1
Then we have a resistive divider
and V
is given by the formula:
key
V
key i
=
(V
DD
R
- VSS)
+
up
i - 1
Σ
j = 0
i - 1
Σ
j = 0
R
R
j
j
So the corresponding voltage of each key i s given by the values of the r esistor s. An equal distribution of voltage between V
To recognize a key, the user will measure V
and VSS is usually recommended.
DD
and will be able to decide which key was
key
pressed.
4.2 PRACTICAL LIMITATIONS
Theoretic ally, wi th an 10- bit ADC , 1023 key s can be decod ed. But po tenti al erro rs must be
taken into account. They can come from the power supply, the key resistivity, the resistor tolerance, the ADC conversion errors.
The resistor toleran ce is the m ain limitati on as us ually 5% tole rance res istors a re us ed. It i s
advised to use a 1% tolerance resistor for the pull-up. Changing this resistor greatly improves
the keyboard as the pull-up has an influence on every key.
The AD C t otal u nad justed e rr or (T UE ) for th e devi ce is spec ifi ed a s 4 LS B. S o, it h as t o be
taken into account to avoid any key decision error.
These parameters will reduce the number of keys that can be efficiently decoded.
5/14
Loading...