Sunday, December 31, 2006

Watch Dog Timers

A watchdog timer is a piece of hardware that can be used to automatically detect software anomalies and reset the processor if any occur. Generally speaking, a watchdog timer is based on a counter that counts down from some initial value to zero.

The embedded software selects the counter's initial value and periodically restarts it. If the counter ever reaches zero before the software restarts it, the software is presumed to be malfunctioning and the processor's reset signal is asserted. The processor (and the software it's running) will be restarted as if a human operator had cycled the power.

Introduction to watch dog timers
DS1232 WDT datasheet
(link updated 050107)

AT Keyboard

The AT keyboard was a keyboard with 84 keys introduced with the IBM PC/AT computer. It succeeded the 83-key PC/XT keyboard and therefore did not have many of the features seen on modern keyboards such as arrow keys and dual ctrl and alt keys.

It was later replaced with the 101-key Enhanced keyboard. Nonetheless, "AT keyboard" remains a popular name for any keyboard that uses the 5-pin DIN connector. Many Enhanced keyboards used this, though it was eventually superseded by the PS/2 connector and many modern computers use Universal Serial Bus (USB) connectors instead.

Interfacing the AT Keyboard

Keyboard Scan Codes

IBM PC Parallel Port

Interfacing the Parallel port : An excellent tutorial by Craig Peacock

How stuff works on Parallel Ports

Thursday, December 28, 2006

ASCII codes

ASCII stands for American Standard Code for Information Interchange. Computers can only understand numbers, so an ASCII code is the numerical representation of a character such as 'a' or '@' or an action of some sort.


BCD to ASCII conversion

The HEX equivalent of ASCII '0'(zero) is 30H.Thus to convert a packed BCD to ASCII we need to add 30H to the unpacked BCD

Packed BCD
0011 0100
Unpacked BCD
03H & 04H
0000 0011 0000 0100
33H & 34 H
0011 0011 0011 0100

Monday, December 18, 2006

I2C Bus events

I2C Bus events - a two page handout on I2C bus events

Build and load process for embedded application programs

The following two page handout is about the build and load process for embedded application programs V/s desktop application programs'