Increase pullup resistor on RX pin #79
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On some GPIO the g_AM_HAL_GPIO_INPUT_PULLUP seems to be strong enough to cause problems with translation buffers like the TXB0104. Limiting to 24k fixing issue.
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This PR makes software serial work reliably on pins connected to translation buffers such as the TXB0104 used on the SparkFun LTE cellular shield.
On some GPIO the g_AM_HAL_GPIO_INPUT_PULLUP seems to be strong enough to cause problems with translation buffers like the TXB0104. Limiting to the pullup to 24k fixes the issue.
With just g_AM_HAL_GPIO_INPUT_PULLUP the TXB0104 has a min voltage of 1.28V:
The Apollo3 can't interpret the 1.3V as low .
But if we use the 24k pullup g_AM_HAL_GPIO_INPUT_PULLUP_24 the signal gets much better:
And just for fun, here is pull up with 12k. The low voltage raises as the pull up resistance decreases we would expect:
This suggests the generic "g_AM_HAL_GPIO_INPUT_PULLUP" uses a strong pullup, however it's not clear in the HAL what "WEAK" means:
I don't believe there is a downside to using the 22k pull up value rather than the 'WEAK' value.