an SPI DAC MCP4921/4922

[jaethelegend]

Hi, I am trying to use MCP 4921 DAC.
I got it to work, but have some questions
1) I tried to use it without connecting one of my pins to LDAC, to save pins, using same code as below leaving out commands associated with LDAC. The spec sheet states that if i coonnect LDAC pin to the ground , the data will transfer on the rising edge of the clock. However, it doesn't seems to be working
2)I don't understand what BUF bit is. The data sheet states that BUF bit is a Vref Input buffer control bit. and when it's 1, it is buffered and when it is 0 it is unbuffered. I don't know the difference between the two.
3) In the below code, when i leave out the
while(!BF)
the program doesn't seem to be working.
The data sheet states

bit 0 BF: Buffer Full Status bit
Receive (SPI and I2 C modes):
1 = Receive complete, SSPBUF is full
0 = Receive not complete, SSPBUF is empty
Transmit (I2 C mode only):
1 = Data transmit in progress (does not include the ACK and Stop bits), SSPBUF is full
0 = Data transmit complete (does not include the ACK and Stop bits), SSPBUF is empty
Since I am transmitting to DAC, in SPI mode(not I2C mode), can't i be care less?

Thank you in advance

[PCM programmer]

I notice you're asking the same question in the Microchip forum (2 times).

The answer is:
Even though the data sheet says the BF bit is only valid in Receive mode
for SPI, it is also valid for transmit mode. The BF bit goes high when
the byte has been transmitted from the MSSP module.

Or, another way to think of it is:
When the MSSP module is transmitting, it is also receiving. That's because
SPI is just two 8-bit shift registers (one in the master and one in the
slave) that are connected to each other in a loop. When the master
transmits a byte to the slave, it also receives a byte from the slave.
So when it receives a byte, the "Buffer Full" bit will go true (high).
At that moment, it has also just finished transmitting a byte.

With regard to your problem:
You are using the Chip Select signal (\CS). You must not set \CS to a
high level before the byte been transmitted. It takes time for the MSSP
to transmit the byte. That's why you must check the Buffer full bit,
and see that it goes high, before you set \CS high. If you set \CS high
before the byte has finished transmitting, the slave will not receive a
complete byte.

A comment about your programming style.
You are writing low-level code, as if you have the Hi-Tech compiler or
some other compiler than CCS. Why ? The CCS spi_write() function
checks the BF bit. It only returns when the BF bit goes high. If you had
used the CCS functions to write your code, you would not have all these
problems. I suggest that you use the CCS functions. They will prevent
errors in your code.

[jaethelegend]

Thanks for the reply.
I always post same question on both forums so that i can get as much resources as possible. Sorry if i offended some of you.

Right now, I am practicing low level coding instead of using the built in functions in CCS-C because I just started with PIC and would like to learn better about how register files and etc. works.

One remaining question is that I am trying to save a port by grounding DAC latch as the data sheet suggests. However, it doesn't seem to be working.

[RLScott]

[Franck26]

DAC latch grounded works fine for me too.

What do you mean by not working?
Is the output = 0V, 5V, or high impedance?

Franck.

[jaethelegend]

It started working fine with a new chip. Thanks