8000 extmod/machine_i2c: Add clock stretching support. · sparkfun/circuitpython@ec078af · GitHub
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deshipudpgeorge
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extmod/machine_i2c: Add clock stretching support.
When the clock is too fast for the i2c slave, it can temporarily hold down the scl line to signal to the master that it needs to wait. The master should check the scl line when it is releasing it after transmitting data, and wait for it to be released. This change has been tested with a logic analyzer and an i2c slace implemented on an atmega328p using its twi peripheral, clocked at 8Mhz. Without the change, the i2c communication works up to aboy 150kHz frequency, and above that results in the slave stuck in an unresponsive state. With this change, communication has been tested to work up to 400kHz.
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‎extmod/machine_i2c.c

Lines changed: 8 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -34,6 +34,9 @@
3434

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#if MICROPY_PY_MACHINE_I2C
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37+
// Clock stretching limit, so that we don't get stuck.
38+
#define I2C_STRETCH_LIMIT 255
39+
3740
typedef struct _machine_i2c_obj_t {
3841
mp_obj_base_t base;
3942
uint32_t us_delay;
@@ -53,6 +56,11 @@ STATIC void mp_hal_i2c_scl_low(machine_i2c_obj_t *self) {
5356

5457
STATIC void mp_hal_i2c_scl_release(machine_i2c_obj_t *self) {
5558
mp_hal_pin_od_high(self->scl);
59+
mp_hal_i2c_delay(self);
60+
// For clock stretching, wait for the SCL pin to be released, with timeout.
61+
for (int count = I2C_STRETCH_LIMIT; mp_hal_pin_read(self->scl) == 0 && count; --count) {
62+
mp_hal_delay_us_fast(1);
63+
}
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}
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5866
STATIC void mp_hal_i2c_sda_low(machine_i2c_obj_t *self) {
@@ -71,7 +79,6 @@ STATIC void mp_hal_i2c_start(machine_i2c_obj_t *self) {
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mp_hal_i2c_sda_release(self);
7280
mp_hal_i2c_delay(self);
7381
mp_hal_i2c_scl_release(self);
74-
mp_hal_i2c_delay(self);
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mp_hal_i2c_sda_low(self);
7683
mp_hal_i2c_delay(self);
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}
@@ -81,7 +88,6 @@ STATIC void mp_hal_i2c_stop(machine_i2c_obj_t *self) {
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mp_hal_i2c_sda_low(self);
8289
mp_hal_i2c_delay(self);
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mp_hal_i2c_scl_release(self);
84-
mp_hal_i2c_delay(self);
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mp_hal_i2c_sda_release(self);
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mp_hal_i2c_delay(self);
8793
}
@@ -108,14 +114,12 @@ STATIC int mp_hal_i2c_write_byte(machine_i2c_obj_t *self, uint8_t val) {
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}
109115
mp_hal_i2c_delay(self);
110116
mp_hal_i2c_scl_release(self);
111-
mp_hal_i2c_delay(self);
112117
mp_hal_i2c_scl_low(self);
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}
114119

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mp_hal_i2c_sda_release(self);
116121
mp_hal_i2c_delay(self);
117122
mp_hal_i2c_scl_release(self);
118-
mp_hal_i2c_delay(self);
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120124
int ret = mp_hal_i2c_sda_read(self);
121125
mp_hal_i2c_delay(self);
@@ -150,7 +154,6 @@ STATIC int mp_hal_i2c_read_byte(machine_i2c_obj_t *self, uint8_t *val, int nack)
150154
uint8_t data = 0;
151155
for (int i = 7; i >= 0; i--) {
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mp_hal_i2c_scl_release(self);
153-
mp_hal_i2c_delay(self);
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data = (data << 1) | mp_hal_i2c_sda_read(self);
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mp_hal_i2c_scl_low(self);
156159
mp_hal_i2c_delay(self);
@@ -163,7 +166,6 @@ STATIC int mp_hal_i2c_read_byte(machine_i2c_obj_t *self, uint8_t *val, int nack)
163166
}
164167
mp_hal_i2c_delay(self);
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mp_hal_i2c_scl_release(self);
166-
mp_hal_i2c_delay(self);
167169
mp_hal_i2c_scl_low(self);
168170
mp_hal_i2c_sda_release(self);
169171

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