Getting Started with LTSpice

LTSpice is a great free circuit simulator. The software and introduction videos can be found at: www.cmosedu.com

New Mac Mini

I finally took the plunge and picked up a Mac. I'm recording a brief diary of pros/cons from my journey: 

Mac Impressions Diary

68HCS12 Heartbeat

It's often useful to have a flashing LED in embedded systems to verify the system is 'running' and healthy. I coded up a 500ms heartbeat using the Motorolla 68HCS12 in C. See below.

#include "vectors12.h"
#include "ioregs12.h"
#include "bits.h" // custom header containing bit values in hex

#define INTERRUPT void __attribute__ ((interrupt))
#define sei() __asm__ __volatile__ ( "sei" )
#define cli() __asm__ __volatile__ ( "cli" )
#define wai() __asm__ __volatile__ ( "wai" )

volatile unsigned int hbcount;

INTERRUPT rti_interrupt(void);

int main(void)
{
// Initialization
sei(); // diable interrupts
UserRTI = (unsigned int) &rti_interrupt;
RTICTL = 0x40; // set RTICTL for 1ms interrupt
CRGINT = BIT8; // enable RTI interrupts
DDRT |= BIT8; // or equal
cli(); // clear interrupt flag

while (1)
{ // idle loop
wai();
};
return 0;
}

INTERRUPT rti_interrupt(void)
{
CRGFLG = 0x80; // acknoledge RTI & clear interrupt flag
hbcount++;
if (hbcount >= 489) // Reset hbcount every 500ms
{
hbcount = 0;
PTT = PTIT ^ BIT8; // exclusive or PTT
}
}

PDP-8 Minicomputer

It's 40 year old technology, but it still pretty cool and complex. I'm working to realize the PDP-8 minicomputer from the early 70s using an FPGA. Stay tuned for progress updates!

Parallel Adder / Multiplier using VHDL

I designed a state machine controller using VHDL. This state machine implemented a parallel adder and multiplier with overflow capability. This project used the Nexys2 board from Digilent. I used Xilinx ISE for the coding and Modelsim for the simulation. For a full report see "Adder / Multiplier in VHDL" under papers.

Current Project: Temperature Controlled Voltage Source

I designed a temperature controlled voltage source. This design uses a discrete BJT operational amplifier design with a AD222100STZ temperature sensor. For a full report, see "Temp Voltage Source" under the papers section.

Modeling Temperature Behavior in LTSpice

Modeling temperature behavior in LTSpice.

I'm working on a temperature controlled amplifier using a thermocouple and operational amplifier. The simple circuit is shown above. The operational amplifier simply amplifies the thermocouple input voltage. The thermocouple input voltage is obviously a function of the temperature (measured between the reference junction and the measuring junction). For modeling purposes, the thermocouple is modeled as a voltage supply.

To simulate the changes in voltage as a function of temperature, setup a sweep in LTSpice using the .step command as shown. Next, establish a parameter using the .PARAM command, creating a new variable as a function of the sweep. This variable can then be used in the circuit. In this example, the "thermocouple" supply voltage changes based on the 'THERM' variable. The voltage graph shows output voltage as a function of temperature.