Help with 4 digit BCD display
I am looking to connect to an existing display in a device that has 4 seven segment displays driven by one 7447 BCD driver chip. I looked in the OBEX for any objects already written to drive such but came up empty.
Don
Don
Comments
Looking at the data sheet for the 7447 BCD chip, if you want to output a 9 then
D = 1
C = 0
B = 0
A = 1
Looks very easy. Do you have a specific programming question?
I know that in order to multiplex all 4 digits that there needs to be a ring counter set up for the digit scan and then it needs to be synchronized to the BCD output.
There are several ways to do this.
You can dedicate 8 prop pins to do it directly. That is, output the bcd digit on 4 pins and the digit select on one of the other 4 pins.
With 6 pins and some external hardware you could output bcd on 4 pins and the binary digit select on 2 pins.
With an external shift register ( 74HC595 or similar ) and 3 pins you can shift out the bcd and digit select serially. A 74HC164 could do this with 2 pins but you might notice some flicker as each digit is shifted out.
You need to pass the address of a four-byte array that holds the digit values.
The problem I have is that the display operates at approx 4 volts so I'll need to operate it at a higher voltage then what the prop can provide so I have attached some transistors.
Attached is the schematic of what I'm thinking of doing. If anyone has any comments about this type of circuit and whether it'll work or not, please speak up.
Thanks.
LED Display.bmp
One advantage to using the 7447 is that the load on the Propeller's I/O pins is less if a lot of segments need to be on (like for 8). You could use driver transistors there (NPNs) too if you're not using the 7447.
LED Display.bmp