Physiological Stimulator

Introduction

The goal of this project was to build a small, cheap, light-weight stimulator for student use in electrophysiology labs. Specifically:

• Computer-controlled pulse trains, with optional manual trigger.

• Times:
  • Pulse duration 1-4000 milliseconds
  • Pulse spacing 1-4000 milliseconds
  • Stimulus interval 1-4000 milliseconds
  • Pulse train setable from 1 pulse to 255 pulses
• Two versions:
  • Controlled via an rs232 port on a host computer.
  • Controlled via a keypad and LCD display.

The traditional stimulator is a box full of analog and digital parts. We replaced all these with a cheap microcontroller.

PC-controlled version

There are four processes which run on the microcontroller:

• The RESET entry point initializes the ports and timers, then waits for the operator to enter stimulation parameters on the host-computer keyboard. When the stimulator is running, a main loop detects a button push (if in manual trigger mode) and detects a 's' keypress on the host-computer keyboard to stop the stimulator and prompt for new parameters.
• The timer 1A compare-match ISR (interrupt service routine) occurs when a new stimulus is ready to start. The ISR initializes pulse counters and emits a synch pulse, then does nothing else until the next repitition of the song. Timer 1 ticks once every 64 microseconds. In manual mode, this ISR is disabled, but is entered as a subroutine call when the trigger button is pushed.
• The timer 1B compare-match ISR occurs whenever a pulse starts or ends. The ISR sets the pulse variables, then sets it's own next occurance by keeping track of the next pulse start/end time.
• The timer zero overflow ISR occurs once per millisecond. This ISR is used for general timing, including debouncing the trigger button. Timer 0 ticks once every 16 microseconds.

This AVR assembler code is the program running on the mcu (march 23 version).

Using the PC-controlled stimulator

This version of the stimulator assumes that you have a terminal emulator (or custom program which can emit rs232 commands) on the host-computer. The communications port should be set to 9600 baud, 8-bits, no parity, one stop-bit, no flow control. If you are using an AVR prototype board, then you must configure it for serial communication.

When the microcontroller is reset, it will prompt the user for 4 numbers with the following message:

Enter number, dur, spacing, interval>

A typical response might be:

4 1 3 500

meaning that a stimulus should be 4 pulses, each 1 millsec long, separated by 3 millsec, and repeating every 500 millsec.

The mcu will then prompt the user for free run vs manual trigger.

Press m for manual, r for freerun>

The only valid responses are 'm' and 'r', for manual and free run respectively. If the manual option is chosen then there will be exactly one stimulus sequence from the mcu when a button is pushed.

PortD.0 and PortD.1 on the AVR mcu are the serial connection to the PC. PortB.0 is the stimulus output. PortB.1 is the synch pulse output. A pushbutton with pullup resistor must be attached to PortD.2 for the manual trigger.

Stand-alone version

There are four processes which run on the microcontroller:

• The RESET entry point initializes the ports and timers, then waits for the operator to enter stimulation parameters on the 16 button keypad. When the stimulator is running, a main loop detects a button push (if in manual trigger mode) and detects a '#' keypress on the keypad to stop the stimulator and prompt for new parameters. All text is displayed on an LCD display.
• The timer 1A compare-match ISR (interrupt service routine) occurs when a new stimulus is ready to start. The ISR initializes pulse counters and emits a synch pulse, then does nothing else until the next repitition of the song. Timer 1 ticks once every 64 microseconds. In manual mode, this ISR is disabled, but is entered as a subroutine call when the trigger button is pushed.
• The timer 1B compare-match ISR occurs whenever a pulse starts or ends. The ISR sets the pulse variables, then sets it's own next occurance by keeping track of the next pulse start/end time.
• The timer zero overflow ISR occurs once per millisecond. This ISR is used for general timing, including debouncing the trigger button. Timer 0 ticks once every 16 microseconds.

The AVR code (march 23 version) and schematics will be here soon...

Using the Stand-alone version

Stay tuned, still under revision...