Max speed SPI Software 16f628

vtrx · Joined: 11 Oct 2017 Posts: 142

what is the maximum speed that is possible with 4MHZ using the 16f628a?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The 16F628 doesn't have a hardware MSSP module, so SPI must be
done in software. A 4 MHz crystal gives an instruction clock of 1 MHz.

Using the test program shown below in MPLAB vs. 8.92, and the
Stopwatch feature, I got 296 instruction cycles to send 16 bits.
That's 296 uSec. 296/16 gives 18.5 uSec per bit. That gives
about 54 KHz as the maximum effective bit frequency. The real
bit frequency will be faster than that, because there are some
cycles used in overhead.

Test program:

vtrx · Joined: 11 Oct 2017 Posts: 142

using 20mhz I can set the uart with 270khz, right?

temtronic · Posted: Sun Jul 11, 2021 6:17 am

yes, X5 clock = X5 SPI speed BUT...
be sure to use xtal/2caps for clock, to get accurate, consistant speed
also since this is a SOFTWARE SPI, nothing else can be done while SPI is being used...

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

No 'uart' involved for SPI. You can get an SPI clock rate of 270KHz at 20Mhz.
The 'A' in the word UART, means 'asynchronous'. The whole point of SPI
is it is a synchronous interface.

temtronic · Posted: Sun Jul 11, 2021 8:17 am

an observation....
You may want to consider a 'pin compatible' PIC that has builtin hardware SSP peripheral. While the software SPI will work, depending on application, it does cost you code space. That PIC only has 2K, so it might be interesting to see how much space IS taken to do software SPI.
As the 628 is an 'older' PIC, odds are pretty good a newer one will have more memory, HW SPI and cost less.
Jay

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

16F1847 and it's cheaper too. It requires Pickit 3 to program it.

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

Absolutely.
The PIC16F18444, is the direct modern replacement for the 628A.
Has hardware SPI, up to 32MHz from the internal clock, and up to 28K of
ROM. It's also about half the price!....
It also supports PPS, so you can move the peripherals to where you want
them.

vtrx · Joined: 11 Oct 2017 Posts: 142

I'm actually going to use 16f648.
The value of this micro on aliexpress, where I buy it, is cheaper than the 16f1847.
The PIC will only receive serial data and write to an 8x8 matrix (15 Cis MAX).

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

Seriously still only gives a 4HHz internal oscillator. Adding a crystal to
give 20MHz, costs more than the 16F1847. This gives 32MHz on the internal
oscillator, and has hardware SPI.

temtronic · Posted: Sun Jul 11, 2021 2:03 pm

It sounds like he's using the PIC to send data to an SPI based 8x8 LED matrix display module. If so, he really doesn't need high speed SPI. Anything faster than say 50Hz will make the LEDs appear to be stable.
A lot would depend on the actual LED module being used, something we don't know.

vtrx · Joined: 11 Oct 2017 Posts: 142

8x8 matrix (MAX7912),15 modules.

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

Thing is that if you are daisy chaining 15 of these modules, you potentially
have 120 characters needing to be stored in the PIC. The actual data
stream needed to be sent to the display for an update is another 30 bytes.
Doesn't allow for much spare RAM in a PIC16F648. Then there will need
to be something like serial data being decoded to actually generate what
is to be sent to the display. Suddenly you are running out of space....
It is a waste of effort, trying to fit code into a smaller PIC. Seriously, using
the 1847, will give you smaller code that is easier to write, and runs a
lot faster, and enough room to actually work in.
The very start of this thread was hoe fast the SPI could be driven, implying
that the poster thinks speed is important. A 1847 at (say) 16MHz, can
generate a SPI stream running over seventy times faster than the 628/648,
and gives space to actually do the work to generate this.

temtronic · Posted: Mon Jul 12, 2021 4:55 am

Points to ponder,while coffeee's being made..

1) given 15 modules, in series...(15 x 15= 240 bits of data.30 bytes of data..)
@the refresh rate of 800Hz(typ in the datasheet),the refresh rate for the chain would be 12,000Hz( 15x 800) ?

2) with 15 module, 330ms per module(spec sheet..) you'll need a solid 5 AMP power supply JUST for the LEDS ! 'Old skool' says to double that for long ter reliablility.

3) data input ,serial (UART) baudrate. will have to be faster than the 'update LED' rate,say a factor of x2, to allow for buffering,parsing,SPI straming.

vtrx · Joined: 11 Oct 2017 Posts: 142

I'm at work now, at night and post the full code, using the 16f628a, to show a fifteen letter word received by the UART(9.600), which is sent by a 18f2550 by USB HID.
It works very well.
The code contains the ASCII table, copied from Arduino, and the value received by the serial is calculated by the ASCII value to be found in the table.