Ultimate gibberish machine from the Art of Electronics

[maelli01 74]

In the Horowitz/Hill Art of Electronics, third edition, design practices with discrete 74HCxxx, FPGA and Microprocessor are compared and discussed.

As an example, the ultimate gibberish machine is described, a circuit that sends out a succession of pseudorandom bytes, as standard RS232 serial data, with 2 selectable speeds, 9600 / 1200 baud. 

 

Independent of the processor type, the implementation with a small micro and program it in C looks like the clear winner here, smallest engineering effort, lowest hardware effort (have to admit that I do not have the faintest idea about FPGA.)

 

The example in the book used some 8051ish device, so I was wondering on how this would look like on a MSP430/launchpad

 

See my code below, based on the 8051 C code from the book, but adapted to run as low power as possible (LPM3)

The power consuption of this thing is: 40uA at 9600baud, 6uA at 1200baud  

//MSP430G2553LP implementation of the ultimate gibberish machine
//from the art of electronics, third edition, Chapter 11.3.
//Produces a pseudorandom datastream at 9600/1200baud, 8n1

//P1.0 output, sync, processor busy
//P1.2 output, serial pseudorandom data
//P1.3 input,  data rate select (button,at start)
//32768Hz crystal is required

//2016-10-23 maelli01

#include <msp430.h>

unsigned char d,c,b,a=1;

int main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT
  P1DIR = ~BIT3; P1REN=BIT3; P1OUT=BIT3;    // all output, except button
  P1SEL = BIT2; P1SEL2=BIT2;                // P1.2=TXD
  P2REN = 0x3F;				    // all unused pins resistor enable
  P3REN = 0xFF;			            // all unused pins resistor enable
  UCA0CTL1 |= UCSSEL_1;                     // CLK = ACLK
  if(P1IN&8){				    // button not pressed: 9600 baud
	  UCA0BR0 = 3;                      // 32768Hz/9600 = 3.41
 	  UCA0MCTL = UCBRS1 + UCBRS0;       // Modulation UCBRSx = 3
  }
  else{					    // button pressed: 1200 baud
	  UCA0BR0 = 27;                     // 32768Hz/1200 = 27.3
	  UCA0MCTL = UCBRS1;                // Modulation UCBRSx = 2
  }
  UCA0BR1 = 0x00;
  UCA0CTL1 &= ~UCSWRST;                     // **Initialize USCI state machine**
  IE2 |= UCA0TXIE;                          // Enable USCI_A0 TX interrupt
  __bis_SR_register(LPM3_bits + GIE);       // Enter LPM3 w/ int
}

#pragma vector=USCIAB0TX_VECTOR
__interrupt void USCI0TX_ISR(void){
  P1OUT |=1;
  UCA0TXBUF=((d<<1)|(c>>7))^c;              // 32 bit pseudorandom generator
  d=c; c=b; b=a; a=UCA0TXBUF;
  P1OUT &=~1;
}