Functional Description
188 Datasheet
the on/off state of the LED. To allow flexibility in pull-up resistor values for power
optimization, the frequency of the transmission is programmable using the DRS field in
the GP_GB_CMDSTS register.
The serial bit stream is Manchester encoded. This choice of transmission ensures that a
transition will be seen on every clock. The 1 or 0 data is based on the transmission
happening during the high or low phase of the clock.
As the clock will be encoded within the data stream, hardware must ensure that the Z-
0 and 0-Z transitions are glitch-free. Driving the pin directly from a flop or through
glitch-free logic are possible methods to meet the glitch-free requirement.
A simplified hardware/software register interface provides control and status
information to track the activity of this block. Software enabling the serial blink
capability should implement an algorithm referenced below to send the serialized
message on the enabled GPIO.
1. Read the Go/Busy status bit in the GP_GB_CMDSTS register and verify it is cleared.
This will ensure that the GPIO is idled and a previously requested message is still
not in progress.
2. Write the data to serialize into the GP_GB_DATA register.
3. Write the DLS and DRS values into the GP_GB_CMDSTS register and set the Go bit.
This may be accomplished using a single write.
The reference diagram shows the LEDs being powered from the suspend supply. By
providing a generic capability that can be used both in the main and the suspend power
planes maximum flexibility can be achieved. A key point to make is that the PCH will
not unintentionally drive the LED control pin low unless a serialization is in progress.
System board connections utilizing this serialization capability are required to use the
same power plane controlling the LED as the PCH GPIO pin. Otherwise, the PCH GPIO
may float low during the message and prevent the LED from being controlled from the
SIO. The hardware will only be serializing messages when the core power well is
powered and the processor is operational.
Care should be taken to prevent the PCH from driving an active ‘1’ on a pin sharing the
serial LED capability. Since the SIO could be driving the line to 0, having the PCH drive
a 1 would create a high current path. A recommendation to avoid this condition
involves choosing a GPIO defaulting to an input. The GP_SER_BLINK register should be
set first before changing the direction of the pin to an output. This sequence ensures
the open-drain capability of the buffer is properly configured before enabling the pin as
an output.
5.15.5.2 Serial Message Format
In order to serialize the data onto the GPIO, an initial state of high-Z is assumed. The
SIO is required to have its LED control pin in a high-Z state as well to allow the PCH to
blink the LED (refer to the reference diagram).
The three components of the serial message include the sync, data, and idle fields. The
sync field is 7 bits of ‘1’ data followed by 1 bit of ‘0’ data. Starting from the high-Z state
(LED on) provides external hardware a known initial condition and a known pattern. In
case one or more of the leading 1 sync bits are lost, the 1s followed by 0 provide a
clear indication of ‘end of sync’. This pattern will be used to ‘lock’ external sampling
logic to the encoded clock.
The data field is shifted out with the highest byte first (MSB). Within each byte, the
most significant bit is shifted first (MSb).
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