© Microhard Systems Inc.
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3.0 Hardware Description
RxD
TxD
RING
…
…
High byte size
Low byte size
Data byte 1
Data byte N
D0 D1 D7
D0 D1 D7
D0 D1 D7
D0 D1 D7
…
…
…
…
CTS
USR_SCK
Don’t care
650µs
Drawing 3-6: Data transfer from Nano Series to user device
3.2.1 Data Transfer — Modem to User Device
When the modem has data to transmit to the user it makes CTS high (if it was low), then transitions
RING low and waits for the clock. It is possible that while CTS is low the user may have just started
sending the clock signal to the modem and the modem didn’t have time to detect it. Due to this
potential conflict when we make CTS high, about 650 µs is required to detect if there was
transmission in progress. If, after waiting 650 µs the modem doesn’t report any bytes received the
modem is reconfigured in transmit mode, loads the message and transitions RING low. The user is
required to start sending clock but there is no requirement on a delay between RING going low and
the clock. When the modem gets enough clocks to output all data we make RING go high, free the
buffer, switch to receive mode and once ready, make CTS low for the new cycle. Once RING is high,
the user must stop sending the clock within 100 µs.
The modem sends a data packet, where the first two bytes represent a packet size: the first byte is a
high byte; the next byte is a low byte. The packet size can’t be zero and can’t exceed 1558.
