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libbase/i2c: use busy_wait_us instead of cdelay 

`cdelay` function is not a proper thing to count time.
It wouldn't count SoC clocks, but CPU clocks.
But even then, there are multiple instructions in `cdelay`:
  - NOP
  - decrement
  - branch with compare
Assuming each instruction takes exactly 1 CPU cycle it is still wrong,
as we wait 3 time longer than requested.
But they don't take exactly 1 CPU cycle.
CPUs have caches, branch predictors, are out-of-order and so on.

So a much better way to count this time would be `busy_wait_us`.

I performed some test using vexriscv and Saleae Logic Analyzer:

vexriscv variant | requested I2C speed | actual (cdelay) | actual (busy_wait_us)
-----------------+---------------------+-----------------+----------------------
minimal          |  50 kHz             |  4 kHz          |  38 kHz
minimal          | 200 kHz             | 15 kHz          |  96 kHz
minimal          | 400 kHz             | 28 kHz          | 137 kHz
-----------------+---------------------+-----------------+----------------------
lite             |  50 kHz             | 12 kHz          |  40 kHz
lite             | 200 kHz             | 43 kHz          | 115 kHz
lite             | 400 kHz             | 74 kHz          | 180 kHz
-----------------+---------------------+-----------------+----------------------
standard         |  50 kHz             | 12 kHz          |  45 kHz
standard         | 200 kHz             | 48 kHz          | 159 kHz
standard         | 400 kHz             | 84 kHz          | 311 kHz

Signed-off-by: Michal Sieron <msieron@antmicro.com>
This commit is contained in:
Michal Sieron 2023-01-23 18:34:37 +01:00
parent b122178876
commit 7f829f9e44
1 changed files with 5 additions and 10 deletions
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15
litex/soc/software/libbase/i2c.c
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@ -7,11 +7,14 @@
#include <generated/soc.h>
#include <generated/csr.h>
#include <system.h>
#ifdef CONFIG_HAS_I2C
#include <generated/i2c.h>
#define I2C_PERIOD_CYCLES (CONFIG_CLOCK_FREQUENCY / I2C_FREQ_HZ)
#define I2C_DELAY(n) cdelay((n)*I2C_PERIOD_CYCLES/4)
#define U_SECOND (1000000)
#define I2C_PERIOD (U_SECOND / I2C_FREQ_HZ)
#define I2C_DELAY(n) busy_wait_us(n * I2C_PERIOD / 4)
int current_i2c_dev = DEFAULT_I2C_DEV;
@ -20,14 +23,6 @@ int get_i2c_devs_count(void) { return I2C_DEVS_COUNT; }
void set_i2c_active_dev(int dev) { current_i2c_dev = dev; }
int get_i2c_active_dev(void) { return current_i2c_dev; }
static inline void cdelay(int i)
{
while(i > 0) {
__asm__ volatile(CONFIG_CPU_NOP);
i--;
}
}
int i2c_send_init_cmds(void)
{
#ifdef I2C_INIT