Nintendo GAME BOY Programming Manual page 33

Chapter 1: System
In the example, 1 byte is sent per frame. (This is not required.) If several bytes are sent
continuously, a transmission interval longer than the processing time of other interrupts (e.g.
V_BLANK) should be used (usually around 1 mS). The reason is that if an attempt is made to
communicate with the slave during another interrupt, the slave cannot receive the data until after
the interrupt is finished. If the next data is transmitted before the other interrupt is finished, the
slave will be unable to receive the initial data of the transmission.
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