Electronic – What happens when data is written to LATCH


This is what I found in PIC16F1947 data sheet:

Reading the PORTB register reads the status of the pins,
whereas writing to it will write to
the PORT latch. All write operations
are read-modify-write operations.
Therefore, a write to a port implies
that the port pins are read, this
value is modified and then written to
the PORT data latch (LATB).

I'm a firmware developer and my background is Computer Science. I still struggle to understand electronics and logic in hardware level. I only have the basic knowledge.

So, I want to understand what happens when data is written to latch in hardware level.

Thank you.

Best Answer

Latch is a kind of memory of one bit.

Let's use the picture in manual:

Generic I/O Port Operation

When you write a bit in a I/O pin, you're storing this bit from Data Bus to the Data Register (D-FlipFlop). If TRISx of this bit is 0, so data from Q of the Data Register will be in the I/O pin. Write in LATx or PORTx is the same. See below in red:

Generic I/O Port Operation Write

On the other hand, read from LATx is different of read from PORTx.

When you're reading from LATx, you're reading what is in the Data Register (D-FlipFlop). See picture below in green:

Generic I/O Port Operation Read LATx

And when you read from PORTx, you're reading the actual I/O pin value. See below in blue:

Generic I/O Port Operation Read PORTx

PIC uses read-modify-write to write operations and this can be a problem, so they use this shadow register to avoid it.