The PortB pins can be used to send data in either direction, they can be used as inputs or outputs. As is common with other microprocessors and microcontrollers, the direction of data movement through each of the PortB lines is individually controlled by a data-direction register. Loading a ‘one’ into the data-direction register will make the corresponding PortB line into an output, on the other hand a ‘zero’, of course, will change it into an input.
Memories
This microcontroller has three memories, or four if we count the general purpose registers. The loading of the memory storage areas employs a serial transfer of data and is achieved by the SPI (serial programming interface).
One ROM storage area is achieved by a Flash memory organized as 1k×16. The program instructions are all either 16 or 32 bits wide and can be cleared and reprogrammed whilst remaining in circuit. It can be cleared and reprogrammed at least 1000 times. The contents of the Flash memory cannot be changed by the program being executed by the CPU and so is free from accidental corruption. The data is held in an EEPROM, which again can be cleared and reprogrammed electronically without removal from the device. It only holds 128 bytes of memory but is able to go through at least 100 000 cycles.
Data corruption can occur in the EEPROM if the supply voltage is reduced too far but this effect can be avoided by any one of the following three methods. The first we have already mentioned – use the Flash memory for critical data. The other two methods are ways to detect the reduction in voltage and immediately put the microcontroller into a safe condition. This is often referred to as ‘brown out protection’. An external circuit detects the falling voltage and applies a low voltage to the reset pin which effectively switches the chip off until the supply voltage recovers. The alternative is to put the microcontroller into a power-down sleep mode which is a power saving mode which has the effect of preventing any decoding or execution of any instructions – which, of course, precludes any ‘writes’ to the EEPROM.
It also has 128 bytes of SRAM (Static RAM) for the temporary storage of data and 32 8-bit general purpose registers that can be connected two at a time with the ALU (arithmetic and logic unit) which is the heart of the ‘brain’ within the microcontroller.
Clock
The AT90S/LS2343 has an internal RC oscillator which runs at 1 MHz, 4 MHz or 10 MHz depending on the version in use. It is one of the few of the micro devices that does not make use of an external crystal although it can use an external clock pulse. This external clock pulse only requires a single pin and hence we have an extra pin to use as an output.
Interrupts
There are only three interrupts. The first, and highest priority, is the reset which is activated by a low voltage applied to pin 1, a power-on reset or a signal from the watchdog. The next is an external interrupt request as described in a moment. Lastly, an overflow from the timer/counter circuit.
Pinout and package
These are shown in Figure 15.6 and we can see that it is available as an 8-pin DIL (dual in line) package which has two lines of pins and also the surface mount version, plastic gull wing SOIC (small outline IC).
Figure 15.6 AT90S/LS2343 pinout
Pin 1 – Active low reset. Must go low for at least 50 ns.
Pin 2 – External clock signal input or PortB 3
Pin 3 – PortB 4. All lines can sink 20 mA and therefore are able to power LEDs directly. Sinking means that the LED or other load is connected between the positive Vcc supply and a low voltage output at the port.
Pin 4 – Ground.
Pin 5 – PortB 0 or MOSI. In serial programming mode, MOSI is the serial data input.
Pin 6 – PortB 1 or MISO/INT0. In serial programming mode, MISO is the serial data output. This pin can also act as the external interrupt described in the previous paragraph.
Pin 7 – PortB 2 or SCK/T0. In serial programming mode, SCK is the serial clock input. This pin can also provide the timer/counter0 clock input.
Pin 8 – Vcc. The LS version requires a positive supply voltage that remains in the range 2.7–6.0 V.
Sleep modes
When the microcontroller is not being used, it can switch off some of its circuitry to save power. Sleep modes are employed in all modern microcontrollers and make an enormous difference to the overall life of an intermittently used device. This enables sealed units in toys and greetings cards to remain active for months or years.
Idle-mode
To see the benefits, this microcontroller has a normal operating current drain of 2.4 mA but when switched to the ‘idle’ mode, the current falls to 0.5 mA. It does this by stopping the CPU activity but allows the timer/counter, watchdog and interrupts to remain operational. This is about an 80% power reduction but we can do a lot better than that otherwise my musical socks would have stopped long ago.
Power-down mode
In this mode, only the external interrupt and watchdog (if enabled) continue to work and current falls to less than 1 microamp, which is a really impressive reduction in power. The microcontroller can be aroused from its sleep only by one of the following: an external reset, the watchdog (if enabled) or INTO external interrupt.
This is another modern RISC development and has many features that are similar to the AVR that we have just looked at. The AT90S/LS2343 was chosen as representative of the very small and basic microcontrollers found embedded in many products. This PIC16F84A example from Microchip Technology is a mid-range device which is larger and more capable than the AVR.
The PIC series ranges from a really simple 8-pin, 4 MHz microcontroller on a level with the AT90S/LS2343 that we have just considered up to a 40-pin 25 MHz device. As mentioned the PIC16F84A is a mid-size version that has 18 pins and runs up to 20 MHz.
The PICs are designed for easy use and are becoming increasingly popular as the first step into the world of microcontrollers. Microchip Technology provides a PICSTART™ PIC development system that provides, at a very reasonable price, an assembler, compiler, EPROM and EEPROM programmer, all the hardware manuals and even a sample PIC to play with. It should be mentioned that other companies have similar systems compatible with the PIC series and for other microcontrollers like the AVR and 8051 series.
It has proved to be such an easy, off the shelf, starting point that to many people ‘PIC’ is not only their first choice but is becoming used as a generic term for any microcontroller.
The general layout of the PIC16F84A is shown in Figure 15.7.
Figure 15.7 PIC16F84A
Supply voltage
The DC supply voltage must remain in the range 2.0–5.5 V for it to work happily though similar versions such as the PIC16F84 can run between 2.0 and 6.0 V.
Sleep mode
To save the current drain, a software instruction can put the PIC into a sleep mode. The supply current is very dependent on the clock frequency and is normally between 1 and 20 mA and when put to sleep the drain is reduced to approximately 1 µA. To obtain the lowest possible sleep current we should hold all I/O pins at VDD or VSS and disable any external clock and hold the master clear pin in a logic high state – not in the reset state. The purpose of all this is to prevent any voltages from floating up and down. If it did so, it would switch and the technology used results in very low currents drawn except at the moment of switching during which it causes a really high spike of current so the higher the frequency, the more often this spike occurs – hence the increased current.