Arbitration

Whenever the bus is free, any unit may start to transmit a message. If 2 or more units start transmitting messages at the same time, the bus access conflict is resolved by bitwise arbitration using the IDENTIFIER. The mechanism of arbitration guarantees that neither information nor time is lost. If a DATA FRAME and a REMOTE FRAME with the same IDENTIFIER are initiated at the same time, the DATA FRAME prevails over the REMOTE FRAME. During arbitration every transmitter compares the level of the bit transmitted with the level that is monitored on the bus. If these levels are equal the unit may continue to send. When a ’recessive’ level is sent and a ’dominant’ level is monitored (see Bus Values), the unit has lost arbitration and must withdraw without sending one more bit.

Sleep Mode / Wake-up

To reduce the system’s power consumption, a CAN-device may be set into sleep mode with-out any internal activity and with disconnected bus drivers. The sleep mode is finished with a wake-up by any bus activity or by internal conditions of the system. On wake-up, the internal activity is restarted, although the transfer layer will be waiting for the system’s oscillator to stabilize and it will then wait until it has synchronized itself to the bus activity (by checking for eleven consecutive ’recessive’ bits), before the bus drivers are set to “onbus” again. In order to wake up other nodes of the system, which are in sleep-mode, a special wake-up message with the dedicated, lowest possible IDENTIFIER (rrr rrrd rrrr; r = ’recessive’, d = ’dominant’) may be used.

The use of TRIS causes concern for some users. The Microchip data sheets recommend not using TRIS instructions for upward compatibility. If you had existing ASM code and it used TRIS then it would be more difficult to port to a new Microchip part without TRIS. C does not have this problem, however; the compiler has a device database that indicates specific characteristics for every part. This includes information on whether the part has a TRIS and a list of known problems with the part. The latter question is answered by looking at the device errata.

CCS makes every attempt to add new devices and device revisions as the data and errata sheets become available.
PCW users can edit the device database. If the use of TRIS is a concern, simply change the database entry for your part and the compiler will not use it.

A switched-mode power supply, switching-mode power supply or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator. While a linear regulator maintains the desired output voltage by dissipating excess power in a “pass” power transistor, the SMPS rapidly switches a power transistor between saturation (full on) and cutoff (completely off) with a variable duty cycle whose average is the desired output voltage. The resulting rectangular waveform is low-pass filtered with an inductor and capacitor. The main advantage of this method is greater efficiency because the switching transistor dissipates little power in the saturated state and the off state compared to the semiconducting state (active region). Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation from the higher efficiency. Disadvantages include greater complexity, the generation of high amplitude, high frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof.