#793 AT89C2051 7-Segment LED Control

Driving a common-cathode 7-Segment LED with the AT89C2051.

Here’s a quick demo..

Notes

I am using a common cathode 7-segment LED similar to the SC56-11 (see the datasheet).

Segment-Pin Mapping

Here’s a summary of how port bits are mapped to LED segments:

Character Encoding

With a common-cathode device, a port bit needs to be asserted for the corresponding segment to light. If a common-anode LED is used, the port bit needs to be low for the segment to light i.e. it is the bitwise complement of the common-cathode value.

Demo Circuit Design

Designed with Fritzing: see Led7SegmentControl.fzz.

With the AT89C2051, Port 1 is an 8-bit bi-directional I/O port. Port pins P1.2 to P1.7 provide internal pull-ups. P1.0 and P1.1 require external pull-ups. The Port 1 output buffers can sink 20 mA and can drive LED displays directly.

With a common-cathode LED display:

• the internal pull-up resistors on P1.2 to P1.7 provide a suitable drive current for an LED segment. They do not require additional external pull-up or current limiting resistors.
• P1.0 and P1.1 require an external pull-up to drive the LED segment. Using 10kΩ appears to provide a similar drive current to the pins with internal pull-ups

Note that if using a common-anode LED display, current limiting resistors will need to be added for the LED segments. The code has a setting to flip between common-anode and common-cathode logic.

The Code

See src/Led7SegmentControl.c

It uses a timer to increment the digit written to the LED display every second.

Programming

The src/Makefile is setup to compile the code using the SDCC compiler .. running on macOS in this instance:

$ cd src
$ make
sdcc -mmcs51 --code-size 2048 Led7SegmentControl.c -o Led7SegmentControl.ihx
packihx Led7SegmentControl.ihx > Led7SegmentControl.hex
packihx: read 20 lines, wrote 29: OK.

Program the chip using at89overlord and the LEAP#394 AT89C2051 Programmer:

$ at89overlord -p /dev/tty.usbserial-2420 -f ./Led7SegmentControl.hex
# Initializing the programmer...
# Initialized!
# Confirming chip ID...
# Confirmed!
# Erasing flash...
# Done!
# Writing flash...
# Done!
# Verifying...
# Done!

Testing

I have the circuit setup on a breadboard with the LEAP#780 AT89C2051 Breadboard Adapter:

Credits and References

• SC56-11 Datasheet
• AT89C2051 product info and datasheet
• Intel MCS-51
• SDCC - Small Device C Compiler
• LEAP#394 AT89C2051 Programmer
• LEAP#780 AT89C2051 Breadboard Adapter