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Micro Mid-term

Terms in this set (23)

Explain the terms active low on an output and draw the circuit for an active low LED.
The output is active (e.g. on) when the line controlling the device is low
Explain the term asynchronous when applied to a USART.
There is no clock signal between the transmitter and receiver. Both systems rely on their internal clocks, the correct baud rate setting and the start bit.
Why is a multiplexer required in the AT90 ADC?
There are 8 ADC inputs, but only 1 ADC convertor, thus a multiplexer is required to switch the ADC inputs
How much time will it take to send 8 bits of data (with no parity) out of a UART if the baud rate is 9600 bits per second
10 bits, including start and stop bit, 1/9600 One bit takes 104us, 10 bits 1.04ms
Explain the term flag and how it is applied in a microcontroller.
A flag shows that an event has occurred. The microcontroller will periodically check the flag and take the appropriate action if the event has occurred
Explain the term odd parity giving an example.
Odd parity is used for error checking when transmitting data. (can detect one bit errors). Odd parity means the number of 1's including the parity bit should be an odd number
Why are pullup resistors required on floating inputs?
Floating inputs have indeterminate values and are susceptible to noise. The pullup resistor will connect Vcc to the input using a 1ok resister.
Why is a while(1) loop required in a microcontroller code but not on a PC
The program must repeat forever in a microcontroller. In a PC your program must be able to be stopped when the user has finished
Explain the terms full and half duplex and state whether the UART is full or half duplex.
Full duplex can transmit and receive at the same time. Half duplex can transmit and receive but not at the same time
UART is full duplex
Explain why, when reading data on the SPI, you must first write to the SPI data register
The SPI is a ring system where the data bits are controlled by the master clock. This clock is activated by the master when the SPI data register is written to. After the write occurs, the clock is activated allowing the data in the slave to be transferred to the master data register
Explain what happens when the TWINT bit in the TWCR (TWI control register) is set to a 1
If the hardware sets it, it means that the current task has been executed
If software sets it, then the flag is cleared and the next task is started
Note either is correct
How many ADC channels does the AT90 have, and how do you select which channel to use
The ADC has 8 channels, the ADMUX register bits 4-0 are used to select which channel
to use
Explain the difference between the UDRE data register empty flag and the TXC, transmit complete flag,
(include the transmit data and transmit serial shift registers in your explanation).
UDRE flag is set when the transmit data register is empty. (used in normal transmission)
TXC flag is set when both the transmit data register and the serial shift register is empty
(used when powering down the transmitter
State the full name for an ISR and explain how it is called.
Interrupt Service Routine, is called by a hardware event such as an ADC conversion
complete, USART receive buffer full.
Explain the term odd parity, and how a receiver would check to see if the parity bit was
correct.
Odd parity is an error checking system, the total number of bytes received including the
parity bit must be an odd number. The receiver will generate the odd parity bit from the
received data, generate a parity bit, and compare it to the parity bit that was transmitted.
Briefly explain why interrupt are preferred to polling giving two disadvantages of using
polling.
Polling requires computer resources to constantly check if the device needs servicing, even
when it is not needed, Polling will have a delay before the device can be serviced, whereas
interrupts will respond immediately.
Explain the term even parity when used with a UART giving an example. Does the UART
receiver generate the parity bit?
Even parity is used for error checking by adding an extra bit called the parity bit. For even
parity the number of bits including the parity bit must be an even number. The transmitter
generates the parity bit, not the receiver. E.g if byte was 00000000 then parity bit is 0
Explain the terms left aligned and right aligned with regard to the ADC result registers and
where you would use each mode.
The ADC is 10-bits which requires two registers ADCH and ADCL. Left aligned means the
bits start from bit 0 of ADCH, XXXXXX11, 11111111 as shown. This is used when the full
10 bits are required (ADCH and ADCL). Right aligned is then the bits start from bit 7 of
ADCH 11111111, 11XXXXXX as shown. This is used when only 8 bits are required (ADCH)
Explain how a PWM output is used to create a varying DC level, and what external
components are required to achieve this?
The mark space of the waveform is varied, which alters the average DC value of the pulsed
signal. A resistor/capacitor network is required to smooth out the pulses
State two advantages of using interrupts over polling
Immediate response to an event, no software overheads required, as opposed to polling
Explain why pull-up resistors are required in the TWI system
The device in control of the line can only pull the line to ground, it cannot drive it high. This
allows multiple master devices to use the line without damage.
State what causes a UART data over run error and two methods that could be used to fix it.
Caused when the UART receive data register is not read quickly enough and received data is
lost. Fix it by reading the data register more often, or slowing the baud rate of the system
Explain why the UART requires a start bit.
The start bit is always low, and the idle or stop bit is always high, allowing the start of the
data to be determined
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