1. Consider the voltage levels and drive strength for the HCS12 family that are

Question

1. Consider the voltage levels and drive strength for the HCS12 family that are discussed in section 7.6 of the textbook.
a. How much current can each output source or sink?
b. What is the highest voltage that can be input and still be considered a logic 0?

2- Answer the following questions about RS232 and the HCS12 SCI (UART)
a. In RS-232, what gets transmitted first – the LSB or the MSB?
b. In RS-232, is MARK a logic 1 or a logic 0?
c. In RS-232, is a START bit MARK or SPACE?
d. For an E clock of 20 MHz, what baud-rate register setting gives a baud rate of approximately 9600 bits per second?
e. For the baud-rate register setting you calculated, what is the exact baud rate that you obtained?
f. Which bit is polled to determine if there is a character ready to be received?

Explanation / Answer

1

a. Source current of an output pin must be equal to or greater than the total current 'FLOWING INTO PERIPHERAL PINS' that are connected.to this pin

Sink current output pin must be equal to or greater than the total current 'FLOWING OUT FROM ALL THE PERIPHERAL PINS' connected to this pin

b. 35% of VDD is considered as the highest voltage that can be input for a logic 0 i.e. 1.75 volts for a VDD of 5 volts

2.

a. LSB is transmitted first in RS-232

b. MARK is a logic 1

c, Start bit is always a SPACE

d. Baud rate = fosc/(12*32*(256-TH1)) where TH1 is contents in transmit Holding register 1

9600 = 20,000,000/(12*32*(256-TH1)

TH1 = 256-5.42535 = 250.57465 = 251 approximately

Baud rate register is set to 251

e. For the Baud rate register value of 251, exact baud rate is

baud rate = 20,000,000/(12*32*(256-251)) = 10,416bits per second

f. RI(reciever Interrupt) bit is polled if there is a character is ready to be recieved

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