My I2CWRITE - timings and tricks
Recent endeavours with a 18F4550 at 48 MHz talking to both a 24LC256 EEPROM and a DAC8571 on the I2C bus have taught me a lot about I2CWRITE which I am now sharing "for the record".
I wont bother mentioning that
I2CWRITE SDA, SCL, $A0, 0, [Buffer\64]
is illegal and wont work - just please use variables ONLY (not constants).
No, my experimentation with the DAC revealed an undocumented feature of I2CWRITE (aka useful bug) - namely that if you don’t bother reading the PBP manual properly and you write:
I2CWRITE SDA, SCL, %10011000, %00010000, DAC
where DAC is a word to send to the DAC, the compiler does not complain and compiles BUT when you run the program, it doesn’t send a stop condition. This can be most useful, leaving the way to leaving the I2C bus connected to the DAC with it listening and waiting for more data which can then be sent as:
I2CWRITE SDA, SCL, DAC
and the bus can be closed down and restored either by doing it properly (note the brackets around the data!!!):
I2CWRITE SDA, SCL, %10011000, %00010000, [DAC]
or by a
GOSUB I2CStop 'see later for this routine
The same technique works fine for the EEPROM also.
Sometimes PicBasic commands are deceptive - take:
I2CWRITE SDA, SCL, ControlByte, Address, [Buffer\64]
This looks like a pretty slick and fast way of sending data to an EEPROM such as the 24LC256. And intuitively it seems that it is best to send the maximum number of bytes in one go..... However:
I2C bus timings for the 24LC256 are defined as a maximum bit rate of 400 KHz (assuming Vcc >2.5v, 100 KHz if Vcc <2.5v), giving a minimum pulse of 2.5 uSec, but also a minimum CLK hi of 0.6 uSec (if Vcc>2.5v) and minimum CLK low of 1.3 uSec. It also needs at least 5 mSec to complete a write, regardless of the size of the write which can be a maximum page size of 64 bytes. In practice this means that in PBP each I2CWrite takes 40 uSec per byte (5 uSec per bit).
And if you think you can do better than MELabs and write your own routines - e.g.:
Don’t bother because this takes longer than I2CWRITE (because of the pacing of SHIFTOUT.Code:I2CWrBuff: 'I2CWRITE SDA, SCL, CntrB, Addr, [str Buffer\8] HIGH SDA 'Start HIGH SCL 'Start LOW SDA 'Start LOW SCL 'Start j = CntrB gosub I2COut j = Addr.highbyte 'Note must send Address Hi, Low GOSUB I2cOut j = Addr.lowbyte GOSUB I2cOut for i = 0 to 7 Value = Buffer[i] 'Buffer[i].Lowbyte is syntax error j = Value.lowbyte 'Send array words Lo, Hi GOSUB I2cOut j = Value.highbyte GOSUB I2cOut next I2CStop: LOW SDA 'Stop HIGH SCL 'Stop HIGH SDA 'Stop RETURN I2COut: SHIFTOUT SDA, SCL, 1, [j] '1=MSBFIRST INPUT SDA HIGH SCL 'Receive Acknowledge LOW SCL RETURN
My task then was to maximise the sample rate of an ADC read while storing the result in a 24LC256 EEPROM. So:
Each I2CWrite requires a 3 byte header (Control byte, Address word), and then the data. Without batching the data for a page write, this means the minimum sample interval is determined by having to wait 5 mSec for the I2CWrite to finish = 200 Hz.
On the other hand if we batch the maximum amount of data to send together with an array write of 64 bytes (64 byte page max), the actual I2CWrite command will take at least:
120 + 40*64 = 2680 uSec = 370 Hz.
So the optimum number of bytes (b) to send in a batch is achieved by solving the equation:
b*(40*b + 120) > 5000
4*b^2 +12b -500 > 0
b > 11.046
So transferring 12 bytes for each I2Cwrite achieves the fastest sample rate of 600 uSec per sample or 1.67 KHz.
However, as the datasheet says:
Write operations are limited to writing bytes within a single physical page, regardless of the number of bytes actually being written. Physical page boundaries start at addresses that are integer multiples of the page buffer size (or ‘page size’) and end at addresses that are integer multiples of [page size - 1].
This means that the answer to my puzzle is the optimum solution is to send 16 bytes (to avoid crossing a page boundary) - this gives a sample interval of 760 uSec or 1.32 KHz.
As my 18F4550 has a 10bit ADC, I am going to need to send 8 words of data at a time.
My routine (including the method of timing the loops using TIMER0) follows:
Enjoy and if I have made any mistakes - be kind.Code:T0CON=%00010011 'Timer 0: Off[0], 16Bit[0], Fosc/4[0], [1], 1:16 prescaler [0011] '16*4/48 usec per tick = 1.333 (4/3) uSec per tick ADC_in: lcdout $FE, 1, "ADC to Memory" pbuff =0 Addr = 0 block = 1 CntrB = (%10100000|(Block<<1)) t0con.7 = 1 'Turn Timer0 on ADC1: Tick.Lowbyte = TMR0L 'Read Low first - High latched Tick.highbyte = TMR0H TMR0H = 0 'Write High first - Latched until write Low TMR0L = 0 high led2 'Indicate acquisition - Menu (gButt) turns off adcin 0, value '10 bit to 16 bit and swap LSB/MSB because 'STR sends LSB,MSB and DAC expects MSB,LSB, ie: 'xxxxxx9876543210 > 210xxxxx,xxxxxxxx + xxxxxxxx,A9876543 'Buffer[pBuff] = (value<<13) + (Value>>2) Buffer[pBuff] = Tick 'Borrow the data area to display timings - remove later pbuff = pbuff +1 if pbuff = 8 then I2CWRITE SDA, SCL, CntrB, Addr, [str Buffer\8] pbuff = 0 addr = addr + 16 ' 8 words if addr=32768 then addr = 0 block = block +1 CntrB = (%10100000|(Block<<1)) endif if block = 3 then Memful endif if pBuff <>0 then pauseus 987 '13 uSec per loop else pauseus 237 '763 uSec per loop endif gosub gButt 'Routine to check for Button press (has two states: short/long) if Butt = 0 then ADC1 goto Menu
Peter Finch
Originally Posted by FinchPJ
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