We shall now
briefly look at the remaining
PicBasic commands in alphabetical order which
are useful during the program development. More details about these
commands can be obtained from the PicBasic manual.
EEPROM
EEPROM Location,
(constant, constant,….., constant)
This command stores constants in consecutive bytes in on-chip
EEPROM memory. The command
only works with the PIC microcontrollers that have EEPROM, such as the PIC16F84, PIC16F877,
etc. Location
is optional,
and if omitted the first EEPROM
location is assumed. Constants can be numeric constants or string constants. Strings are stored as consecutive bytes of ASCII values. An example is given below.
EEPROM 3, (5, 2, 8) ‘ Store 5 in location 3,
‘ 2 in location 4, and 8 in
‘ location 5
END
END
Stops
execution
and enters low power
mode. The command has no parameters.
HIGH
HIGH Pin
Makes the specified pin an output pin and sets it to logic 1. Pin only applies to PORTB pins and it can take values from 0 to 7. In the following example, bit 1 of PORTB is configured as an out- put pin and is set to logic 1:
HIGH
1
I2CIN
I2CIN Control, Address, Var, (,Var)
This command is used to read data from serial EEPROMs with a 2-wire I2C interface. A list of some compatible devices is given in Table 4.4. The lower 7 bits of the Control byte contain a 4-bit control
code, followed by
the chip
select or additional address information,
depending on the device used.
As shown in Table 4.4, the 4-bit control code for EEPROMs is “1010”. The high-order bit (MSB) of the Control byte is a flag indicating whether the Address is to be sent as 8 bits or 16 bits. If the flag is low, the Address is sent as 8 bits, and if it is high, the Address is sent as 16 bits. (,Var) shown in the command list is used only for 1-bit information.
The I2C data and clock lines are predefined in the PicBasic library as bit 0 of PORTA (RA0) and bit 1 of PORTA (RA1), respectively.
For example, when communicating with a 24LC02B
EEPROM, the required Address is
8 bits, the control code is “1010” and chip select or additional address
information is not required and can be
assumed to be 0. The required Control byte
is then “01010000”.
Figure 4.2 shows how
the 24LC02B (or any other serial EEPROM) can be connected to a PIC
microcontroller. In this example, a PIC16F84 is used and pin RA0
and RA1 are connected to the data and clock pins of the EEPROM, respectively. These
are the only connections required to communicate with an I2C-compatible device.
As shown in the figure, the I2C lines should be con-
nected to Vdd ( 5 supply)
with 4.7K resistors.
Some
I2C compatible EEPROM
Device
|
Capacity
|
Control
|
Address
size
|
24LC01B
|
128 bytes
|
01010xxx
|
8 bits
|
24LC02B
|
256 bytes
|
01010xxx
|
8 bits
|
24LC04B
|
512 bytes
|
01010xxb
|
8 bits
|
24LC08B
|
1 K bytes
|
01010xbb
|
8 bits
|
24LC16B
|
2 K bytes
|
01010bbb
|
8 bits
|
24LC32B
|
4 K bytes
|
11010ddd
|
16 bits
|
24LC65B
|
8 K bytes
|
11010ddd
|
16 bits
|
bbb block
select bits (each block is 256 bytes)
ddd device
select bits
xxx don’t care
I2C Connections
to a PIC microcontroller
In the following example, a data byte is read from address 20 of the serial EEPROM and stored in variable B1. Note that the Control byte is set to “01010000”, Address is
assigned variable B0 and
value 20 stored in it, and the byte read from the EEPROM is stored in data register B1.
Symbol con %01010000
Symbol addr B0
addr 20 ‘ Set address to 20
I2CIN con, addr,
B1 ‘ Read from address 20 to B1
I2COUT
I2COUT Control, Address, Value (,Value)
This command is used to send data to an I2C compatible device such as a serial EEPROM
described in command I2CIN. The (,Value)
in the command is used for 16-bit information.
When writing data to an EEPROM, it is necessary to wait about 10 ms (device dependent) for the write operation to complete before attempting to write again. In the example given below, data byte
10 is written to address 30, and also data byte in variable B5 is written to address 31 of an EEPROM.
Symbol con %01010000
Symbol addr B0
addr 30 ‘ Set address to 30
I2COUT con, addr, (10)
‘ Write
byte 10 to address 30
PAUSE 10
‘ Wait 10ms
addr 31 ‘ Set address to 31
I2COUT con, addr, (B5)
‘ Write
byte in B5 to address 31
PAUSE 10
‘ Wait 10
ms
INPUT
INPUT Pin
This makes the specified PORTB
pin an input. Pin is from 0 to 7. For
example, INPUT 2 ‘ Make
RB2 an input pin
LOOKDOWN
LOOKDOWN Search, (Constant, Constant,…..), Var
This command provides a look-up table. It looks down a list of Constants and
compares each one with
the Search value. If a match is found, the position of the match is stored in Var. Note that the first Constant is assumed to be at position 0. The Constant list can be numeric or string constants. In the following example, if we assume that variable B0 has value 5 then variable B1 will contain
3 which is the position
of 5 in the table: LOOKDOWN B0, (0, 8, 9, 5, 12, 0, 1), B1
LOOKUP
LOOKUP Index,
(Constant, Constant,….), Var
This command is used to retrieve values from a table. When Index is
0, Var is loaded
with the first
Constant; when Index is 1, Var is loaded with the second Constant and so on. In the following
example, if we assume
that variable B0 has value 3, variable B1 will be loaded with 8 which is the
3rd element in the table starting
from 0:
LOOKUP B0, (0, 9, 0, 8, 12,
32), B1
LOW
LOW Pin
This command makes the specified pin an output pin and clears it to logic 0. Pin only
applies to PORTB pins and it can take values from 0 to 7. In the following example,
bit 2 of PORTB is con- figured
as an output pin and is cleared to logic
0:
LOW 2
NAP
NAP Period
The NAP command
places the PIC microcontroller in low-power mode for a while to save power in
battery applications. The Period is a variable from 0 to 7 and the approximate
delay is given in Table
Delay in NAP command
Period
|
Delay (s,
approx)
|
0
|
18 10 3
|
1
|
36 10 3
|
2
|
72 10 3
|
3
|
144 10 3
|
4
|
288 10 3
|
5
|
576 10 3
|
6
|
1.152
|
7
|
2.304
|
In
the following example the microcontroller is put into low power mode for just over 1 s: NAP
6
OUTPUT
OUTPUT Pin
This command
makes the specified pin of PORTB an output
pin. Pin can take values
from 0 to 7. In the following example,
bit 2 of PORTB (RB2) is made an
output pin:
OUTPUT 2
PAUSE
PAUSE Period
This is one of the
commonly used
commands to delay a program by a specified
amount. Period is in milliseconds and can range from 1 to 65,535
ms (i.e. just over one minute). PAUSE does not put the microcontroller into low-power mode. In the following example, the program is delayed by 1 s:
PAUSE 1000
PEEK
PEEK Address, Var
This command is used to read the value of a RAM register at the specified Address and then put the value into variable Var. The PEEK command can be used to access all registers
of the PIC microcontroller including the Port registers,
A/D converter registers, etc.
In the following example, the 8-bit value of PORTB is read
and stored in variable B0: Symbol PORTB 6 ‘ PORTB
register address
PEEK
PORTB,
B0 ‘ Read PORTB into B0
POKE
POKE Address, Var
This command is used to send data to a RAM register at the specified Address.
The POKE com- mand
can be used to send data to all accessible registers of the PIC microcontroller, including the PORT
registers, PORT direction registers, A/D converter
registers, etc.
In the following example, TRISB is cleared to 0 so that all PORTB pins are outputs.
The hexa-
decimal value 24 is sent to PORTB.
Symbol TRISB
$86
‘ TRISB
register address
Smbol PORTB
6 ‘ PORTB
register address
POKE TRISB,
0 ‘ Clear TRISB
POKE PORTB, $24 ‘ Send $24 to PORTB
POT
POT Pin, Scale, Var
This command could be useful to read an analogue voltage
if the microcontroller has no built-in A/D converter. Pin is a PORTB pin and can take a value between 0 and 7. For this command to work, a resistor and a capacitor are serially connected to a port pin as shown in Figure 4.3. When a
voltage is applied to a resistor–capacitor circuit, the voltage across the capacitor
rises exponen- tially as the capacitor
is charged through the resistor. The charge time is dependent
on the value of the resistor
and the capacitor.
Resistor and capacitor
connected to an I/O pin
POT 1, 255, B0
PULSIN
PULSIN Pin, State, Var
The PULSIN command measures
the pulse width
of any signal connected to a PORTB pin. With a 4 MHz crystal or resonator, the pulse width will be measured in 10 s units. If State is 0, the width
of a low pulse is measured;
if Scale is 1, the width if a high pulse is measured. The meas-
ured value in 10 s units is stored in variable Var.
Var can be a byte or a word. If a word is used,
it can take values 1 to 65,535,
i.e. the minimum
pulse width that can be measured is 10 s and the maximum is 655,350 s. If a byte is used, the
range of the measurement is 10 to 2550 s.
PULSOUT
PULSOUT Pin, Period
This command generates a pulse on a PORTB pin (Pin can
be 0 to 7) of specified Period in 10 s
units. The Period is a word and thus pulses of up to 655,350
s can be generated. The specified pin is automatically made an output pin.
For example, to generate a 500- s pulse on pin 1 of PORTB,
we need the command
PULSOUT 1, 50
PWM
PWM Pin, Duty, Cycle
This command outputs a
Pulse-Width-Modulated (PWM) signal on
the specified PORTB pin (Pin can be 0 to 7). The Duty is the pulse duty-cycle and can range from 0 to 255. 0 corresponds
to a 0% duty-cycle, and 255 corresponds to a 100% duty-cycle. The generated
PWM pulse is repeated
Cycle times. The specified port Pin is made an output just before the command is executed and reverts
to an input after the pulse is generated.
In the following example,
a 200-cycle PWM signal is generated
on bit 0 of PORTB with a duty- cycle of 50%:
PWM 0, 127, 200
Another use of this
command is to generate an
analogue signal by
sending the output to a resistor–capacitor circuit
as shown in Figure 4.4.
In this circuit,
the voltage across
the capacitor will vary depending on the Duty and the Cycle of the pulses.
Using PWM signal for D/A conversion
RANDOM
RANDOM Var
This command generates
a random number and stores in word variable Var.
For example, to gen-
erate a random number and store in W1
use the command:
RANDOM W1
READ
READ Address, Var
This command is used to read a byte
from the specified Address of the built-in EEPROM mem- ory. The byte read is stored in variable Var.
This command can only be used with PIC microcon-
trollers that have built-in EEPROM memory (such as
PIC16F84, or PIC16F877).
In the following example, the byte at address 10 of EEPROM
is read and stored in variable B1:
READ 10, B1 ‘ Read
byte at address 10
‘ and store in B1
REVERSE
REVERSE Pin
This command reverses the mode of a PORTB pin (Pin
can be from 0 to 7). If the pin is an input,
it is made an output. Similarly, if the pin is an output, it is made an
input.
In the following example, bit 2 of PORTB is first made an output pin, then changed to an input pin:
OUTPUT 2 ‘ RB2
is output pin
REVERSE 2 ‘ RB”
is an input pin
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